What is concurrency?

Concurrency in terms of databases means allowing multiple users to access the data contained within a database at the same time. If concurrent access is not managed by the Database Management System (DBMS) so that simultaneous operations don't interfere with one another problems can occur when various transactions interleave, resulting in an inconsistent database.

Concurrency is achieved by the DBMS, which interleaves actions (reads/writes of DB objects) of various transactions. Each transaction must leave the database in a consistent state if the DB is consistent when the transaction begins. Concurrent execution of user programs is essential for good DBMS performance. Because disk accesses are frequent, and relatively slow, it is important to keep the CPU humming by working on several user programs concurrently. Interleaving actions of different user programs can lead to inconsistency: e.g., check is cleared while account balance is being computed. DBMS ensures such problems don’t arise: users can pretend they are using a single-user system.

Define Transaction A transaction is a sequence of read and write operations on data items that logically functions as one unit of work. • It should either be done entirely or not at all. • If it succeeds, the effects of write operations persist (commit); if it fails, no effects of write operations persist (abort). • These guarantees are made despite concurrent activity in the system, and despite failures that may occur.

ACID Properties of Transaction

• Atomic Process all of a transaction or none of it; transaction cannot be further subdivided

(like an atom) • Consistent Data on all systems reflects the same state

• Isolated Transactions do not interact/interfere with one another; transactions act as if they are independent

• Durable Effects of a completed transaction are persistent

Concurrent Execution

1. Improved throughput and resource utilization. (THROUGHPUT = Number of Transactions executed per unit of time.) The CPU and the Disk can operate in parallel. When a Transaction Read/Write the Disk another Transaction can be running in the CPU. The CPU and Disk utilization also increases. 2. Reduced waiting time. In a serial processing a short Transaction may have to wait for a long transaction to complete. Concurrent execution reduces the average response time; the average time for a Transaction to be completed.

What is concurrency control?

Concurrency control is needed to handle problems that can occur when transactions execute concurrently. The following are the concurrency issues:- Lost Update: An update to an object by some transaction is overwritten by another interleaved transaction without knowledge of the initial update.

Lost Update Example:-

Transaction A’s update is lost

Uncommitted Dependency: a transaction reads an object updated by another transaction that later falls. Uncommitted Dependency Example:-

Transaction B reads an uncommitted value for R

Inconsistent Analysis: a transaction calculating an aggregate function uses some but not all updated objects of another transaction.

Inconsistent Analysis Example:-

The value in SUM will be inconsistent

Main goals of Database Concurrency Control • To point out problem areas in earlier performance analyses • To introduce queuing network models to evaluate the baseline performance of transaction processing systems • To provide insights into the relative performance of transaction processing systems • To illustrate the application of basic analytic methods to the performance analysis of various concurrency control methods • To review transaction models which are intended to relieve the effect of lock contention • To provide guidelines for improving the performance of transaction processing systems due to concurrency control; and to point out areas for further investigation.

Transaction properties

To ensure data integrity the DBMS, should maintain the following transaction properties- atomicity, consistency, isolation and durability. These properties often referred to as acid properties an acronym derived from the first letter of the properties. We will consider a banking system that contains several accounts and a set of transactions that accesses and updates accounts. Access to a database is accomplished by two operations given below:- 1. Read(x)-This operation transfers the data item x from the database to a local buffer belonging to the transaction that executed the read operation 2. Write(x)-the write operation transfers the data item x from the local buffer of the transaction that executed the write operation to the database. Now suppose that Ti is a transaction that transfers RS. 2000/- from account CA2090 to SB2359. This transaction is defined as follows:-

Ti:Read(CA2090);CA2090:=CA2090-2000;Write(CA2090);Read(SB2359);SB2359:=SB2359+2000;Write(SB2359);

Implementing Atomicity

Let’s assume that before the transaction take place the balances in the account is Rs. 50000/- and that in the account SB2359 is Rs. 35000/-. Now suppose that during the execution of the transaction a failure (for example, a power failure) occurred that prevented the successful completion of the transaction. The failure occurred after the Write(CA2090); operation was executed, but before the execution of Write(SB2359); in this case the value of the accounts CA2090 and SB2359 are reflected in the database are Rs. 48,000/- and Rs. 35000/- respectively. The Rs. 2000/- that we have taken from the account is lost. Thus the failure has created a problem. The state of the database no longer

reflects a real state of the world that the database is supposed to capture. Such a state is called an inconsistent state. The database system should ensure that such inconsistencies are not visible in a database system. It should be noted that even during the successful execution of a transaction there exists points at which the system is in an inconsistent state. But the difference in the case of a successful transaction is that the period for which the database is in an inconsistent state is very short and once the transaction is over the system will be brought back to a consistent state. So if a transaction never started or is completed successfully, the inconsistent states would not be visible except during the execution of the transaction. This is the reason for the atomicity requirement. If the atomicity property provided all actions of the transaction are reflected in the database of none are. The mechanism of maintaining atomicity is as follows The DBMS keeps tracks of the old values of any data on which a transaction performs a Write and if the transaction does not complete its execution, old values are restored or make it appear as though the transaction never took place. The transaction management component of the DBMS ensures the atomicity of each transaction.

Implementing Consistencies The consistency requirement in the above eg is that the sum of CA2090 and SB2359 be unchanged by the execution of the transaction. Before the execution of the transaction the amounts in the accounts in CA2090 and SB2359 are 50,000 and 35,000 respectively. After the execution the amounts become 48,000 and 37,000. In both cases the sum of the amounts is 85,000 thus maintaining consistency. Ensuring the consistency for an individual transaction is the responsibility of the application programmer who codes the transaction.

Implementing the Isolation Even if the atomicity and consistency properties are ensured for each transaction there can be problems if several transactions are executed concurrently. The different transactions interfere with one another and cause undesirable results. Suppose we are executing the above transaction Ti. We saw that the database is temporarily inconsistent while the transaction is being executed. Suppose that the transaction has performed the Write(CA2090) operation, during this time another transaction is reading the balances of different accounts. It checks the account CA2090 and finds the account balance at 48,000. Suppose that it reads the account balance of the other account(account SB2359, before the first transaction has got a chance to update the account.So the account balance in the account Sb2359 is 35000. After the second transaction has read the account balances, the first transaction reads the account balance of the account SB2359 and updates it to 37000.But here we are left with a problem. The first transaction has executed successfully and the database is back to a consistent state. But while it was in an inconsistent state, another transaction performed some operations( May be updated the total account balances). This has left the database in an inconsistent state even after both the transactions have been executed successfully. On solution to the situation( concurrent execution of transactions) is to execute the transactions serially- one after the other. This can create many problems. Suppose long transactions are being executed first. Then all other transactions will have to wait in the queue. There might be

many transactions that are independent(or that do not interfere with one another). There is no need for such transactions to wait in the queue. Also concurrent executions of transactions have significant performance advantages. So the DBMS have found solutions to allow multiple transactions to execute concurrency with out any problem. The isolation property of a transaction ensures that the concurrent execution of transactions result in a system state that is equivalent to a state that could have been obtained if the transactions were executed one after another. Ensuring isolation property is the responsibility of the concurrency-control component of the DBMS.

