Tuesday, 17 March 2015

SOA Dehydration Process


Oracle BPEL Process Manager uses the dehydration store database to maintain long-running asynchronous processes and their current state information in a database while they wait for asynchronous callbacks. Storing the process in a database preserves the process and prevents any loss of state or reliability if a system shuts down or a network problem occurs. There are two types of processes in Oracle BPEL Process Manager. These processes impact the dehydration store database in different ways.
  • Transient processes: this process type does not incur any intermediate dehydration points during process execution. If there are unhandled faults or there is system downtime during process execution, the instances of a transient process do not leave a trace in the system. Instances of transient processes cannot be saved in-flight (whether they complete normally or abnormally). Transient processes are typically short-lived, request-response style processes.
  • Durable processes: this process type incurs one or more dehydration points in the database during execution because of the following activities:
    • Receive activity
    • OnMessage branch in a pick activity
    • OnAlarm branch in a pick activity
    • Wait activity
    Instances of durable processes can be saved in-flight (whether they complete normally or abnormally). These processes are typically long-living and initiated through a one-way invocation. Because of out-of-memory and system downtime issues, durable processes cannot be memory-optimized
  • An idempotent activity is an activity that can be retried (for example, an assign activity or an invoke activity). Oracle BPEL Server saves the instance after a nonidempotent activity.

Idempotent BPEL Property

A BPEL invoke activity is by default an idempotent activity, meaning that the BPEL process does not dehydrate instances immediately after invoke activities. Therefore, if idempotent is set to true and Oracle BPEL Server fails right after an invoke activity executes, Oracle BPEL Server performs the invoke again after restarting. This is because no record exists that the invoke activity has executed. This property is applicable to both durable and transient processes.

If idempotent is set to false, the invoke activity is dehydrated immediately after execution and recorded in the dehydration store. If Oracle BPEL Server then fails and is restarted, the invoke activity is not repeated, because Oracle BPEL Process Manager sees that the invoke already executed.

When idempotent is set to false, it provides better failover protection, but at the cost of some performance, since the BPEL process accesses the dehydration store much more frequently. This setting can be configured for each partner link in the bpel.xml file.

Setting this parameter to true can significantly improve throughput. However, as mentioned previously, you must ensure that the partner's service can be safely retried in the case of a server failure. Some examples of where this property can be set to true are read-only services (for example, CreditRatingService) or local EJB/WSIF invocations that share the instance's transaction

This property has the following values:

false: activity is dehydrated immediately after execution and recorded in the dehydration store

true (default): If Oracle BPEL Server fails, it performs the activity again after restarting. This is because the server does not dehydrate immediately after the invoke and no record exists that the activity executed.

inMemoryOptimization BPEL Property

This property indicates to Oracle BPEL Server that this process is a transient process and dehydration of the instance is not required. When set to true, Oracle BPEL Server keeps the instances of this process in memory only during the course of execution. This property can only be set to true for transient processes (that is, those that do not contain any middle process receive, pick, or wait activities).

The default for this property is false, which means that instances are persisted completely and recorded in the dehydration store database for a synchronous BPEL process.

When inMemoryOptimization is set to true, dehydration is deactivated, and Oracle BPEL Process Manager keeps instances in memory only. The settings for the completionPersistPolicy and completionPersistLevel properties are also examined to determine persistence behavior. The inMemoryOptimization property can improve throughput when set to true and, in conjunction with these two other properties, can minimize database growth.

BPEL - CallBack Message Handling

BPEL engine maintains all Async call back messages into database table called dlv_message. Wecan see such all messages in BPEL console call-back manual recovery area.The query being used by bpel console is joined on dlv_message and work_item tables.This query simply picks up all call back messages which are undelivered and have not been modified with in certain threshold time.

Call-back messages are processed in following steps
  • BPEL engine assigns the call-back message to delivery service
  • Delivery service saves the message into dlv_message table with state 'UNDELIVERED-0'
  • Delivery service schedules a dispatcher thread to process message asynchronously
  • Dispatcher thread enqueues message into JMS queue
  • Message is picked up by MDB
  • MDB delivers the message to actual BPEL process  waiting for call-back and changes state to 'HANDLED=2'
So given above steps, there is always possibility that message is available in dlv_message table but MDB is failed in delivering it to BPEL process which keeps message always in state= 0.