- Scheduling
- Activity Automaton
- Bookmarks Revisited
- Activity Initialization and Uninitialization
- Composite Activity Execution
- WF Threads
- Where are We?
This chapter is from the book
This chapter is from the book
This chapter is from the book
Activity Initialization and Uninitialization
In the activity automaton, Initialized is the start state in which all activities begin their lifecycle. When the WorkflowRuntime.CreateWorkflow method returns, all activities in the newly created WF program instance are in the Initialized state.
Within the implementation of CreateWorkflow, the WF runtime calls the Initialize method of the root activity in the WF program. There are other interesting details related to the creation of new WF program instances, and they will be covered in Chapter 5; here we will focus only on activity initialization.
Activities can use the Initialize method to perform whatever initialization is necessary when a WF program instance is created. Custom services added to the WF runtime (and also the WorkflowQueuingService) can be obtained by the activity via the IServiceProvider that is passed as a parameter to Initialize. ActivityExecutionContext is not available because the activity (indeed, the WF program) has not yet begun its execution.
The CompositeActivity class overrides Initialize and in its implementation invokes the Initialize method of all enabled child activities. If you develop a composite activity, or indeed any activity that requires initialization logic, you should always call base.Initialize within your implementation of the Initialize method to ensure proper initialization of the WF program instance.
The WF runtime's scheduler machinery is not used during initialization to dispatch the calls to Initialize. It would be overkill to do so because the WF program instance is not yet running. Because invocation of Initialize is synchronous, the WF runtime can guarantee that when the WorkflowRuntime.CreateWorkflow method returns, the WF program instance is fully initialized and ready for execution.
If an exception is thrown from any activity's Initialize method, the initialization of the WF program instance fails, and the WorkflowRuntime.CreateWorkflow method will throw an exception indicating that this has occurred.
So, what can an activity do in its Initialize method? Initialize carries one parameter of type System.IServiceProvider. No execution context exists at this time for the activity, so it is not correct for the WF runtime to provide AEC. Still, the IServiceProvider of Initialize does the same service chaining that AEC does. Any custom services that you add to the WorkflowRuntime are proffered by this service provider so that an activity may do whatever resource initialization is required. The WorkflowQueuingService is available too, so that WF program queues may be created.
To summarize, the Initialized state is the start state of the activity automaton. Activities in this state have not started their execution, and can be said to be in a latent form, but do get a chance to perform initialization logic in their Initialize method.
Listing 3.14 updates the ReadLine activity so that it creates its WF program queue within its Initialize method.
Listing 3.14. The ReadLine Activity with Initialization Logic
using System;
using System.Workflow.ComponentModel;
using System.Workflow.Runtime;
namespace EssentialWF.Activities
{
public class ReadLine : Activity
{
private string text;
public string Text
{
get { return this.text; }
}
protected override void Initialize(
IServiceProvider provider)
{
WorkflowQueuingService qService =
(WorkflowQueuingService) provider.GetService(
typeof(WorkflowQueuingService));
if (!qService.Exists(this.Name))
qService.CreateWorkflowQueue(this.Name, true);
}
protected override ActivityExecutionStatus Execute(
ActivityExecutionContext context) {
WorkflowQueuingService qService =
context.GetService<WorkflowQueuingService>();
WorkflowQueue queue = qService.GetWorkflowQueue(Name);
if (queue.Count > 0)
{
this.text = (string) queue.Dequeue();
return ActivityExecutionStatus.Closed;
}
queue.QueueItemAvailable += this.ContinueAt;
return ActivityExecutionStatus.Executing;
}
void ContinueAt(object sender, QueueEventArgs e)
{
ActivityExecutionContext context =
sender as ActivityExecutionContext;
WorkflowQueuingService qService =
context.GetService<WorkflowQueuingService>();
WorkflowQueue queue = qService.GetWorkflowQueue(Name);
this.text = (string) queue.Dequeue();
context.CloseActivity();
}
protected override void Uninitialize(IServiceProvider provider)
{
WorkflowQueuingService qService =
(WorkflowQueuingService) provider.GetService(
typeof(WorkflowQueuingService));
if (qService.Exists(this.Name))
qService.DeleteWorkflowQueue(this.Name);
}
}
}
The implementation of Execute accounts for the fact that by the time the activity executes, there may already be an item in its WF program queue. If an item is indeed available, there is no need to subscribe to the QueueItemAvailable event. The ReadLine activity also contains an implementation of the Uninitialize method, in which the WF program queue is deleted.