Implementing durability The durability property guarantees that, once a transaction completes successfully, all updates that it carried out on the database persist, even if there is a system failure after the transaction completes execution. We can guarantee durability by ensuring that either the updates carried out by the transaction have been written to the disk before the transaction completes or information about the updates that are carried out by the transaction and written to the disk are sufficient for the data base to reconstruct the updates when the data base is restarted after the failure. Ensuring durability is the responsibility of the recovery- management component of the DBMS

Transaction management and concurrency control components of a DBMS

Transaction States

Once a transaction is committed, we cannot undo the changes made by the transactions by rolling back the transaction. Only way to undo the effects of a committed transaction is to execute a compensating transaction. The creating of a compensating transaction can be quite complex and so the task is left to the user and it is not handled by the DBMS. The transaction must be in one of the following states:- 1. Active:- This is a initial state, the transaction stays in this state while it is executing 2. Partially committed:- The transaction is in this state when it has executed the final statement 3. Failed: - A transaction is in this state once the normal execution of the transaction cannot proceed. 4. Aborted: - A transaction is said to be aborted when the transaction has rolled back and the database is being restored to the consistent state prior to the start of the transaction. 5. Committed: - A transaction is in this committed state once it has been successfully executed and the database is transformed in to a new consistent state. Different transactions states are given in following figure.

Concurrency Control

Concurrency control in database management systems permits many users (assumed to be interactive) to access a database in a multiprogrammed environment while preserving the illusion that each user has sole access to the system. Control is needed to coordinate concurrent accesses to a DBMS so that the overall correctness of the database is maintained. For example, users A and B both may wish to read and update the same record in the database at about the same time. The relative timing of the two transactions may have an impact on the state of the database at the end of the transactions. The end result may be an inconsistent database.

Why Concurrent Control is needed?

• Several problems can occur when concurrent transactions execute in an uncontrolled manner. – The lost update problem : This occurs when two transactions that access the same database items have their operations interleaved in a way that makes the value of same database item incorrect. – The temporary update (or dirty read) problem : This occurs when one transaction updates a database item and then the transaction fails for some reason. The updated item is accessed by another transaction before it is changed back to its original value. – The incorrect summary problem : If one transaction is calculating an aggregate function on a number of records while other transaction is updating some of these records, the aggregate function may calculate some values before they are updated and others after they are updated. • Whenever a transaction is submitted to a DBMS for execution, the system must make sure that : – All the operations in the transaction are completed successfully and their effect is recorded permanently in the database; or

– the transaction has no effect whatever on the database or on the other transactions in the case of that a transaction fails after executing some of operations but before executing all of them.

Concurrency Anomalies Following are the problems created due to the concurrent execution of the transactions:-

Multiple update problems In this problem, the data written by one transaction (an update operation) is being overwritten by another update transaction. This can be illustrated using our banking example. Consider our account CA2090 that has Rs. 50000 balance in it. Suppose a transaction T1 is withdrawing RS. 10000 from the account while another transaction T2 is depositing RS. 20000 to the account. If these transactions were executed serially (one after another), the final balance would be Rs. 60000, irrespective of the order in which the transactions are performed. In other words, if the transactions were performed serially, then the result would be the same if T1 is performed first or T2 is performed first-order is not important. But if the transactions are performed concurrently, then depending on how the transactions are executed the results will vary. Consider the execution of the transactions given below

Figure 1

Both transactions start nearly at the same time and both read the account balance of 50000. Both transactions perform the operations that they are supposed to perform-T1

will reduce the amount by 10000 and will write the result to the data base; T 2 will increase the amount by 20000 and will write the amount to the database overwriting the previous update. Thus the account balance will gain additional 10000 producing a wrong result. If T2 were to start execution first, the result would have been 40000 and the result would have been wrong again. This situation could be avoided by preventing T2 from reading the value of the account balance until the update by T1 has been completed.

Incorrect Analysis Problem Problems could arise even when a transaction is not updating the database. Transactions that read the database can also produce wrong result, if they are allowed to read the database when the database is in an inconsistent state. This problem is often referred to

as dirty read or unrepeatable data. The problem of dirty read occurs when a transaction reads several values from the data base while another transactions are updating the values. Consider the case of the transaction that reads the account balances from all accounts to find the total amount in various account. Suppose that there are other transactions, which are updating the account balances-either reducing the amount (withdrawals) or increasing the amount (deposits). So when the first transaction reads the account balances and finds the totals, it will be wrong, as it might have read the account balances before the update in the case of some accounts and after the updates in other accounts. This problem is solved by preventing the first transaction (the one that reads the balances) from reading the account balances until all the transactions that update the accounts are completed.

Inconsistent Retrievals

consider two users A and B accessing a department database simultaneously. The user A is updating the database to give all employees a 5% salary raise while user B wants to know the total salary bill of a department. The two transactions interfere since the total salary bill would be changing as the first user updates the employee records. The total salary retrieved by the second user may be a sum of some salaries before the raise and others after the raise. Such a sum could not be considered an acceptable value of the total salary (the value before the raise or after the raise would be).

Figure 2: An Example of Inconsistent Retrieval

The problem illustrated in the last example is called the inconsistent retrieval anomaly. During the execution of a transaction therefore, changes made by another transaction that has not yet committed should not be visible since that data may not be consistent.

Uncommitted Dependency

Consider the following situation:

Figure 3: An Example of Uncommitted Dependency

Transaction A reads the value of Q that was updated by transaction B but was never committed. The result of Transaction A writing Q therefore will lead to an inconsistent state of the database. Also if the transaction A doesn't write Q but only reads it, it would be using a value of Q which never really existed! Yet another situation would occur if the roll back happens after Q is written by transaction A. The roll back would restore the old value of Q and therefore lead to the loss of updated Q by transaction A. This is called the uncommitted dependency anomaly.

Serializability

The concept of a transaction has been discussed in the last chapter. A transaction must have the properties of atomicity, consistency, isolation and durability. In addition, when more than one transaction are being executed concurrently, we must have serializability.

