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Transcript of Transaction Management and Recovery, 2 nd Edition. R. Ramakrishnan and J. Gehrke1 Transaction...
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 1
Transaction Management Overview
Chapter 18
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 2
Transactions
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.
A user’s program may carry out many operations on the data retrieved from the database, but the DBMS is only concerned about what data is read/written from/to the database.
A transaction is the DBMS’s abstract view of a user program: a sequence of reads and writes.
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 3
Concurrency in a DBMS Users submit transactions, and can think of each
transaction as executing by itself. 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. DBMS will enforce some ICs, depending on the ICs
declared in CREATE TABLE statements. Beyond this, the DBMS does not really understand the
semantics of the data. (e.g., it does not understand how the interest on a bank account is computed).
Issues: Effect of interleaving transactions, and crashes.
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 4
Atomicity of Transactions
A transaction might commit after completing all its actions, or it could abort (or be aborted by the DBMS) after executing some actions.
A very important property guaranteed by the DBMS for all transactions is that they are atomic. That is, a user can think of a Xact as always executing all its actions in one step, or not executing any actions at all. DBMS logs all actions so that it can undo the actions
of aborted transactions.
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 5
Example
Consider two transactions (Xacts):
T1: BEGIN A=A+100, B=B-100 ENDT2: BEGIN A=1.06*A, B=1.06*B END
Intuitively, the first transaction is transferring $100 from B’s account to A’s account. The second is crediting both accounts with a 6% interest payment.
There is no guarantee that T1 will execute before T2 or vice-versa, if both are submitted together. However, the net effect must be equivalent to these two transactions running serially in some order.
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 6
Example (Contd.)
Consider a possible interleaving (schedule):
T1: A=A+100, B=B-100 T2: A=1.06*A, B=1.06*B
This is OK. But what about:T1: A=A+100, B=B-100 T2: A=1.06*A, B=1.06*B
The DBMS’s view of the second schedule:
T1: R(A), W(A), R(B), W(B)T2: R(A), W(A), R(B), W(B)
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 7
Scheduling Transactions
Serial schedule: Schedule that does not interleave the actions of different transactions.
Equivalent schedules: For any database state, the effect (on the set of objects in the database) of executing the first schedule is identical to the effect of executing the second schedule.
Serializable schedule: A schedule that is equivalent to some serial execution of the transactions.
(Note: If each transaction preserves consistency, every serializable schedule preserves consistency. )
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 8
Anomalies with Interleaved Execution
Reading Uncommitted Data (WR Conflicts, “dirty reads”):
Unrepeatable Reads (RW Conflicts):
T1: R(A), W(A), R(B), W(B), AbortT2: R(A), W(A), C
T1: R(A), R(A), W(A), CT2: R(A), W(A), C
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 9
Anomalies (Continued)
Overwriting Uncommitted Data (WW Conflicts):
T1: W(A), W(B), CT2: W(A), W(B), C
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 10
Lock-Based Concurrency Control
Strict Two-phase Locking (Strict 2PL) Protocol: Each Xact must obtain a S (shared) lock on object
before reading, and an X (exclusive) lock on object before writing.
All locks held by a transaction are released when the transaction completes
If an Xact holds an X lock on an object, no other Xact can get a lock (S or X) on that object.
Strict 2PL allows only serializable schedules.
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 11
Aborting a Transaction If a transaction Ti is aborted, all its actions have to
be undone. Not only that, if Tj reads an object last written by Ti, Tj must be aborted as well!
Most systems avoid such cascading aborts by releasing a transaction’s locks only at commit time. If Ti writes an object, Tj can read this only after Ti commits.
In order to undo the actions of an aborted transaction, the DBMS maintains a log in which every write is recorded. This mechanism is also used to recover from system crashes: all active Xacts at the time of the crash are aborted when the system comes back up.
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 12
The Log
The following actions are recorded in the log: Ti writes an object: the old value and the new value.
Log record must go to disk before the changed page! Ti commits/aborts: a log record indicating this action.
Log records are chained together by Xact id, so it’s easy to undo a specific Xact.
Log is often duplexed and archived on stable storage. All log related activities (and in fact, all CC related
activities such as lock/unlock, dealing with deadlocks etc.) are handled transparently by the DBMS.
