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Workflow

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Introduction

Business enterprises need to reduce the cost of doing business and continually develop new services and products. Enterprise Content Management helps with storing business-critical content (customer data, documents, etc.) in a central repository and in a unified way. Workflow Management provides the methodologies and software that help with organizing the processes that operate on this content inside an organization.

The relationship between Enterprise Content Management and Workflow Management is analogous to that of attributes (data) and methods (operations on the data) that are encapsulated in a class in object-oriented programming.

The Workflow component provides a virtual machine that executes workflows represented through object graphs. These object graphs can be created programmatically through the software component's Workflow Definition API. Alternatively, a workflow definition can be loaded from an XML file. Object graph and XML file are two different representations of a workflow definition that uses a backend language built on the workflow patterns.

More information on the theoretical background is available here.

Class Overview

This section gives you an overview of the main classes of the Workflow component.

ezcWorkflow

An object of the ezcWorkflow class represents a workflow.

Workflow Execution

ezcWorkflowExecutionNonInteractive

An object of the ezcWorkflowExecutionNonInteractive class can execute non-interactive workflows, in other words workflows that contains no input nodes and no sub-workflows.

ezcWorkflowDatabaseExecution

An object of the ezcWorkflowDatabaseExecution class can execute interactive workflows. The persistence data is stored using a relational database.

Workflow Definition

ezcWorkflowDefinitionStorageXml

The ezcWorkflowDefinitionStorageXml class provides the functionality to save workflow definitions to and load workflow definitions from XML files.

ezcWorkflowDatabaseDefinitionStorage

The ezcWorkflowDatabaseDefinition Storageclass provides the functionality to save workflow definitions to and load workflow definitions from a relational database.

Graph Node Classes

Objects of the ezcWorkflowNode classes represent the nodes of a workflow.

ezcWorkflowNode

ezcWorkflowNode is the abstract base class for all graph node classes.

ezcWorkflowNodeStart

An object of the ezcWorkflowNodeStart class represents the one and only start node of a workflow. The execution of the workflow starts here.

Creating an object of the ezcWorkflow class automatically creates the start node for this new workflow. It can be accessed through the startNode property.

ezcWorkflowNodeEnd

An object of the ezcWorkflowNodeEnd class represents an end node of a workflow. A workflow must have at least one end node. The execution of the workflow ends when an end node is reached.

Creating an object of the ezcWorkflow class automatically creates a default end node for this new workflow. It can be accessed through the endNode property.

ezcWorkflowNodeCancel

As soon as a node of the ezcWorkflowNodeCancel type is activated, the complete workflow instance is removed. This includes currently executing nodes, those which may execute at some future time, and all parent and sub-workflows. The workflow instance is recorded as having completed unsuccessfully.

ezcWorkflowNodeFinally

Start node of a sequence of final activities that is executed when a workflow execution is cancelled.

Creating an object of the ezcWorkflow class automatically creates the finally node for this new workflow. It can be accessed through the finallyNode property.

ezcWorkflowNodeAction

An object of the ezcWorkflowNodeAction class represents an activity node. When the node is reached, the business logic that is implemented by the associated service object is executed.

ezcWorkflowNodeSubWorkflow

An object of the ezcWorkflowNodeSubWorkflow class represents a sub-workflow. When the node is reached, the specified sub-workflow is started. The workflow is suspended until the sub-workflow has finished executing.

ezcWorkflowNodeInput

An object of the ezcWorkflowNodeInput class represents an input node. When the node is reached, the workflow engine will suspend the workflow execution if the specified input data is not available (first activation). While the workflow is suspended, the application that embeds the workflow engine may supply the input data and resume the workflow execution (second activation of the input node). Input data is stored in a workflow variable.

ezcWorkflowNodeVariableSet

An object of the ezcWorkflowNodeVariableSet class sets a specified workflow variable to a given value.

ezcWorkflowNodeVariableUnset

An object of the ezcWorkflowNodeVariableUnset class unsets a specified workflow variable.

ezcWorkflowNodeVariableAdd

An object of the ezcWorkflowNodeVariableAdd class adds a given operand, either a constant or the value of another workflow variable, to a specified workflow variable.

ezcWorkflowNodeVariableSub

An object of the ezcWorkflowNodeVariableSub class subtracts a given operand, either a constant or the value of another workflow variable, from a specified workflow variable.

ezcWorkflowNodeVariableMul

An object of the ezcWorkflowNodeVariableMul class multiplies a specified workflow variable with a given operand, either a constant or the value of another workflow variable.

ezcWorkflowNodeVariableDiv

An object of the ezcWorkflowNodeVariableDiv class divides a specified workflow variable by a given operand, either a constant or the value of another workflow variable.

ezcWorkflowNodeVariableIncrement

An object of the ezcWorkflowNodeVariableIncrement class increments the value of a specified workflow variable.

ezcWorkflowNodeVariableDecrement

An object of the ezcWorkflowNodeVariableDecrement class decrements the value of a specified workflow variable.

ezcWorkflowNodeParallelSplit

The ezcWorkflowNodeParallelSplit class implements the Parallel Split workflow pattern.

