Does Microsoft's Service Bus replicate message for every subscription in a topic?

2
votes

Does the Azure Service Bus and its on-premise version, Service Bus for Windows Server, replicate a message for every subscriber?

For example, let's say that there is a single topic with five subscribers, then is that message stored in the service bus' database five times - once for each subscriber - or is that message only stored once with business logic to determine which subscribers have read the message?

It would be nice if there is an official site and/or documentation to provide as a reference.

2
azureazureservicebusservicebus
Storage is cheap. As such, it makes sense to duplicate those messages rather than trying to create and maintain an extremely complex logic that would keep in sync information associated with multiple subscriptions. Think of DLQs, lock duration, auto-deletion on idling, message TTL v& subscription default TTL, sessions, batching, etc. - Sean Feldman

2 Answers

3
votes

The behavior the Azure Service Bus seems to be that it is keeping a copy per subscriber. I tested this by creating a topic with two subscriptions. I sent in a single message and I see that the size of the Topic in Bytes is 464 (using topic.SizeInBytes). When I receive one message of a subscription the size the drops in half to 232. I tested it with three subscriptions and same behavior occurred: 696 bytes.

Even if they aren't keeping a copy of the message per subscription they are counting the size of the message times the number of subscriptions against the maximum size of the topic, which may be what you were trying to determine.

I agree it would be nice if they documented the behavior, especially for Service Bus for Windows Server since that could affect planning for the amount of storage you need to set aside. As for the Azure Service Bus side, I'm not sure the implementation behind the scenes matters as much as knowing how it factors towards the max size of the topic.

2
votes

A subscription to a topic resembles a virtual queue that receives copies of the messages that were sent to the topic. You can optionally register filter rules for a topic on a per-subscription basis, which allows you to filter/restrict which messages to a topic are received by which topic subscriptions.

I think it copies messages. If it does not copy, it should check always, did all subscribers get the messages ? Additionally, if there is filter, it should check just these subscribers to delete message. I think, copying and applying simple consume implemation cost is less than without copying cost.

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