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 13
Recovering From a Crash
There are 3 phases in the Aries recovery algorithm: Analysis: Scan the log forward (from the most recent
checkpoint) to identify all Xacts that were active, and all dirty pages in the buffer pool at the time of the crash.
Redo: Redoes all updates to dirty pages in the buffer pool, as needed, to ensure that all logged updates are in fact carried out and written to disk.
Undo: The writes of all Xacts that were active at the crash are undone (by restoring the before value of the update, which is in the log record for the update), working backwards in the log. (Some care must be taken to handle the case of a crash occurring during the recovery process!)
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 14
Summary
Concurrency control and recovery are among the most important functions provided by a DBMS.
Users need not worry about concurrency. System automatically inserts lock/unlock requests and
schedules actions of different Xacts in such a way as to ensure that the resulting execution is equivalent to executing the Xacts one after the other in some order.
Write-ahead logging (WAL) is used to undo the actions of aborted transactions and to restore the system to a consistent state after a crash. Consistent state: Only the effects of commited Xacts
seen.
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 15
Concurrency Control
Chapter 19
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 16
Conflict Serializable Schedules
Two schedules are conflict equivalent if: Involve the same actions of the same
transactions Every pair of conflicting actions is ordered
the same way
Schedule S is conflict serializable if S is conflict equivalent to some serial schedule
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 17
Example
A schedule that is not conflict serializable:
The cycle in the graph reveals the problem. The output of T1 depends on T2, and vice-versa.
T1: R(A), W(A), R(B), W(B)T2: R(A), W(A), R(B), W(B)
T1 T2A
B
Dependency graph
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 18
Dependency Graph
Dependency graph: One node per Xact; edge from Ti to Tj if Tj reads/writes an object last written by Ti.
Theorem: Schedule is conflict serializable if and only if its dependency graph is acyclic
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 19
Review: Strict 2PL
Strict Two-phase Locking (Strict 2PL) Protocol: Each Xact must obtain a S (shared) lock on
object before reading, and an X (exclusive) lock on object before writing.
All locks held by a transaction are released when the transaction completes
If an Xact holds an X lock on an object, no other Xact can get a lock (S or X) on that object.
Strict 2PL allows only schedules whose precedence graph is acyclic
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 20
Two-Phase Locking (2PL)
Two-Phase Locking Protocol Each Xact must obtain a S (shared) lock on
object before reading, and an X (exclusive) lock on object before writing.
A transaction can not request additional locks once it releases any locks.
If an Xact holds an X lock on an object, no other Xact can get a lock (S or X) on that object.
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 21
View Serializability Schedules S1 and S2 are view equivalent
if: If Ti reads initial value of A in S1, then Ti also
reads initial value of A in S2 If Ti reads value of A written by Tj in S1, then
Ti also reads value of A written by Tj in S2 If Ti writes final value of A in S1, then Ti also
writes final value of A in S2T1: R(A) W(A)T2: W(A)T3: W(A)
T1: R(A),W(A)T2: W(A)T3: W(A)
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 22
Crash Recovery
Chapter 20
If you are going to be in the logging business, one of the things that you have to do is to learn about heavy equipment.
Robert VanNatta, Logging History of Columbia
County
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 23
Review: The ACID properties
AA tomicity: All actions in the Xact happen, or none happen.
CC onsistency: If each Xact is consistent, and the DB starts consistent, it ends up consistent.
II solation: Execution of one Xact is isolated from that of other Xacts.
D D urability: If a Xact commits, its effects persist.
The Recovery Manager guarantees Atomicity & Durability.
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 24
Motivation
Atomicity: Transactions may abort (“Rollback”).
Durability: What if DBMS stops running? (Causes?)
crash!• Desired Behavior after
system restarts:– T1, T2 & T3 should be
durable.– T4 & T5 should be
aborted (effects not seen).
T1T2T3T4T5
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 25
Assumptions
Concurrency control is in effect. Strict 2PL, in particular.
Updates are happening “in place”. i.e. data is overwritten on (deleted from) the
disk.
A simple scheme to guarantee Atomicity & Durability?
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 26
Handling the Buffer Pool
Force every write to disk? Poor response time. But provides durability.
