Transactional Workflow Chapter 9. © Jim Gray, Andreas Reuter Transaction Processing - Concepts and...

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Transactional Workflow

Chapter 9

© Jim Gray, Andreas Reuter Transaction Processing - Concepts and Techniques WICS August 2 - 6, 1999 2

What Is the Problem ?

“A workflow management system is an active system that manages the flow of business processes performed by multiple persons in multiple steps. It gets the right data to the right people with the right tools at the right time.”

(This definition omits a umber of aspects: roles, events, errors, cooperation, ...)


In More Technical Terms: What Is Workflow ?

WF is a long-lived execution involving a potentially large number of autonomous agents such as programs, databases, sensors, actors, humans.

Control flow and data flow are (partially) pre-defined and may evolve over time.

There are numerous interdependent consistency criteria.

A WF must be kept alive by “the system” under all circumstances.


Source

Step

case

case

fork

loop

compensationstep

Components of a Workflow


Steps ...

execute application logic of any kind, can interact with human operators, access shared data in databases, depend on events and can create their own

events, have a short duration and (should) behave like

classical transactions, are invoked depending on the execution history.


The Script ...

specifies the control flow and the workflow, defines the event and data conditions under which a

step is to be executed, defines the synchronization criteria for accessing

shared data, maintains the local execution context of a workflow

instance, handles resource conflicts, in particular on shared

data, represents a persistent execution.


What Is Transactional Workflow ?

There are different interpretations: Extended transaction models adapted to the

needs of workflow: Sagas, Flex transactions, etc.

Application of some transactional properties such as isolation and durability to workflows.

Using classical distributed transactions to implement the control flow machinery of a workflow system.


What Happens to the Transactional Properties ?

A: Atomicity does not apply to an entire workflow

C: Consistency must be redefined including the temporal dimension.

I: Isolation must be limited in time; cooperation must be allowed.

D: Rather than the effects of transaction, the execution itself must be durable.


Correctness

Transactional correctness guarantees a consistent overall state if each individual step is executed correctly (or not at all) and there was a consistent initial state.

In long-lived executions, this definition cannnot be used, because strict isolation is not feasible and rollback is not option at the workflow level.


Rollback vs. Compensation

Rollback is based on the assumption that an erroneous state can be reverted to the previous (correct) state without affecting anybody => Isolation.

Compensation tries to modify an erroneous state such that all the consistency constraints work as though the faulty operation was never executed => Formal definition of consistency.


Correctness For Long-Lived Activities

Transactions can be executed iff the possibility of rollback is maintained.

A step in a workflow can be executed iff the individual step can be rolled back and its commitment does not block any of the previously

executed steps from being compensated if needed.


What Are Invariants For?

Compensation must be guaranteed for completed steps => certain predicates on the shared and local state must

be maintained. The requirements for a state to be executable

are formalized as combined event / state predicates called “invariants”.

Invariants are alos useful to describe correctness criteria for forward execution.


Types of Invariants

Entry invariants guard the execution of a step. If an entry invariant is violated, there are different

options: give up (compensate), negotiate, resolve conflict.

Exit invariants formalize the new consistent state. Its protection can be strict (must), moderate (want), lose (hope).


Invariants: Virtual Objects

Invariants may contain expressions like: obj_1 + obj_2 rel_op value. The objects are not necessarily managed by the

same RM. To support such invariants, they are established

as virtual objects, which: have a special name, have a “value” method, are stored at each participating RM, are evaluated locally whenever possible.


Dynamic Aspects of Invariants

p1

pa2 pa3 pa4

pb2 pb3 pb4

Individual invariants established by each step


p1

p1&pa2 p1&pa2&pa3 p1&pa2&pa3&pa4

p1&pb2 p1&pb2&pb3 p1&pb2&pb3&pb4

Accumulated invariants for a “case of” control flow




Invariants can be deleted if the step with the corresponding entry invariant will never be executed. This implies: All invariants become obsolete at the end of a

workflow. Dead code must be detected dynamically. We need special loop invariants.

If a step´s compensation step is dynamically modified, this may cause problems.


Supporting Invariants

In order to support invariants, database systems must give up some their autonomy. In particular, they have to: provide notification about lock conflicts, implement recoverable locks, implement semantic locks (e.g. escrow), implement existence locks at the tuple and at the

schema level.


Using Transactions in a Workflow-System

Application-level transactions for grouping multiple steps

Transaction 1

Transaction 2


B C

System-level transactions for transferring control from one step to the next



B C




C


B

CM

transfercontrol

receiverequest

notifyCM

notifyCM

System transaction A



B C

Input queuecommit

localprocessing

post

Version 1: Synchronous transfer

Workflow and Transactional Queues


B C

Input queuecommit

localprocessing

Version 2: Asynchronous transfer

outputqueue

Commit-Post-Transaction

Transfer Transaction

Workflow and Transactional Queues


C

Input queue Output queue

Local code

1. dequeue

2. local execution

3. post

Completelocal transactionincl. transferof control

Queue-Driven Step Processing


C

Normal processing

dequeuelocalprocessing

Transaction abort

localrollbackpost

Queues And Rollback


Problems Related to Rollback

Why did the local transaction fail (system abort or application-initiated rollback?)

In which cases should the TA be re-posted (and how often)?

Who gets notified about an abort (source or CM)? Who gets notified about the eventual failure to restart a

transaction? Should application-level TAs be treated as distributed or

nested transactions at the system level? Which programming level should handle these issues

(step or script)?


Summing It Up - 1

Transactional concepts can be carried over to workflow management in multiple ways and at differet levels.

The most obvious application of transaction technology is at the level of the workflow engine, where transactions provide persistent execution of a script, local recovery in case of partial failures, reliable state transitions, recoverable events, and consistent context.


Summing It Up - 2

Transactions at the system level need a number of extensions: nesting, chaining, leave-resume, transfer.

Participating resource managers need to be able to support an open distributed two-phase-commit protocol.

Persistent storage managers need a number of functional extensions such as recoverable locks.


Summing It Up - 3

At the step level, transactions provide atomicity for short-term related computations.

The TM must be able to support dependen-cies among transcations.

At the script level, the concepts of atomicity and consistency have to be translated into more abstract notions (compensation, invariants).

Transactional Workflow Chapter 9. © Jim Gray, Andreas Reuter Transaction Processing - Concepts and...

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