Active queues

Work queues are available in the Blue Prism product to enable cases to be queued and worked in a predefined order within a session.

Using the traditional session management model, sessions are started on resources which poll the work queue for cases to work. These sessions are started in Control Room manually, or via a scheduler service which is running on a Blue Prism Server instance.

Active queues introduce an alternative mechanism for managing the sessions which work the queues, made possible by creating a closer association between work queues and sessions.

Instead of creating sessions separately in Control Room and then moving to the queue management page to see the results, active queues allow you to set a target number of resources which should be working the queue, Blue Prism uses the active queue configuration to determine how to achieve that target.

Active queue requirements

In order for Blue Prism to determine how to start and stop sessions on behalf of a queue, it needs to know two things about the queue:

• Which process works the queue?
• Which resources should be used to work the queue?

This information is set in the Work queue configuration page in the System section of the Blue Prism client, by associating a published process and a resource group with the queue.

With this information, Blue Prism can find an available resource and create a process on it which will then work the queue.

In order to stop a session which is running on behalf of an active queue, a 'stop request' is made to the session, allowing it to ensure it does not stop in the middle of a transaction.

Process requirements

In order to operate on behalf of an active queue, a process must meet some basic requirements.

• The process must be published before it can be set as the assigned process in an active queue.

• Assuming that the process contains a main loop, the function IsStopRequested() must be queried using a decision stage within that loop in order to ascertain if a safe stop has been requested.

  This is the mechanism used by the active queue controller to stop sessions running on behalf of active queues that it is controlling.

How it works

When the Active Queues section of Control Room is entered, an Active Queue Controller is created for each of the queues being displayed. These controllers remain in memory until the user logs out of the client.

Each controller monitors the sessions running on the resources assigned to their queue and polls the database for the latest statistics for the queue that they are responsible for.

When a target resource value is set, the controller determines whether it needs to create new sessions or stop existing sessions to reach that target. If new sessions are required, it will create and start the new sessions; if existing sessions must be stopped, it will send a stop request to the appropriate number of sessions to reach the target value.

Create a session

If the queue controller determines that new sessions must be created in order to reach the target, it will create and start new sessions on the available resources in the resource group assigned to its queue.

It will create sessions on the least busy resource first, eg. if the group has three available resources and two of them are running a background process, the first session it creates will be on the resource which is not running any sessions.

If any sessions fail to be created or started for some reason, it will retry on other resources. After three attempts, it will abandon the session and decrement the target value to indicate that it will no longer be attempting to create the session.

If any errors occur which cause a session to be abandoned by the controller, they will be recorded in the Blue Prism event log and will be visible using the Windows Event Viewer.

Stop a session

If the queue controller decides that existing sessions must be stopped in order to reach the target, it will iterate through the sessions and choose the oldest sessions (ie. the sessions that were started at the earliest times) to send a stop request to.

Once a controller sends a stop request to a session, the session is marked as Stopping. In this state, it is still 'running' – it is contributing to the 'Active Sessions' count in the queue, and it is not available to run further sessions on.

In order to stop correctly, the worker process must check the IsStopRequested() function regularly, cleaning up its environment and exiting the process when the function indicates that a stop has been requested.

Session completions/terminations

When a session which is running on behalf of an active queue completes or terminates, it is taken out of the 'Active Sessions' count for the queue and becomes available again for further sessions.

If the completion / termination occurs after a stop request has been sent by a queue controller, the target resource value is unchanged (ie. the session was expected to stop in order for the target value to be reached).

If the completion / termination occurs before any stop request has been sent by a queue controller, the target resource value is decremented – no other session is created to take its place.

Active queues and resource pools

To a certain extent, resource pools are trying to solve the same problem as the assigned resource group in an active queue – that of finding an available resource and executing a process on it.

As such, it is not recommended to use resource pools within active queues – the active queue controller assumes that each resource represents a single slot for running sessions on behalf of its queue. Resource pools are transparent in that they appear to be single resources which can run multiple exclusive sessions and the queue controller can't drill down into the member resources to ascertain availability.

If you do include a resource pool in a resource group assigned to an active queue, it will be treated as a single resource in the 'Available Resources' count and it will only have a single session assigned to it, at which point it will be taken out of consideration for that queue.