Steal buffer-pool frames from uncommited Xacts? If not, poor throughput. If so, how can we ensure
atomicity?
Force
No Force
No Steal Steal
Trivial
Desired
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 27
More on Steal and Force
STEAL (why enforcing Atomicity is hard) To steal frame F: Current page in F (say P) is written
to disk; some Xact holds lock on P. What if the Xact with the lock on P aborts? Must remember the old value of P at steal time
(to support UNDOing the write to page P).
NO FORCE (why enforcing Durability is hard) What if system crashes before a modified page is
written to disk? Write as little as possible, in a convenient place, at
commit time,to support REDOing modifications.
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 28
Basic Idea: Logging
Record REDO and UNDO information, for every update, in a log. Sequential writes to log (put it on a separate disk). Minimal info (diff) written to log, so multiple updates fit
in a single log page.
Log: An ordered list of REDO/UNDO actions Log record contains:
<XID, pageID, offset, length, old data, new data>
and additional control info (which we’ll see soon).
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 29
Write-Ahead Logging (WAL)
The Write-Ahead Logging Protocol: Must force the log record for an update before
the corresponding data page gets to disk. Must write all log records for a Xact before
commit. #1 guarantees Atomicity. #2 guarantees Durability.
Exactly how is logging (and recovery!) done? We’ll study the ARIES algorithms.
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 30
WAL & the Log Each log record has a unique Log Sequence
Number (LSN). LSNs always increasing.
Each data page contains a pageLSN. The LSN of the most recent log record
for an update to that page.
System keeps track of flushedLSN. The max LSN flushed so far.
WAL: Before a page is written, pageLSN flushedLSN
LSNs
DB
pageLSNs
RAM
flushedLSN
pageLSN
Log recordsflushed to disk
“Log tail” in RAM
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 31
Log RecordsPossible log record types: Update Commit Abort End (signifies end of
commit or abort) Compensation Log
Records (CLRs) for UNDO actions
prevLSNXIDtype
lengthpageID
offsetbefore-imageafter-image
LogRecord fields:
updaterecordsonly
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 32
Other Log-Related State
Transaction Table: One entry per active Xact. Contains XID, status
(running/commited/aborted), and lastLSN.
Dirty Page Table: One entry per dirty page in buffer pool. Contains recLSN -- the LSN of the log record
which first caused the page to be dirty.
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 33
Normal Execution of an Xact
Series of reads & writes, followed by commit or abort. We will assume that write is atomic on disk.
In practice, additional details to deal with non-atomic writes.
Strict 2PL. STEAL, NO-FORCE buffer management, with
Write-Ahead Logging.
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 34
Checkpointing
Periodically, the DBMS creates a checkpoint, in order to minimize the time taken to recover in the event of a system crash. Write to log: begin_checkpoint record: Indicates when chkpt began. end_checkpoint record: Contains current Xact table and
dirty page table. This is a `fuzzy checkpoint’: Other Xacts continue to run; so these tables accurate
only as of the time of the begin_checkpoint record. No attempt to force dirty pages to disk; effectiveness of
checkpoint limited by oldest unwritten change to a dirty page. (So it’s a good idea to periodically flush dirty pages to disk!)
Store LSN of chkpt record in a safe place (master record).
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 35
The Big Picture: What’s Stored Where
DB
Data pageseachwith apageLSN
Xact TablelastLSNstatus
Dirty Page TablerecLSN
flushedLSN
RAM
prevLSNXIDtype
lengthpageID
offsetbefore-imageafter-image
LogRecords
LOG
master record
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 36
Simple Transaction Abort
For now, consider an explicit abort of a Xact. No crash involved.
We want to “play back” the log in reverse order, UNDOing updates. Get lastLSN of Xact from Xact table. Can follow chain of log records backward via the
prevLSN field. Before starting UNDO, write an Abort log record.
For recovering from crash during UNDO!
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 37
Abort, cont.
To perform UNDO, must have a lock on data! No problem!
Before restoring old value of a page, write a CLR: You continue logging while you UNDO!! CLR has one extra field: undonextLSN
Points to the next LSN to undo (i.e. the prevLSN of the record we’re currently undoing).
