Nuno Filipe Godinho

PDC09 – Day Two – Windows Workflow Foundation 4.0 from the Inside out

Bob Schmidt – Program Manager

For WF4:

• Programs are data
• Scheduler-based program execution
  • stackless & serializable
• Runtime-mediated code rendezvous
• Natural Control flow
  • run it the way you think of it

Authoring

• WF is an activity
• Can author programmatically or using XAML
• Can contain other activities
• Separate base classes for activities that return a value
• WF program is a definition from which many instances can be created
• Each instance of an activity has a unique environment (visible data values)

Execution

• WF runtime just sees activities and not sequence or parallel or recurrence
• WF runtime is a referee that enforces the rules of the work
• CacheMetadata is how an activity describes itself
• WF runtime knows when an activity is done
• An activity can schedule the execution of a child activity and be notified upon its completion
• There can be multiple, distinct method invocations per activity
• Can Express all kinds of patterns (control flow)
• There can be multiple paths of invocation …

Scheduler

• WorkItem represents the invocation of a method
• WorkItems can be created by:
• Activity Code
• External Code
• Flow of execution
1. The Activity Code or the External Code Add a WorkflowItem
2. Them the elements get Pushed or Enqueued for execution
3. Afterwards the Scheduler will Pop it and execute it
• Details
• Scheduler per program instance
• Manages an ordered list of work items

Threading Model

• One thread at a time is used to process work items for a program instance
• Simplifies the programming model of activities
• Thread may differ work items in the same instance
• WF runtime uses host-provided SynchronizationContext
• Standard TLS mechanisms do not apply to WF
• Instead use WF Execution Properties – names properties visible for a part of a workflow
• Attached / detached from the current environment by the WF runtime before / after work item invocation
• Activities should not block this thread

Bookmarks

• Problems
• What happens when your programs need to wait for input?
• We want a “Zero footprint” WaitForInput()
• Bookmark
• A named resumption point in a WF program
• Resumption will schedule an activity’s callback method
• WF runtime mediates resumption – no need for the instance to be in memory
• WCF Receive activity is built on top of bookmarks in order to give access to this possibility when using Workflow Services

Persistence

• lets you pause an instance, save it somewhere & resume it later
• has no affinity to a WF host instance, CLR instance, thread, process or machine
• instances of WF are serializable
• A CLR Stack is not serializable, but in WF program a stack only exists transiently, during work item execution
• Sometimes you want a “no persist zone”
• Automatic during execution of an async activity
• Details of persistence are deliberately separate from the machinery of WF runtime

Serializable Instances

• Contains
• Work Item list
• Bookmarks
• Data (arg and values)
• Environment for all executing activity instances
• Activity instance info (callbacks, execution props)
• Custom Data from persistence participants
• Doesn’t contain
• Workflow Definition
• Why?
• several instances can share the same workflow definition
• management of definitions is responsibility of the host