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What does the persistence subsystem do?

The persistence subsystem is responsible for making a workflow instance durable. Durable workflow instances can be unloaded from memory and reloaded on the same or a different machine at a later point in time. This way, the persistence subsystem enables scenarios such as long-running workflow applications, increase system load by unloading idle instance, migration of instances between machines, scale out and load-balancing, and recovery of failed instances.

The persistence subsystem performs the following tasks:

·         Storing the state of workflow instances in a persistence store.

·         Recovery of durable workflow instances.

·         Activation of durable workflow instances with expired durable timers.

·         Instantaneous reactivation of durable workflow instances after a shutdown or crash of the Workflow Service Host.

Persistence for a workflow is enabled by defining an instance store for the Workflow Service Host. The .NET 4 framework comes with the SQL Workflow Instance Store, which is a SQL Server implementation of an instance store.
When a workflow instance persists, the SQL Workflow Instance Store (SWIS) saves the current instance state into the persistence store together with additional metadata that is required for activation, recovery and control. After the instance is persisted, the Workflow Service Host can unload the instance from memory. At a later point in time the Workflow Service Host may instruct the instance store to load the instance again. For example, the Workflow Service Host will automatically reload the workflow instance when a new message arrives for that instance or if any of the instance's timers expire.

Persistence is triggered in multiple ways:

·         Some activities persist the instance. The SendReplyToReceive activityhas a PersistBeforeSend property, which can be set to persist the Workflow state before a reply is sent.

·         The user defines the Workflow Idle behavior and sets the PersistOnIdle time. When specified the Workflow Service Host persists the instance after the instance has been idle for the specified time. An instance can go idle when it is waiting on a receive or delay activity.

·         The user defines the Workflow Idle behavior and sets the UnloadOnIdle time. When specified the Workflow Service Host persists and unloads the instance after the instance has been idle for the specified time.

In addition to these mechanisms, the workflow can contain explicit persistence activities. Those explicit persist activities are only required to persist a workflow throughout a long episode of computation or to guarantee that certain well-known persist points are present.