Serializability is a given set of interleaved transactions is said to be serial sable if and only if it produces the same results as the serial execution of the same transactions

Serializability is an important concept associated with locking. It guarantees that the work of concurrently executing transactions will leave the database state as it would have been if these transactions had executed serially. This requirement is the ultimate criterion for database consistency and is the motivation for the two-phase locking protocol, which dictates that no new locks can be acquired on behalf of a transaction after the DBMS releases a lock held by that transaction. In practice, this protocol generally means that locks are held until commit time. Serializability is the classical concurrency scheme. It ensures that a schedule for executing concurrent transactions is equivalent to one that executes the transactions serially in some order. It assumes that all accesses to the database are done using read and write operations. A schedule is called ``correct'' if we can find a serial schedule that is ``equivalent'' to it. Given a set of transactions T1...Tn, two schedules S1 and S2 of these transactions are equivalent if the following conditions are satisfied:

Read-Write Synchronization: If a transaction reads a value written by another transaction in one schedule, then it also does so in the other schedule.

Write-Write Synchronization: If a transaction overwrites the value of another transaction in one schedule, it also does so in the other schedule.

These two properties ensure that there can be no difference in the effects of the two schedules

Serializable schedule

A schedule is serial if, for every transaction T participating the schedule, all the operations of T are executed consecutively in the schedule. Otherwise it is called non-serial schedule.

• Every serial schedule is considered correct; some non serial schedules give erroneous results.

• A schedule S of n transactions is serializable if it is equivalent to some serial schedule of the same n transactions; a non serial schedule which is not equivalent to any serial schedule is not serializable. • The definition of two schedules considered “equivalent”: – result equivalent: producing same final state of the database (is not used) – conflict equivalent: If the order of any two conflicting operations is the same in both schedules. – view equivalent: If each read operation of a transaction reads the result of the same write operation in both schedules and the write operations of each transaction must produce the same results. • Conflict serializable: if a schedule S is conflict equivalent to some serial schedule. we can reorder the non-conflicting operations S until we form the equivalent serial schedule, and S is a serializable schedule. • View Serializability: Two schedules are said to be view equivalent if the following three conditions hold. The same set of transactions participate in S and S'; and S and S' include the same operations of those transactions. A schedule S is said to be view serializable if it is view equivalent to a serial schedule.

When two or more transactions are running concurrently, the steps of the transactions would normally be interleaved. The interleaved execution of transactions is decided by the database system software called the scheduler which receives a stream of user requests that arise from the active transactions. A particular sequencing (usually interleaved) of the actions of a set of transactions is called a schedule. A serial schedule is a schedule in which all the operations of one transaction are completed before another transaction can begin (that is, there is no interleaving).

It is the responsibility of the scheduler to maintain consistency while allowing maximum concurrency. The scheduler therefore must make sure that only correct schedules are allowed. We would like to have a mechanism or a set of conditions that define a correct schedule. That is unfortunately not possible since it is always very complex to define a consistent database. The assertions defining a consistent database (called integrity constraints or consistency constraints) could well be as complex as the database itself and checking these assertions, assuming one could explicitly enumerate them, after each transaction would not be practical. The simplest approach to this problem is based on the notion of each transaction being correct by itself and a schedule of concurrent executions being able to preserve their correctness.

A serial schedule is always correct since we assume transactions do not depend on each other. We further assume that each transaction when run in isolation transforms a consistent database into a new consistent state and therefore a set of transactions executed

one after another (i.e. serially) must also be correct. A database may however not be consistent during the execution of a transaction but it is assumed the database is consistent at the end of the transaction. Although using only serial schedules to ensure consistency is a possibility, it is often not a realistic approach since a transaction may at times be waiting for some input/output from secondary storage or from the user while the CPU remains idle, wasting a valuable resource. Another transaction could have been running while the first transaction is waiting and this would obviously improve the system efficiency. We therefore need to investigate schedules that are not serial. However, since all serial schedules are correct, interleaved schedules that are equivalent to them must also be considered correct. There are in fact n! different serial schedules for a set of any n transactions. Note that not all serial schedules of the same set of transactions result in the same consistent state of the database. For example, a seat reservation system may result in different allocations of seats for different serial schedules although each schedule will ensure that no seat is sold twice and no request is denied if there is a free seat available on the flight. However one serial schedule could result in 200 passengers on the plane while another in 202 as shown below.

Let us consider an airline reservation system. Let there be 12 seats available on flight QN32. Three requests arrive; transaction T1 for 3 seats, transaction T2 for 5 seats and transaction T3 for 7 seats. If the transactions are executed in the order { T1, T2, T3} then we allocate 8 seats but cannot meet the third request since there are only 4 seats left after allocating seats for the first two transactions. If the transactions are executed in the order { T2 , T3 , T1}, we allocate all the 12 remaining seats but the request of transaction T1

cannot be met. If the transactions are instead executed in the order { T3 , T1 , T2} then we allocate 10 seats but are unable to meet the request of Transaction T2 for 5 seats. In all there are 3!, that is 6, different serial schedules possible. The remaining three are { T1, T3,

T2}, { T2, T1 ,T3}, and { T3, T2 ,T1}. They lead to 10, 8 and 12 seats being sold respectively. All the above serial schedules must be considered correct although any one of the three possible results may be obtained as a result of running these transactions.

We have seen that some sets of transactions when executed concurrently may lead to problems, for example, lost update anomaly. Of course, any sets of transactions can be executed concurrently without leading to any difficulties if the read and write sets of these transactions do not intersect. We want to discuss sets of transactions that do interfere with each other. To discuss correct execution of such transactions, we need to define the concept of serializability.

Let T be a set of n transactions T1, T2,……. ,Tn . If the n transactions are executed serially (call this execution S), we assume they terminate properly and leave the database in a consistent state. A concurrent execution of the n transactions in T (call this execution C) is called serializable if the execution is computationally equivalent to a serial execution. There may be more than one such serial execution. That is, the concurrent execution C always produces exactly the same effect on the database as some serial execution S does where S is some serial execution of T, not necessarily the order T1, T2,……. ,Tn . Thus if a transaction Ti writes a data item A in the interleaved schedule C before another transaction Tj reads or writes the same data item, the schedule C must be equivalent to a serial schedule in which Ti appears before Tj . Therefore in the interleaved transaction Ti

appears logically before it Tj does; same as in the equivalent serial schedule. The concept of serializability defined here is sometimes called final state serializability; other forms of serializability exist. The final state serializability has sometimes been criticized for being too strict while at other times it has been criticized for being not strict enough! Some of these criticisms are valid though and it is therefore necessary to provide another definition of serializability called view serializability. Two schedules are called view serializable if the order of any two conflicting operations in the two schedules is the same. As discussed earlier, two transactions may have a RW-conflict or a WW-conflict when they both access the same data item.

If a schedule of transactions is not serializable, we may be able to overcome the concurrency problems by modifying the schedule so that it serializable. The modifications are made by the database scheduler.