The Parallel Split workflow pattern divides one thread of execution unconditionally into multiple parallel threads of execution.

Use Case Example: After the credit card specified by the customer has been successfully charged, the activities of sending a confirmation email and starting the shipping process can be executed in parallel.

ezcWorkflowNodeSynchronization

The ezcWorkflowNodeSynchronization class implements the Synchronization workflow pattern.

The Synchronization workflow pattern synchronizes multiple parallel threads of execution into a single thread of execution. Workflow execution continues once all threads of execution that are to be synchronized have finished executing (exactly once).

Use Case Example: After the confirmation email has been sent and the shipping process has been completed, the order can be archived.

ezcWorkflowNodeExclusiveChoice

The ezcWorkflowNodeExclusiveChoice class implements the Exclusive Choice workflow pattern.

The Exclusive Choice workflow pattern defines multiple possible paths for the workflow of which exactly one is chosen.

Use Case Example: After an order has been received, the payment can be performed by credit card or bank transfer.

ezcWorkflowNodeSimpleMerge

The ezcWorkflowNodeSimpleMerge class implements the Simple Merge workflow pattern.

The Simple Merge workflow pattern is to be used to merge the possible paths that are defined by a preceding Exclusive Choice. It is assumed that of these possible paths exactly one is taken and no synchronization takes place.

Use Case Example: After the payment has been performed by either credit card or bank transfer, the order can be processed further.

ezcWorkflowNodeLoop

The ezcWorkflowNodeLoop class is a specialization of the ezcWorkflowNodeExclusiveChoice class and may be used to conveniently express loops.

ezcWorkflowNodeMultiChoice

The ezcWorkflowNodeMultiChoice class implements the Multi Choice workflow pattern.

The Multi-Choice workflow pattern defines multiple possible paths for the workflow of which one or more are chosen. It is a generalization of the Parallel Split and Exclusive Choice workflow patterns.

ezcWorkflowNodeSynchronizingMerge

The ezcWorkflowNodeSynchronizingMerge class implements the Synchronizing Merge workflow pattern.

The Synchronizing Merge workflow pattern is to be used to synchronize multiple parallel threads of execution that are activated by a preceding Multi-Choice.

ezcWorkflowNodeDiscriminator

The ezcWorkflowNodeDiscriminator class implements the Discriminator workflow pattern.

The Discriminator workflow pattern can be applied when the assumption we made for the Simple Merge workflow pattern does not hold. It can deal with merge situations where multiple incoming branches may run in parallel. It activates its outgoing node after being activated by the first incoming branch and then waits for all remaining branches to complete before it resets itself. After the reset the Discriminator can be triggered again.

Use Case Example: To improve response time, an action is delegated to several distributed servers. The first response proceeds the flow, the other responses are ignored.

Condition Classes

The ezcWorkflowCondition classes can be used to express branch conditions and input validation.

ezcWorkflowConditionVariable

An object of the ezcWorkflowConditionVariable loads a workflow variable and evaluates another ezcWorkflowCondition object for it.

ezcWorkflowConditionNot

An object of the ezcWorkflowConditionNot decorates an ezcWorkflowCondition object and negates its expression.

ezcWorkflowConditionAnd

An object of the ezcWorkflowConditionAnd class represents a boolean AND expression. It can hold an arbitrary number of ezcWorkflowCondition objects.

ezcWorkflowConditionOr

An object of the ezcWorkflowConditionOr class represents a boolean OR expression. It can hold an arbitrary number of ezcWorkflowCondition objects.

ezcWorkflowConditionXor

An object of the ezcWorkflowConditionXor class represents a boolean XOR expression. It can hold an arbitrary number of ezcWorkflowCondition objects.

ezcWorkflowConditionIsTrue

The condition represented by an ezcWorkflowConditionIsTrue object evaluates to true when the associated workflow variable has the value true.

ezcWorkflowConditionIsFalse

The condition represented by an ezcWorkflowConditionIsFalse object evaluates to true when the associated workflow variable has the value false.

ezcWorkflowConditionIsEqual

The condition represented by an ezcWorkflowConditionIsEqual object evaluates to true when the associated workflow variable is equal to the comparison value.

ezcWorkflowConditionIsNotEqual

The condition represented by an ezcWorkflowConditionIsNotEqual object evaluates to true when the associated workflow variable is not equal to the comparison value.

