What is a potential downside of using durable subscribers in JMS?

Using durable subscribers in JMS (Java Message Service) can introduce higher latency as a potential downside. This stems from the way durable subscriptions work. When a subscriber is designated as durable, it allows messages to be retained by the message broker even when the subscriber is not actively connected. As a result, the broker must manage message storage and retrieval, which can add overhead to the process of delivering messages. This management increases the time it takes for messages to move from the producer to the subscriber, particularly if the subscriber has disconnected for a period of time and the messages are queued to be sent when it reconnects.

In contrast, while durable subscribers ensure message delivery even if the subscriber is offline, this reliability comes at the cost of performance. The broker needs to maintain the state of the messages and ensure they are delivered correctly once the subscriber reconnects, which inherently requires additional processing time compared to non-durable subscriptions where messages are processed and discarded once consumed.