We now consider a number of examples of possible schedules of two transactions running concurrently. Consider a situation where a couple has three accounts (A, B, C) with a bank. The husband and the wife maintain separate personal savings accounts while they also maintain a loan account on their house. A is the husband's account, B is the wife's account and C is the housing loan. Each month a payment of $500 is made to the account C. To make the payment on this occasion, the couple walk to two adjoining automatic teller machines and the husband transfers $200 from account A to C (Transaction 1) while the wife on the other machine transfers $300 from account B to C (Transaction 2). Several different schedules for these two transactions are possible. We present the following four schedules for consideration.

Figure 4: A serial schedule without interleaving

Figure 5: An interleaved serializable schedule

Figure 6: An interleaved non-serializable schedule

Figure 7: Another interleaved serializable schedule

It is therefore clear that many interleaved schedules would result in a consistent state while many others will not. All correct schedules are serializable since they all must be equivalent to either serial schedule (T1 , T2) or (T2 ,T1).

TESTING FOR SERIALIZABILITY

Since a serializable schedule is a correct schedule, we would like the DBMS scheduler to test each proposed schedule for serializability before accepting it. Unfortunately most concurrency control method do not test for serializability since it is much more time-consuming task than what a scheduler can be expected to do for each schedule. We therefore resort to a simpler technique and develop a set of simple criteria or protocols that all schedules will be required to satisfy. These criteria are not necessary for serializability but they are sufficient.

There is a simple technique for testing a given schedule S for conflict serializability. The testing is based on constructing a directed graph in which each of the transactions is represented by one node and an edge between Ti and Tj exists if any of the following conflict operations appear in the schedule:

1. Ti executes WRITE( X) before Tj executes READ( X), or 2. Ti executes READ( X) before Tj executes WRITE( X) 3. Ti executes WRITE( X) before Tj executes WRITE( X).

[Needs fixing] Three possibilities if there are two transactions T i , Tj that interfere with each other.

This is not serializable since is has a cycle.

Basically this graph implies that Ti ought to happen before Tj and Tj ought to happen before Ti-- an impossibility. If there is no cycle in the precedence graph, it is possible to build an equivalent serial schedule by traversing the graph.

The above conditions are derived from the following argument. Let T i and Tj both access X and Tj consist of either a Read(X) or Write(X). If T j access is a Read(X) then there is conflict only if Ti had a Write(X) and if Ti did have a Write(X) then must come before Tj. If however Tj had a Write(X) and Ti had a Read(X) (Ti would have a Read(X) even if it had a Write(X)) then must come before . [Needs fixing]

Enforcing Serializability

As noted earlier, a schedule of a set of transactions is serializable if computationally its effect is equal to the effect of some serial execution of the transactions. One way to enforce serializability is to insist that when two transactions are executed, one of the transactions is assumed to be older than the other. Now the only schedules that are accepted are those in which data items that are common to the two transactions are seen by the junior transaction after the older transaction has written them back. The three basic techniques used in concurrency control (locking, timestamping and optimistic concurrency control) enforce this in somewhat different ways. The only schedules that these techniques allow are those that are serializable. We now discuss the techniques.

Recoverability

So far we have studied what schedules are acceptable from the viewpoint of consistency of the database, assuming implicitly that there are no transaction failures. We now address the effect of transaction failures during concurrent execution. If a transaction Ti fails, for whatever reasons, we need to undo the effect of this transaction to ensure the atomicity property of the transaction. In a system that allows concurrent execution, it is necessary also to ensure that any transaction Tj that is dependent on Ti (that is Tj has read data written by Ti) is also aborted. To achieve this surety, we need to place restrictions on the type of schedules permitted in the system No we are going to look at what schedules are acceptable from the view point of recovery from transaction failure.

Recoverable schedules A recoverable schedule is one where, for each pair of transaction Ti and Tj such that Tj

reads a data item previously written by Ti, the commit operation of Ti appears before the commit operation of Tj.

Cascade less Schedules Even if a schedule is recoverable, to recover correctly from the failure of transaction Ti, we may have to rollback several transactions. This phenomenon, in which a single transaction failure which leads to a series of transaction rollbacks, is called cascading rollbacks. Cascading rollback is undesirable, since it leads to the undoing of a significant amount of work. It is desirable to restrict the schedules to those where cascading rollbacks cannot occur. Such schedules are called cascadeless schedules. Then we can say that a Cascadeless Schedule is one where, for each pair of transactions Ti and Tj such that Tj reads a data item previously written by Ti., the commit operation of Ti appears before the read operation of Tj. It is easy to verify that every cascadeless schedule is also recoverable

Transaction Definition in SQL A data manipulation language must include a construct for specifying the set of actions that constitute a transaction. The SQL standard specifies that a transaction begins implicitly. Transactions are ended with one of these SQL statements. Commit work – commit the current transaction and begins a new one Rollback work – causes the current transaction to abort. The keyword is optional. If a program terminates without either of these commands, the updates are either committed or rolled back-which of the two happens is not specified by the standard and depends on the implementation.

Locking

Locking is a common technique by which a database may synchronize execution of concurrent transactions. Using locking a transaction can obtain exclusive or shareable access rights (called locks) to an object. If the access provided by the lock is shareable, the lock is often called a shared lock (sometime called a read lock). On the other hand if the access is exclusive, the lock is called an exclusive lock (sometime called a write lock). If a number of transactions need to read an object and none of them wishes to write that object, a shared lock would be the most appropriate. Of course, if any of the transactions were to write the object, the transaction must acquire an exclusive lock on the object to avoid the concurrency anomalies that we have discussed earlier. A number of locking protocols are possible. Each protocol consists of

1. a set of locking types

2. a set of rules indicating what locks can be granted concurrently

3. a set of rules that transactions must follow when acquiring or releasing locks

We will use the simplest approach that uses shared and exclusive locks, a transaction would set a read lock on the data item that it reads and exclusive lock on the data item that it needs to update. As the names indicate, a transaction may obtain a shared lock on a data item even if another transaction is holding a shared lock on the same data item at the same time. Of course, a transaction cannot obtain a shared or exclusive lock on a data item if another transaction is holding an exclusive lock on the data item. The shared lock may be upgraded to an exclusive lock (assuming no other transaction is holding a shared lock on the same data item at that time) for items that the transaction wishes to write. This technique is sometime also called blocking because if another transaction requests access to an exclusively locked item, the lock request is denied and the requesting transaction is blocked.

