Guaranteed Messaging can be used to ensure the delivery of a message between two applications even in cases where the receiving application is off line, or there is a failure of a piece of network equipment. As well, those messages are delivered in the order they were published.
Two key points about Guaranteed Messaging:
- It keeps messages that are tagged as persistent or non-persistent (rather than Direct) across event broker restarts by spooling them to persistent storage. Although a persistent delivery mode is typically used for Guaranteed messages, a non-persistent delivery mode is provided to offer compatibility with Java Message Service (JMS), and to allow the delivery mode of the messages to be modified to accommodate the persistence requirements of an endpoint or a client subscription when there is a topic match (refer to Topic Matching and Message Delivery Modes).
- It keeps a copy of the message until successful delivery to all clients and downstream event brokers has been verified
To support Guaranteed Messaging, an event broker must have message spooling enabled, and a Solace PubSub+ appliance must also have an Assured Delivery Blade (ADB) installed.
Messages accepted by Solace PubSub+ through Guaranteed Messaging for delivery to clients are never lost, but might not get accepted if system resource limits are exceeded. If an ingress message cannot be received by Solace PubSub+ (for example, if the spool quota is exceeded), the publisher is not acknowledged, and the appropriate event broker statistic is incremented.