The Uninitialize method is the logical counterpart of the Initialize method.
Uninitialize is called (synchronously, not via a work item in the scheduler work queue) as the final part of an activity's transition to the Closed state from the Executing state. It is also called when it is determined by the WF runtime that an activity in the Initialized state will never be executed. The latter case occurs when the parent of an activity transitions to the Closed state without having requested the execution of that child activity.
Activities cannot assume that they will always be executed, just as the program statements in all but one branch of a C# if statement will be passed over. Any resources created in an activity's Initialize method should therefore be cleaned up in its Uninitialize method.
As part of an activity's transition to the Closed state (and just prior to the invocation of Uninitialize), the WF runtime synchronously invokes the OnClosed method that is defined by Activity. In this method, activities can clean up the resources they allocated during their execution (as opposed to during their initialization).
You might wonder why OnClosed exists when we also have Uninitialize. The simple answer is that Uninitialize should clean up resources allocated in Initialize, whereas the purpose of OnClosed is to clean up resources allocated during the execution of the activity. An executing activity can transition to the Closed state from several different states (which will be discussed more in the next chapter), and the OnClosed method will be called in each of these cases.
To summarize, when we execute a ReadLine activity, ReadLine has its methods invoked in the following order:
- Initialize
- Execute
- ContinueAt
- OnClose
- Uninitialize
If a ReadLine activity is present in a WF program, but never executes, it will only have its Initialize and Uninitialize methods called.
Activities as CLR Objects
Because Activity implements System.ComponentModel.IComponent, which extends System.IDisposable, activities are given yet another opportunity to perform cleanup of resources. The IDisposable.Dispose method, however (like an activity's constructor), is a practicality necessitated by the fact that a WF program instance is transiently realized as a set of CLR objects when that program instance is in memory. These objects, like any objects, are created and destroyed subject to the rules of the CLR. However, transitions in the CLR object lifecycle are logically unrelated to the execution lifecycle of the WF program instance (and the activities within it). In other words, the calling of the Activity.Dispose method reflects the passivation cycles of a WF program instance—every time a WF program instance is passivated, the activity objects that represent the program instance while it is in memory are disposed because they no longer represent the (passivated) program instance.
The WF runtime will call the Dispose method on the CLR object representing an activity every time the WF program instance containing the activity is passivated. In contrast, Initialize and Uninitialize are called exactly once during the logical lifetime of an activity, which can span any number of passivation cycles. In contrast, Dispose may be invoked multiple times for an activity during its lifetime.
It is recommended that activities not perform any resource management in their object constructors. CLR objects that transiently represent an activity may be constructed and disposed multiple times during the course of the activity's execution lifetime. The constructor of an activity may be called multiple times even during the creation of a single program instance (or reactivation of an instance). It is crucial to understand that because an activity is an intrinsically resumable entity, its logical lifespan is governed by the activity automaton and not by the lifetime of any CLR object.
In order to provide well-defined points for resource allocation and cleanup, Activity defines two additional methods, OnExecutionContextLoad and OnExecutionContextUnload, which bracket the lifetime of a CLR object representing an activity in a WF instance. You can rely upon the WF runtime to call OnExecutionContextLoad during the creation (or reactivation) and OnExecutionContextUnload during the passivation of a WF instance. OnExecutionContextUnload is essentially just like Dispose except that it accepts an IServiceProvider as a parameter and therefore has access to runtime services.
Dispose, OnExecutionContextLoad, and OnExecutionContextUnload are side effects of the fact that the WF runtime is layered on top of the CLR, and are related to the management of CLR objects which transiently represent a WF program instance. In contrast, Initialize, Uninitialize, and OnClose are related to the lifetime of the activity as described by the activity automaton. It is crucial to understand this difference between CLR programs and WF programs. From the perspective of the CLR, a CLR program instance is defined by its in-memory existence and lifetime. From the point of view of the WF runtime, a WF program instance is defined on an altogether different plane, and in fact can spend most of its lifetime in persistent storage. Because a WF program instance may passivate and reactivate many times (perhaps on different machines), objects that represent the activities in that instance in memory might need to be constructed and disposed of many times before the WF program instance completes (see Figure 3.9).
)
Figure 3.9 Lifecycle of a WF program instance
Many activities require only an empty constructor and Dispose method, but it is important nonetheless to know when and why they will be called.