A transaction can hold a lock on one or more items of information. The data item must be unlocked before the transaction is completed. The locks are granted by a database system software called the lock manager which maintains information on all locks that are active and controls access to the locks. As noted earlier, several modes of locks may be available. For example, in shared mode, a transaction can read the locked data item but cannot update it. In exclusive lock mode, a transaction has exclusive access to the locked data item and no other transaction is allowed access to it until the lock is released. A transaction is allowed to update the data item only as long as an exclusive lock is held on it.

If transactions running concurrently are allowed to acquire and release locks as data item is read and updated, there is a danger that incorrect results may be produced. Consider the following example:

Table 8 :- Unserializable Schedule using Locks Page No 2

XL (called exclusively lock) is a request for an exclusive lock and UL is releasing the lock (sometimes called unlock). SL is a request for a shared lock. In the example above, the result displayed by transaction 2 is incorrect because it was able to read B before the update but read A only after the update. The above schedule is therefore not serializable. As discussed earlier, the problem with above schedule is inconsistent retrieval.

The problem arises because the above schedule involves two RW-conflicts and the first one that involves A involves Transaction 1 logically appearing before Transaction 2 while the second conflict involving B involves Transaction 2 logically appearing before Transaction 1.

To overcome this problem a two-phase locking ( 2PL) scheme is used in which a transaction cannot request a new lock after releasing a lock. Two phase locking involves the following two phases:

1. Growing Phase ( Locking Phase) -- During this phase locks may be acquired but not released.

2. Shrinking Phase ( Unlocking Phase) - During this phase locks may be released but not acquired.

In summary, the main feature of locking is that conflicts are checked at the beginning of the execution and resolved by waiting.

Using the two-phase locking scheme in the above example, we obtain the following schedule by modifying the earlier schedule so that all locks are obtained before any lock is released:

Table 9: - A Schedule using Two-Phase Locking

Unfortunately, although the above schedule would not lead to incorrect results, it has another difficulty: both the transactions are blocked waiting for each other; a classical deadlock situation has occurred! Deadlocks arise because of circular wait conditions involving two or more transactions as above. A system that does not allow deadlocks to occur is called deadlock-free. In two-phase locking, we need a deadlock detection mechanism and a scheme for resolving the deadlock once a deadlock has occurred. We will look at such techniques in Section 5.

The attraction of the two-phase algorithm derives from a theorem which proves that the two-phase locking algorithm always leads to serializable schedules that are equivalent to serial schedules in the order in which each transaction acquires its last lock. This is a sufficient condition for serializability although it is not necessary.

To show that conditions of serializability are met if 2PL is used we proceed as follows. We know that a test of serializability requires building a directed graph in which each of the transactions is represented by one node and an edge between Ti and Tj exists if any of the following conflict operations appear in the schedule:

1. Ti executes WRITE( X) before Tj executes READ( X), or

2. Ti executes READ( X) before Tj executes WRITE( X)

3. Ti executes WRITE( X) before Tj executes WRITE( X).

A schedule will not be serializable if a cycle is found in the graph. We assume a simple concurrent execution of two transactions and assume that the graph has a cycle. The cycle is possible only if there is a edge between Ti and Tj because one of the above conditions was met followed by the schedule meeting one of the following conditions:

1. Tjexecutes WRITE( Y) before Ti executes READ( Y), or

2. Tj executes READ( Y) before Ti executes WRITE( Y)

3. Tj executes WRITE( Y) before Ti executes WRITE( Y).

These two sets of conditions provide a total of nine different combinations of conditions which could lead to a cycle. It can be shown that none of these combinations are possible. For example, assume the following two conditions have been satisfied by a schedule:

1. Ti executes WRITE( X) before Tjexecutes READ( X),

2. Tj executes WRITE( Y) before Ti executes WRITE( Y).

For these conditions to have met, Ti must have had an XL(X) which it would release allowing Tj to acquire SL(X). Also, for the second condition to have met, T j must have had an XL(Y) which it would release allowing T i to acquire XL(Y). This is just not possible using 2PL. Similarly, it can be shown that none of the nine combinations of conditions are possible if 2PL is used.

Several versions of two-phase locking have been suggested. Two commonly used versions are called the static two-phase locking and the dynamic two-phase locking. The static technique basically involves locking all items of information needed by a transaction before the first step of the transaction and unlocking them all at the end of the transaction. The dynamic scheme locks items of information needed by a transaction only immediately before using them. All locks are released at the end of the transaction. [which is better??]

To use the locking mechanism, the nature and size of the individual data item that may be locked must be decided upon. A lockable data item could be as large as a relation or as small as an attribute value of a tuple. The lockable item could also be of a size in between, for example, a page of a relation or a single tuple. Larger the size of the items that are locked, the smaller the concurrency that is possible since when a transaction wants to access one or more tuples of a relation, it is allowed to lock the whole relation and some other transaction wanting to access some other tuples of the relation is denied access. On the other hand, coarse granularity reduces the overhead costs of locking since the table that maintains information on the locks is smaller. Fine granularity of the locks involving individual tuple locking or locking of even smaller items allows greater concurrency at the cost of higher overheads. Therefore there is a tradeoff between level of concurrency and overhead costs. Perhaps a system can allow variable granularity so that in some situations, for example when computing the projection of a relation, the whole relation may be locked while in other situations only a small part of the relation needs locking.

Timestamping Control

The two-phase locking technique relies on locking to ensure that each interleaved schedule executed is serializable. Now we discuss a technique that does not use locks and works quite well when level of contention between transactions running concurrently is not high. It is called Timestamping.

Timestamp techniques are based on assigning a unique timestamp (a number indicating the time of the start of the transaction) for each transaction at the start of the transaction and insisting that the schedule executed is always serializable to the serial schedule in the chronological order of the timestamps. This is in contrast to two-phase locking where any schedule that is equivalent to some serial schedule is acceptable.

Since the scheduler will only accept schedules that are serializable to the serial schedule in the chronological order of the timestamps, the scheduler must insist that in case of conflicts, the junior transaction must process information only after the older transaction has written them. The transaction with the smaller timestamp being the older transaction. For the scheduler to be able to carry this control, the data items also have read and write timestamps. The read timestamp of a data item X is the timestamp of the youngest transaction that has read it and the write timestamp is the timestamp of the youngest transaction that has written it. Let timestamp of a transaction be TS(T).

Consider a transaction T1 with timestamp = 1100. Suppose the smallest unit of concurrency control is a relation and the transaction wishes to read or write some tuples from a relation named R. Let the read timestamp of R be RT(R) and write timestamp be WT(R). The following situations may then arise:

1. T1 wishes to read R. The read by T1 will be allowed only if WT(R)< TS(T1)or WT(R)< 1100 that is the last write was by an older transaction. This condition ensures that data item read by a transaction has not been written by a younger transaction. If the write timestamp of R is larger than 1100 (that is, a younger transaction has written it), then the older transaction is rolled back and restarted with a new timestamp.