ezcWorkflowConditionIsGreaterThan

The condition represented by an ezcWorkflowConditionIsGreaterThan object evaluates to true when the associated workflow variable is greater than the comparison value.

ezcWorkflowConditionIsEqualOrGreaterThan

The condition represented by an ezcWorkflowConditionIsEqualOrGreaterThan object evaluates to true when the associated workflow variable is equal to or greater than the comparison value.

ezcWorkflowConditionIsLessThan

The condition represented by an ezcWorkflowConditionIsLessThan object evaluates to true when the associated workflow variable is less than the comparison value.

ezcWorkflowConditionIsEqualOrLessThan

The condition represented by an ezcWorkflowConditionIsEqualOrLessThan object evaluates to true when the associated workflow variable is equal to or less than the comparison value.

ezcWorkflowConditionInArray

The condition represented by an ezcWorkflowConditionInArray object evaluates to true when the associated workflow variable is in the specified array.

ezcWorkflowConditionIsAnything

The condition represented by an ezcWorkflowConditionIsAnything object always evaluates to true.

ezcWorkflowConditionIsArray

The condition represented by an ezcWorkflowConditionIsArray object evaluates to true when the associated workflow variable is an array.

ezcWorkflowConditionIsBool

The condition represented by an ezcWorkflowConditionIsBool object evaluates to true when the associated workflow variable is a boolean.

ezcWorkflowConditionIsFloat

The condition represented by an ezcWorkflowConditionIsFloat object evaluates to true when the associated workflow variable is a float.

ezcWorkflowConditionIsInteger

The condition represented by an ezcWorkflowConditionIsInteger object evaluates to true when the associated workflow variable is an integer.

ezcWorkflowConditionIsObject

The condition represented by an ezcWorkflowConditionIsObject object evaluates to true when the associated workflow variable is an object.

ezcWorkflowConditionIsString

The condition represented by an ezcWorkflowConditionIsString object evaluates to true when the associated workflow variable is a string.

Workflow Definition Usage

This section gives a few basic examples on how to create, save and load workflows. It also demonstrates how to make a graphical representation of your workflow.

Creating a simple workflow programmatically

This example shows how to set up a fairly simple workflow that will execute differently based on the variable 'choice'.

  1. <?php
  2. // Create new workflow of name "Test".
  3. $workflow = new ezcWorkflow( 'Test' );
  4. // Create an Input node that expects a boolean workflow variable of name "choice".
  5. $input = new ezcWorkflowNodeInput(
  6. array( 'choice' => new ezcWorkflowConditionIsBool )
  7. );
  8. // Add the previously created Input node
  9. // as an outgoing node to the start node.
  10. $workflow->startNode->addOutNode( $input );
  11. // Create a new Exclusive Choice node and add it as an
  12. // outgoing node to the previously created Input node.
  13. // This node will choose which output to run based on the
  14. // choice workflow variable.
  15. $branch = new ezcWorkflowNodeExclusiveChoice;
  16. $branch->addInNode( $input );
  17. // Either $true or $false will be run depending on
  18. // the above choice.
  19. // Note that neither $true nor $false are valid action nodes.
  20. // see the next example
  21. $trueNode = new ezcWorkflowNodeAction( 'PrintTrue' );
  22. $falseNode = new ezcWorkflowNodeAction( 'PrintFalse' );
  23. // Branch
  24. // Condition: Variable "choice" has boolean value "true".
  25. // Action: PrintTrue service object.
  26. $branch->addConditionalOutNode(
  27. new ezcWorkflowConditionVariable( 'choice', new ezcWorkflowConditionIsTrue ),
  28. $trueNode );
  29. // Branch
  30. // Condition: Variable "choice" has boolean value "false".
  31. // Action: PrintFalse service object.
  32. $branch->addConditionalOutNode(
  33. new ezcWorkflowConditionVariable( 'choice', new ezcWorkflowConditionIsFalse ),
  34. $falseNode
  35. );
  36. // Create SimpleMerge node and add the two possible threads of
  37. // execution as incoming nodes of the end node.
  38. $merge = new ezcWorkflowNodeSimpleMerge;
  39. $merge->addInNode( $trueNode );
  40. $merge->addInNode( $falseNode );
  41. $merge->addOutNode( $workflow->endNode );
  42. ?>

Adding custom PHP code to your workflow and manipulating workflow variables

In the previous example the $trueNode and $falseNodes where action objects. However, in order to make them work we must add service objects. The following example replaces the lines where $trueNode and $falseNode are created.

