WebSphere MQ Telemetry Transport delivers messages according to the levels defined in a Quality of Service (QoS). The levels are described below:
The table below shows the QoS level 0 protocol flow.
Client | Message and direction | Broker |
---|---|---|
QoS = 0 | PUBLISH |
Action: Publish message to subscribers |
A message with QoS level 1 has a Message ID in the message header.
The table below shows the QoS level 1 protocol flow.
Client | Message and direction | Broker |
---|---|---|
QoS = 1 |
PUBLISH |
Actions:
|
Action: Discard message | PUBACK |
If the client does not receive a PUBACK message (either within a time period defined in the application, or if a failure is detected and the communications session is restarted), the client resends the PUBLISH message with the DUP flag set.
When it receives a duplicate message from the client, the broker republishes the message to the subscribers, and sends another PUBACK message.
A message with QoS level 2 has a Message ID in the message header.
The table below shows the QoS level 2 protocol flow.
Client | Message and direction | Broker |
---|---|---|
QoS = 2 |
PUBLISH |
Action: Store message in database |
PUBREC |
Message ID = x | |
Message ID = x | PUBREL |
Actions:
|
Action: Discard message | PUBCOMP |
Message ID = x |
If a failure is detected, or after a defined time period, each part of the protocol flow is retried with the DUP bit set. The additional protocol flows ensure that the message is delivered to subscribers once only.
Because QoS1 and QoS2 indicate that messages must be delivered, the broker stores messages in a database. If the broker has problems accessing this data, messages might be lost. For more details, and actions you can take to reduce these problems, see Designing Telemetry applications.
In any network, it is possible for devices or communication links to fail. If this happens, one end of the link might not know what is happening at the other end; these are known as in doubt windows. In these scenarios assumptions have to be made about the reliability of the devices and networks involved in message delivery.
WebSphere MQ Telemetry Transport assumes that the client and broker are generally reliable, and that the communications channel is more likely to be unreliable. If the client device fails, it is typically a catastrophic failure, rather than a transient one. The possibility of recovering data from the device is low. Some devices have non-volatile storage, for example flash ROM. The provision of more persistent storage on the client device protects the most critical data from some modes of failure.
Beyond the basic failure of the communications link, the failure mode matrix becomes complex, resulting in more scenarios than the specification for WebSphere MQ Telemetry Transport can handle.
The time delay (retry interval) before resending a message that has not been acknowledged is specific to the application, and is not defined by the protocol specification.
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