2. T1 wishes to read R. The read by T1 will be allowed if it satisfies the above condition even if RT(R)> TS(T1), that is the last read was by a younger transaction. Therefore if data item has been read by a younger transaction that is quite acceptable.

3. T1 wishes to write some tuples of a relation R. The write will be allowed only if read timestamp RT(R)< TS(T1)or RT(R)< 1100, that is the last read of the transaction was by an older transaction and therefore no younger transaction has read the relation. 4. T1 wishes to write some tuples of a relation R. The write need not be carried out if the above condition is met and if the last write was by a younger transaction, that is WT(R)> TS(T1), . The reason this write is not needed is that in this case the younger transaction has not read R since if it had, the older transaction T1 would have been aborted.

If the data item has been read by a younger transaction, the older transaction is rolled back and restarted with a new timestamp. It is not necessary to check the write (??) since if a younger transaction has written the relation, the younger transaction would have read the data item and the older transaction has old data item and may be ignored. It is possible to ignore the write if the second condition is violated since the transaction is attempting to write obsolete data item. [expand!]

Let us now consider what happens when an attempt is made to implement the following schedule:

???

We assume transaction T1 to be older and therefore .

Read (A) by transaction T1 is permitted since A has not been written by a younger transaction. Write (A) by transaction T1 is also permitted since A has not been read or written by a younger transaction. Read (B) by transaction T2 is permitted since B has not been written by a younger transaction. Read (B) by transaction T1 however is not allowed since B has been read by (younger) transaction T2 and therefore transaction T1 is rolled back. [needs to be read again and fixed]

Refer to page 383 of Korth and Silberchatz for a schedule that is possible under the timestamping control but not under 2PL

Deadlocks

A deadlock may be defined as a situation in which each transaction in a set of two or more concurrently executing transactions is blocked circularly waiting for another transaction in the set, and therefore none of the transactions will become unblocked unless there is external intervention.

A deadlock is clearly undesirable but often deadlocks are unavoidable. We therefore must deal with the deadlocks when they occur. There are several aspects of dealing with deadlocks: one should prevent them if possible (deadlock prevention), detect them when they occur (deadlock detection) and resolve them when a deadlock has been detected (deadlock resolution). Deadlocks may involve two, three or more transactions. To resolve deadlocks, one must keep track of what transactions are waiting and for which transaction they are waiting for.

Once a deadlock is identified, one of the transactions in the deadlock must be selected and rolled back (such transaction is called a victim) thereby releasing all the locks that that transaction held breaking the deadlock.

Since locking is the most commonly used (and often the most efficient) technique for concurrency control, we must deal with deadlocks. The deadlocks may either be prevented or identified when they happen and then resolved. We discuss both these techniques.

Deadlock Prevention

Deadlocks occur when some transactions wait for other transactions to release a resource and the wait is circular (in that T1 waits for T2 which is waiting for T4 which in turn is waiting for T1 or in the simplest case T1 waits for T2and T2 for T1). Deadlocks can be prevented if circular waits are eliminated. This can be done either by defining an order on who may wait for whom or by eliminating all waiting. We first discuss two techniques that define an ordering on transactions. [Further info .. Ullman page 373]

Wait-die Algorithm

When a conflict occurs between T1 and T2 (T1 being the older transaction), if T1 possesses the lock then T2 (the younger one) is not allowed to wait. It must rollback and restart. If however T2 possessed the lock at conflict, the senior transaction is allowed to wait. The technique therefore avoids cyclic waits and generally avoids starvations (a transaction's inability to secure resources that it needs). A senior transaction may however find that it has to wait for every resource that it needs, and it may take a while to complete.

Wound-die Algorithm

To overcome the possibility of senior transaction having to wait for every item of data that it needs, the wound-die scheme allows a senior transaction to immediately acquire a data item that it needs and is being controlled by a younger transaction. The younger transaction is then restarted.

Immediate Restart

In this scheme, no waiting is allowed. If a transaction requests a lock on a data item that is being held by another transaction (younger or older), the requesting transaction is restarted immediately. This scheme can lead to starvation if a transaction requires several popular data items since every time the transaction restarts and seeks a new item, it finds some data item to be busy resulting in it being rolled back and restarted.

Deadlock Detection

As noted earlier, when locking is used, one needs to build and analyse a waits-for graph every time a transaction is blocked by a lock. This is called continuous detection. One may prefer a periodic detection scheme although results of experiments seem to indicate that the continuous scheme is more efficient.

Once a deadlock has been detected, it must be resolved. The most common resolution technique requires that one of the transactions in the waits-for graph be selected as a victim and be rolled back and restarted. The victim may be selected as

1. randomly

2. the youngest transaction since this is likely to have done the least amount of work.

3. the one that has written the least data back to the database since all the data written must be undone in the rollback.

4. the one that is likely to affect the least number of other transactions. That is, the transaction whose rollback will lead to least other rollbacks (in cascade rollback).

5. the one with the fewest locks.

One experimental study has suggested that the victim with the fewest locks provides the best performance. Also it has been suggested that if deadlock prevention with immediate restart is to be used, the database performance suffers.

Evaluation of control mechanisms

A number of researchers have evaluated the various concurrency control mechanisms that have been discussed here. It is generally accepted that a technique that minimizes the number of restarts (like the two-phase locking technique) is superior when the number of conflicts is high. The timestamping and the optimistic algorithms usually performed

much worse under high conflict levels since they wasted the most resources through restarts. Deadlock prevention techniques like those based on immediate restart tend to make the database slow down somewhat. One therefore must have deadlock detection technique in place. At low loads, it doesn't really matter much which technique is used although some studies show that the optimistic techniques then are more efficient. It is of course possible to use more than one technique in a system. For example, two-phase locking may be combined with optimistic technique to produce hybrid techniques that work well in many situations.

Backup and Recovery

INTRODUCTION

An enterprise's database is a very valuable asset. It is therefore essential that a DBMS provide adequate mechanisms for reducing the chances of a database failure and suitable procedures for recovery when the system does fail due to a software or a hardware problem. These procedures are called recovery techniques.

In this chapter, as before, we assume that we are dealing with a multiple user database system and not a single user system that are commonly used on personal computers. The problems of recovery are much simpler in a single user system than in a multiple user system.