  1. <?php
  2. class MyServiceObject implements ezcWorkflowServiceObject
  3. {
  4. private $message;
  5. public function __construct( $message )
  6. {
  7. $this->message = $message;
  8. }
  9. public function execute( ezcWorkflowExecution $execution )
  10. {
  11. echo $this->message;
  12. // Manipulate the workflow.
  13. // Does not affect the workflow, for illustration only.
  14. $execution->setVariable( 'choice', true );
  15. // Return true to signal that the service object has finished
  16. // executing.
  17. return true;
  18. }
  19. public function __toString()
  20. {
  21. return "MyServiceObject, message {$this->message}";
  22. }
  23. }
  24. $trueNode = new ezcWorkflowNodeAction( array( 'class' => 'MyServiceObject',
  25. 'arguments' => array( 'message: TRUE' ) )
  26. );
  27. $falseNode = new ezcWorkflowNodeAction( array( 'class' => 'MyServiceObject',
  28. 'arguments' => array( 'message: FALSE' ) )
  29. );
  30. ?>

When this action node is executed it will load the service object with the parameters specified and run the execute() method. Our execute method will print the message passed to the constructor and change the workflow variable 'choice'.

The execute() method of a service object may return false to indicate that it is not finished yet. The workflow execution will then be suspended. The next time the workflow execution is resumed the service object's execute() method will be invoked again.

Saving a workflow to an XML file

Typically a workflow is built once and executed many times. It is also common that workflows are interupted during execution and resumed later in another PHP request. To accomodate this it is possible to save and load workflows to XML files (or database through the WorkflowDatabaseTiein).

This example shows you how to store a workflow to an XML file. The example assumes the workflow can be found in the $workflow variable.

  1. <?php
  2. // Set up workflow definition storage (XML).
  3. $definition = new ezcWorkflowDefinitionStorageXml( '/path/to/directory' );
  4. // Save workflow definition to database.
  5. $definition->save( $workflow );
  6. ?>

Loading a workflow from an XML file

It is of course also possible to load a workflow that has been saved to disk.

  1. <?php
  2. // Set up workflow definition storage (XML).
  3. $definition = new ezcWorkflowDefinitionStorageXml( '/path/to/directory' );
  4. // Load latest version of workflow named "Test".
  5. $workflow = $definition->loadByName( 'Test' );
  6. ?>

Visualizing a workflow using GraphViz

It is possible to write visitors that traverse the workflow graph. One example of this is the GraphViz visitor that generates a viewable graph representation of the workflow. The GraphViz visitor uses the __tostring methods on the nodes to get names for the nodes.

  1. <?php
  2. // Generate GraphViz/dot markup for workflow "Test".
  3. $visitor = new ezcWorkflowVisitorVisualization;
  4. $workflow->accept( $visitor );
  5. print $visitor;
  6. ?>
digraph Test {
node1 [label="Start"]
node3 [label="Input"]
node4 [label="Exclusive Choice"]
node5 [label="PrintTrue"]
node7 [label="Simple Merge"]
node2 [label="End"]
node6 [label="PrintFalse"]

node1 -> node3
node3 -> node4
node4 -> node5 [label="choice is true"]
node4 -> node6 [label="choice is false"]
node5 -> node7
node7 -> node2
node6 -> node7
}

img/tutorial_example_06.png

Workflow Execution Usage

For examples on how to execute workflows see the WorkflowDatabaseTiein tutorial.

Execution Plugins

The workflow engine can be customized and extended through a plugin system. A plugin is an object of a class that extends ezcWorkflowExecutionPlugin and that is registered with the execution object using the addPlugin() method.

The following methods are automatically invoked during the workflow execution after the respective event has been handled:

  • afterExecutionStarted()
  • afterExecutionSuspended()
  • afterExecutionResumed()
  • afterExecutionCancelled()
  • afterExecutionEnded()
  • afterNodeActivated()
  • afterNodeExecuted()
  • afterThreadStarted()
  • afterThreadEnded()
  • afterVariableSet()
  • afterVariableUnset()

The above methods only allow read access to the events that they handle and are usually used to log workflow execution.

The following methods are automatically invoked during the workflow execution before the respective event is handled:

  • beforeNodeActivated()
  • beforeVariableSet()
  • beforeVariableUnset()

The above methods allow both read and write access to the events that they handle and may be used to change the behaviour of the workflow execution.

The WorkflowSignalSlotTiein component allows you to hook into the workflow execution into using the SignalSlot component. Details can be found in its documentation.

Execution Listeners

Objects of classes that implement the ezcWorkflowExecutionListener interface can be registered as listeners with an ezcWorkflowExecution object to receive messages during the execution of a workflow.

The ezcWorkflowEventLogListener class that is provided by the WorkflowEventLogTiein component is the reference implementation of this interface. You can find an example on how to register execution listeners in its documentation.

Last updated: Mon, 21 Dec 2009