When workflows are processed in high-scale, high-availability cloud systems, the workflow handler associated with a particular session must be able to recover from unexpected failures and can resume partially completed work on a different process or machine from where the work began. Abandoning a message causes the same message to be served again with the next receive operation. A receiver may have many in-flight messages, but the messages will be received in order. Only one receiver can have a lock on a session. The session lock held by the session receiver is an umbrella for the message locks used by the peek-lock settlement mode. A session acts in many ways like a sub queue. One Session with SessionId = 4 has no active, owning client, which means that no messages are delivered from this specific session. The previous illustration shows three concurrent session receivers. With that operation, an interleaved message stream in one queue or subscription is cleanly de-multiplexed to different receivers and those receivers can also live on different client machines, since the lock management happens service-side, inside Service Bus. When multiple concurrent receivers pull from the queue, the messages belonging to a particular session are dispatched to the specific receiver that currently holds the lock for that session. The session lock should be treated like an exclusive lock on a file, meaning that the application should close the session as soon as it no longer needs it and/or doesn't expect any further messages. Instead, you can use the automatic lock renewal feature where you can specify the time duration for which you want to keep getting the lock renewed. There are methods on the receiver to renew the locks as well. The lock is released when you call close methods on the receiver or when the lock expires. It will also hold exclusive locks on all messages with the session ID that will arrive later. When the session is accepted and held by a client, the client holds an exclusive lock on all messages with that session's session ID in the queue or subscription. Sessions provide concurrent de-multiplexing of interleaved message streams while preserving and guaranteeing ordered delivery.Ī session receiver is created by a client accepting a session. There's an imperative model that controls when sessions and messages are received, and a handler-based model that hides the complexity of managing the receive loop.įor samples, use links in the Next steps section. The APIs for sessions exist on queue and subscription clients. All messages must be sent as part of a session (by setting the session id) and received by accepting the session. When sessions are enabled on a queue or a subscription, the client applications can no longer send/receive regular messages. The relative position of the content messages can be computed as the current message SequenceNumber delta from the start message SequenceNumber. For example, your application could set the Label property for the first message to start, for intermediate messages to content, and for the last message to end. Service Bus doesn't set any specific rules. Typically, however, an application has a clear notion of where a set of related messages starts and ends. Theoretically, a message can be received for a session today, the next message in a year's time, and if the session ID matches, the session is the same from the Service Bus perspective. Once a session exists, there's no defined time or API for when the session expires or disappears. On session-aware queues or subscriptions, sessions come into existence when there's at least one message with the session ID. At the AMQP 1.0 protocol level, this value maps to the group-id property. Service Bus isn't prescriptive about the nature of the relationship between messages, and also doesn't define a particular model for determining where a message sequence starts or ends.Īny sender can create a session when submitting messages into a topic or queue by setting the session ID property to some application-defined identifier that's unique to the session. To realize a FIFO guarantee in Service Bus, use sessions. For differences between these tiers, see Service Bus pricing. The standard and premium tiers support sessions. The basic tier of Service Bus doesn't support sessions.
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