Before we discuss causes of database failures and the techniques to recover from them, we should note that in our discussion in this chapter we shall assume that the database is resident on a disk and is transferred from the disk to the main memory when accessed. Modifications of data are initially made in the main memory and later propagated to the disk. Disk storage is often called nonvolatile storage because the information stored on a disk survives system crashes like processor and power failure. Non-volatile storage may be destroyed by errors like a head crash but that happens only infrequently. A part of the database needs to be resident in the main memory. Main memory is volatile storage because the contents of such storage are lost with a power failure. Given that a database on disk may also be lost, it is desirable to have a copy of the database stored in what is called stable storage. Stable storage is assumed to be reliable and unlikely to lose information with any type of system failures. No media like disk or tape can be considered stable, only if several independent nonvolatile storage media are used to replicate information can we achieve something close to a stable storage

A database system failure may occur due to:

1. power failures -- perhaps the most common cause of failure. These result in the loss of the information in the main memory. 2. operating system or DBMS failure -- this often results in the loss of the information in the main memory. 3. user input errors -- these may lead to an inconsistent data base.

4. hardware failure (including disk head crash) -- hardware failures generally result in the loss of the information in the main memory while some hardware failures may result in the loss of the information on the disk as well. 5. other causes like operator errors, fire, flood, etc. -- some of these disasters can lead to loss of all information stored in a computer installation.

A system failure may be classified as soft-fail or a hard-fail. Soft-fail is a more common failure involving only the loss of information in the volatile storage. A hard-fail usually occurs less frequently but is harder to recover from since it may involve loss or corruption of information stored on the disk.

Definition - Failure

A failure of a DBMS occurs when the database system does not meet its specification i.e. the database is in an inconsistent state.

Definition - Recovery

Recovery is the restoration of the database, after a failure, to a consistent state.

Recovery from a failure is possible only if the DBMS maintains redundant data (called recovery data) including data about what users have been doing. To be able to recover from failures like a disk head crash, a backup copy of the database must be kept in a safe place. A proper design of recovery system would be based on clear assumptions of the types of failures expected and the probabilities of those failures occurring.

Of course we assume that the recovery process will restore the database to a consistent state as close to the time of failure as possible but this depends on the type of failure and the type of recovery information that the database maintains. If for example the disk on which the database was stored is damaged, the recovery process can only restore the database to the state it was when the database was last archived.

Most modern database systems are able to recover from a soft-fail by a system restart which takes only a few seconds or minutes. A hard-fail requires rebuilding of the database on the disk using one or more backup copies. This may take minutes or even hours depending on the type of the failure and the size of the database. We should note that no set of recovery procedures can cope with all failures and there will always be situations when complete recovery may be impossible. This may happen if the failure results in corruption of some part of the database as well as loss or corruption of redundant data that has been saved for recovery.

We note that recovery requirements in different database environments are likely to be different. For example, in banking or in an airline reservation system it would be a requirement that the possibility of losing information due to a system failure be made very very small. On the other hand, loss of information in a inventory database may not be quite so critical. Very important databases may require fault-tolerant hardware that may for example involve maintaining one or more duplicate copies of the database coupled with suitable recovery procedures.

The component of DBMS that deals with recovery is often called the recovery manager.

The concept of a transaction

Before we discuss recovery procedures, we need to define the concept of a transaction. A transaction may be defined as a logical unit of work which may involve a sequence of steps but which normally will be considered by the user as one action. For example, transferring an employee from Department A to Department B may involve updating several relations in a database but would be considered a single transaction. Transactions are straight-line programs devoid of control structures. The sequence of steps in a transaction may lead to inconsistent database temporarily but at the end of the transaction, the database is in a consistent state. It is assumed that a transaction always carries out correct manipulations of the database.

The concept of transaction is important since the user considers them as one unit and assumes that a transaction will be executed in isolation from all other transactions running concurrently that might interfere with the transaction. We must therefore require that actions within a transaction be carried out in a prespecified serial order and in full and if all the actions do not complete successfully for some reason then the partial effects must be undone. A transaction that successfully completes all the actions is said to commit. It otherwise aborts and must be rolled back. For example, when a transaction involves transferring an employee from Department A to Department B, it would not be acceptable if the result of a failed transaction was that the employee was deleted from Department A but not added to Department B. Haerder and Reuter (1983) summarise the properties of a transaction as follows:

(a) Atomicity - although a transaction is conceptually atomic, a transaction would usually consist of a number of steps. It is necessary to make sure that other transactions do not see partial results of a transaction and therefore either all actions of a transaction are completed or the transaction has no effect on the database. Therefore a transaction is either completed successfully or rolled back. This is sometime called all-or-nothing.

(b) Consistency - although a database may become inconsistent during the execution of a transaction, it is assumed that a completed transaction preserves the consistency of the database.

(c) Isolation - as noted earlier, no other transactions should view any partial results of the actions of a transaction since intermediate states may violate consistency. Each transaction must be executed as if it was the only transaction being carried out.

(d) Durability - once the transaction has been completed successfully, its effects must persist and a transaction must complete before its effects can be made permanent. A committed transaction cannot be aborted. Also a transaction would have seen the effects of other transactions. We assume those transactions have been committed before the present transaction commits.

When an application program specifies a transaction we will assume that it is specified in the following format:

Begin Transaction (details of the transaction) Commit

All the actions between the Begin and Commit are now considered part of a single transaction. If any of these actions fail, all the actions carried out before would need to be undone. We assume that transactions are not nested.

We will use the classical example of a bank account transfer transaction to illustrate some of the concepts used in recovery procedures. The transactions is:

Begin Transaction Transfer 100 from Account A to Account B Commit

It is clear that there needs to be a transfer program in the system that will execute the transaction. There are at least the following steps involved in carrying out the transaction:

(a) Read Account A from Disk

(b) If balance is less then 100, return with an appropriate message.

(c) Subtract 100 from balance of Account A.

(d) Write Account A back to Disk.

(e) Read Account B from Disk.

(f) Add 100 to balance of Account B.

(g) Write Account B back to Disk.

We have ignored several small details; for example, we do not check if accounts A and B exist. Also we have assumed that the application program has the authority to access the accounts and transfer the money.

In the above algorithm, a system failure at step (e) or (f) would leave the database in an inconsistent state. This would result in 100 been subtracted from Account A and not added to Account B. A recovery from such failure would normally involve the incomplete transaction being rolled back.

In the discussion above we ignored how the disk manager transfers blocks from and to the disk. For example, in step (1) above, the block containing information on account A may already be in main memory buffers and then a read from disk is not required. More importantly, in Step (d), the write may not result in writing to the disk if the disk manager decides to modify the buffer but not write to disk. Also, in some situations, a write may first lead to a read if the block being modified is not resident in the main memory.

Now that the basic concept of transaction has been described, we can classify the different failures modes that we have discussed earlier.