CLRs never Undone (but they might be Redone when repeating history: guarantees Atomicity!)
At end of UNDO, write an “end” log record.
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 38
Transaction Commit
Write commit record to log. All log records up to Xact’s lastLSN are
flushed. Guarantees that flushedLSN lastLSN. Note that log flushes are sequential,
synchronous writes to disk. Many log records per log page.
Commit() returns. Write end record to log.
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 39
Crash Recovery: Big Picture
Start from a checkpoint (found via master record).
Three phases. Need to: Figure out which Xacts
committed since checkpoint, which failed (Analysis).
REDO all actions. (repeat history)
UNDO effects of failed Xacts.
Oldest log rec. of Xact active at crash
Smallest recLSN in dirty page table after Analysis
Last chkpt
CRASH
A R U
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 40
Recovery: The Analysis Phase
Reconstruct state at checkpoint. via end_checkpoint record.
Scan log forward from checkpoint. End record: Remove Xact from Xact table. Other records: Add Xact to Xact table, set
lastLSN=LSN, change Xact status on commit.
Update record: If P not in Dirty Page Table,
Add P to D.P.T., set its recLSN=LSN.
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 41
Recovery: The REDO Phase We repeat History to reconstruct state at crash:
Reapply all updates (even of aborted Xacts!), redo CLRs. Scan forward from log rec containing smallest
recLSN in D.P.T. For each CLR or update log rec LSN, REDO the action unless: Affected page is not in the Dirty Page Table, or Affected page is in D.P.T., but has recLSN > LSN, or pageLSN (in DB) LSN.
To REDO an action: Reapply logged action. Set pageLSN to LSN. No additional logging!
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 42
Recovery: The UNDO Phase
ToUndo={ l | l a lastLSN of a “loser” Xact}Repeat:
Choose largest LSN among ToUndo. If this LSN is a CLR and undonextLSN==NULL
Write an End record for this Xact. If this LSN is a CLR, and undonextLSN != NULL
Add undonextLSN to ToUndo Else this LSN is an update. Undo the update, write a
CLR, add prevLSN to ToUndo.
Until ToUndo is empty.
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 43
Example of Recovery
begin_checkpoint
end_checkpoint
update: T1 writes P5
update T2 writes P3
T1 abort
CLR: Undo T1 LSN 10
T1 End
update: T3 writes P1
update: T2 writes P5
CRASH, RESTART
LSN LOG
00
05
10
20
30
40
45
50
60
Xact TablelastLSNstatus
Dirty Page TablerecLSN
flushedLSN
ToUndo
prevLSNs
RAM
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 44
Example: Crash During Restart!
begin_checkpoint, end_checkpoint
update: T1 writes P5
update T2 writes P3
T1 abort
CLR: Undo T1 LSN 10, T1 End
update: T3 writes P1
update: T2 writes P5
CRASH, RESTART
CLR: Undo T2 LSN 60
CLR: Undo T3 LSN 50, T3 end
CRASH, RESTART
CLR: Undo T2 LSN 20, T2 end
LSN LOG00,05
10
20
30
40,45
50
60
70
80,85
90
Xact TablelastLSNstatus
Dirty Page TablerecLSN
flushedLSN
ToUndo
undonextLSN
RAM
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 45
Additional Crash Issues
What happens if system crashes during Analysis? During REDO?
How do you limit the amount of work in REDO? Flush asynchronously in the background. Watch “hot spots”!
How do you limit the amount of work in UNDO? Avoid long-running Xacts.
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 46
Summary of Logging/Recovery
Recovery Manager guarantees Atomicity & Durability.
Use WAL to allow STEAL/NO-FORCE w/o sacrificing correctness.
LSNs identify log records; linked into backwards chains per transaction (via prevLSN).
pageLSN allows comparison of data page and log records.
Transaction Management and Recovery, 2nd Edition. R. Ramakrishnan and J. Gehrke 47
Summary, Cont.
Checkpointing: A quick way to limit the amount of log to scan on recovery.
Recovery works in 3 phases: Analysis: Forward from checkpoint. Redo: Forward from oldest recLSN. Undo: Backward from end to first LSN of
oldest Xact alive at crash. Upon Undo, write CLRs. Redo “repeats history”: Simplifies the
logic!