Specifying the Maximum Number of Concurrently Active Processes
Incoming events may not be evenly distributed over time. That is, there may be periods where a large number of incoming events occur and other periods where relatively few events occur. To prevent your system from being overwhelmed by incoming events, the Max Jobs (Flow Limit) field limits the number of process instances created by a process starter. This allows you to control the flow of processing so that incoming events are no longer accepted when the limit is reached.
Controlling the flow of processing is especially useful when you are using protocols that can store unsent messages on the server until the receiver is ready to process them. For example, if your process definition polls an email server for new messages (that is, Receive Mail is the process starter), and then you can set Max Jobs (Flow Limit) to control the number of process instances created for each new email. Email that has not been processed remains on the email server until the process engine is ready to create more process instances. Other protocols where this approach are useful are TIBCO Rendezvous Certified Messaging (RVCM), JMS durable topic subscriptions, and JMS queues.
When a process engine reaches the specified Max Jobs (Flow Limit), it is placed in a FLOW_CONTROLLED state. In this state, the process engine can continue executing existing process instances, but new process instances are not allowed. Incoming messages can then be directed to another process engine. A process engine will resume creating new process instances once a sufficient number of its current process instances have completed.