TYPES OF FAILURES

At the beginning of this chapter we discussed a number of failure modes. Gray et al (1981) classifies these failures in the following four classes:

1. Transaction failure - a transaction may fail to complete for a number of reasons. For example, a user may cancel the transaction before it completes or the user may enter erroneous data or the DBMS may instruct the transaction to be abandoned and rolled back because of a deadlock or an arithmetic divide by zero or an overflow. Normally a transaction failure does not involve the loss of the contents of the disk or the main memory storage and recovery procedure only involves undoing changes caused by the failed transaction.

2. System failure - most of these failures are due to hardware, database management system or operating system failures. A system failure results in the loss of the contents of the main memory but usually does not affect the disk storage. Recovery from system

failure therefore involves reconstructing the database using the recovery information saved on the disk. 3. Media failure - media failures are failures that result in the loss of some or all the information stored on the disk. Such failures can occur due to hardware failures, for example disk head crash or disk dropped on the floor, or by software failures, for example, bugs in the disk writing routines in the operating system. Such failures can be recovered only if the archive version of the database plus a log of activities since the time of the archive are available. 4. Unrecoverable Failures - these are failures that result in loss of data that cannot be recovered and happen usually because of operations errors, for example, failure to make regular archive copies of the database or disasters like an earthquake or a flood.

Gray et al (1981) notes that in their experience 97 per cent of all transactions were found to execute successfully. Most of the remaining 3 per cent failed because of incorrect user input or user cancellation. All system crashes were found to occur every few days and almost all of these crashes were due to hardware or operating system failures. Several times a year, the integrity of the disk was lost.

The concept of a log

We now discuss some techniques of rolling back a partially completed transaction when a failure has occurred during its execution. It is clear that to be able to roll back a transaction we must store information about what that transaction has done so far. This is usually done by keeping a log or a journal although techniques that do not use a log exist. A log is an abstraction used by the recovery manager to store information needed to implement the atomicity and durable properties of transactions. The log manager is the component of a DBMS that implements the log abstraction. Logs are logically an append-only sequence of unstructured records stored on disk to which an insert is generated at each insert, delete and update. A log can therefore become very large quickly and may become a system performance problem. Each record appended to the log is assigned a unique log sequence number for identification.

When a transaction failure occurs, the transaction could be in one of the following situations:

(a) the database was not modified at all and therefore no roll back is necessary. The transaction could be resubmitted if required.

(b) the database was modified but the transaction was not completed. In this case the transaction must be rolled back and may be resubmitted if required.

(c) the database was modified and the transaction was completed but it is not clear if all the modifications were written to the disk. In this case there is a need to ensure that all updates carried out by the completed transaction are durable.

To ensure that the uncompleted transactions can be rolled back and the completed transactions are durable, most recovery mechanisms require the maintenance of a log on a non-volatile medium. Without the log or some other similar technique it would not be possible to discover whether a transaction updated any items in the database. The log maintains a record of all the changes that are made to the database, although different recovery methods that use a log may require the maintenance of somewhat different log. Typically, a log includes entries of the following type:

1. Transaction Number 12345 has begun.

2. Transaction Number 12345 has written x that had old value 1000 and new value 2000.

3. Transaction Number 12345 has committed.

4. Transaction Number 12345 has aborted.

Once a transaction writes commit to the log, it is assumed to commit even if all changes have not been propoagated to the disk. It should however be noted that when an entry is made to the log, the log record may not be immediately written to the disk since normally a system will only write a log block when it is full. A system crash therefore may result in some part of the log that was not yet written to disk also being lost but this does not create any problems. Most systems insist that log blocks be forced out at appropriate times.

If the log maintains information about the database before the updates and after the updates, the information in the log may be used to undo changes when a failure occurs and redo changes that a committed trasaction has made to the database that may have not been written to the disk. Another approach is possible in which the log only maintains information about the database after the updates but the changes are made to the database only if the transaction is completed successfully. We consider both techniques.

Recovery is often a significant cost of maintaining a database system. A major component of the recovery cost is the cost of maintaining the log which is needed only when a crash occurs.

We note that log is the complete history of the database and the maintenance of a log is sometimes needed for auditing purposes and the log then is used for both auditing and recovery. The log may also be used for performance analysis. If a database contained sensitive data, it may be necessary to maintain a log of what information was read by whom and what was written back. This could then be used for auditing the use of the system. This is sometimes called an audit-trail. We do not discuss audit aspect of log maintenance any further. Since log is the complete history of the database, access to the log should be carefully controlled. Read access to a log could provide a user indirect access to the whole database.

At recovery time we cannot normally look at all the log since the last failure since the log might be very very big. Markers are therefore often put on the log to ensure that not all the log needs to be looked at when a failure occurs. We will discuss this further later.

Before we discuss the recovery techniques it is necessary to briefly discuss buffer management.

RECOVERY AND BUFFER MANAGEMENT

The part of main memory that is available for storage of copies of disk blocks is called the buffer. A software usually is needed to manage the buffer. The primary purpose of a buffer management strategy is to keep the number of disk accesses to a minimum. An easy way to reduce the disk traffic is to allocate a large fraction of the primary memory to storing blocks from disks but there clearly is a limit to how much primary storage can be assigned to buffers. Buffer manager is a specialised virtual memory manager since needs of a database are somewhat more specialised than an ordinary operating system.

1. Replacement strategy

2. Pinned blocks

3. Forced output of blocks

Buffer manager interacts with the crash recovery system to decide on what can be written back to disk. Crash recovery system may insist that some other blocks be forced output before a particular buffer can be output.

To understand the various recovery mechanisms thoroughly, it is essential that we understand the buffer management strategies used in a DBMS. An operating system uses algorithms like the LRU (least recently used) and MRU (most recently used) in managing virtual memory. Similar strategies can be used in database buffer management but a database management does have some peculiar requirements that make some techniques that are suitable for operating systems unsuitable for database management.

Given that the main memory of the computer is volatile, any failure is likely to result in the loss of its contents which often includes updated blocks of data as well as output buffers to the log file.

[Does it need to be rewritten???] All log based recovery algorithms follow the write-ahead log principle. This involves writing the recovery data for a transaction to the log before a transaction commits and recovery data for a recoverable data page must be written to the log before the page is written from main memory to the disk. To enforce the write-ahead principle, the recovery manager needs to be integrated with the buffer manager. Some recovery algorithms permit the buffer manager to steal dirty main memory pages (a dirty page is a page in which data has been updated by a transaction that has not been committed) by writing them to disk before the transactions that modify them commit. The write-ahead principle requires the log records referring to the dirty pages must be written before the pages are cleaned. No-steal buffer management policies result in simpler recovery algorithms but limit the length of the transactions. Some recovery algorithms require the buffer manager to force all pages modified by a transaction to disk when the transaction commits. Recovery algorithms with no-force buffer management policies are more complicated, but they do less I/O than those which force buffers at commit.