The short answer
Solace Direct messaging is reliable but not guaranteed: messages are not spooled or retained for disconnected clients and can be discarded on disconnects, slow-consumer buffer overflow or congestion. Guaranteed messaging spools messages to persistent storage, delivers them in publish order, and survives offline receivers and broker restarts.
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Solace PubSub+ offers two delivery modes, and the wording in the documentation is precise in a way that is easy to skim past: Direct messaging provides reliable, but not guaranteed, delivery [1]. The difference decides whether a message can disappear.
Direct messaging
- Direct messages are not spooled on the message bus, and are not retained for a client that is not connected to the event broker [1].
- They can be discarded before delivery when a client disconnects while messages are being published, when a client fails to consume at the published rate and its egress buffer overflows, or under congestion or system failures [1].
- Solace positions it for extremely high message rates and low latency, where consumers can tolerate loss under congestion and messages do not need to be kept for slow or offline consumers [1].
Guaranteed messaging
- Guaranteed messaging can ensure delivery even when the receiving application is offline or a piece of network equipment fails [2].
- Messages are delivered in the order they were published [2].
- Messages are kept across event broker restarts by spooling them to persistent storage [2].
What we ask
We ask Solace engineers which flows in their last system ran Direct and what they did about slow consumers. The useful answer names the flows that could tolerate loss and the ones that were moved to Guaranteed queues because they could not.
Hiring for this?
Conntal places middleware engineers on contract, contract-to-hire or direct hire, each screened by a practitioner before you see a CV.
Sources
- [1] Solace, Direct Messages. Retrieved 7 October 2026.Primary source
- [2] Solace, Guaranteed Messages. Last updated 1 October 2026.Primary source
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Related questions
Can Solace Direct messages be lost?
Yes. Direct messaging is reliable but not guaranteed; messages are not spooled and can be discarded if a client disconnects, a slow consumer overflows its egress buffer, or under congestion or system failures.
Does Solace Guaranteed messaging survive a broker restart?
Yes. Guaranteed messages are spooled to persistent storage, kept across event broker restarts, and delivered in publish order.