After a few months of collaborative work across several members in our Tellago crew, Hermes is finally here. Hermes, also known as the great messenger of god in the Greek mythology, was the name we gave to this new open source alternative for doing durable pub/sub messaging over http.
There is no doubt that MSMQ has been the predominant technology for doing Pub/Sub within the Microsoft stack given the reliability it provides as a temporary storage for messages. However, the main problem of using a queue technology like that is that you sacrifice interoperability for better reliability, and interoperability usually matters a lot in the enterprise. How many times you have heard stories of people doing crazy things in the enterprise with queue connectors or similar technologies for decoupling systems implemented in different platform stacks.
Http is something universal accepted in all the existing platforms and makes the things really simple in that sense. Almost all the existing programming languages in the world provide libraries for consuming Http endpoints or Web Apis with a few lines of code.
There are some Pub/Sub alternatives using Http and RESTful services in the cloud like the AppFabric Service Bus or PubNub, but Hermes is also suited to be used within the boundaries of a single organization. Every time you want to decouple two systems or provide valuable business events to other systems you don't know upfront, Hermes becomes a great candidate for implementing that kind of scenario.
As I said before, you might sacrifice reliability in the sense that you are using Http for publishing the events into the Hermes repository, and a http channel must be available at that time, but that’s an issue on the publisher side only. The subscription side is implemented through simple http polling with Atom, so the subscriber can read the messages when an Http channel is available. Also, all the transformation responsibilities are moved to the subscriber side simplifying a lot the core messaging engine in Hermes.
Hermes uses MongoDB as the backend storage for the messages, making extremely easy to store, partition and index high volumes of messages for a large number of topics and subscriptions.
You can read more details about the underline architecture in the announcement made by Jesus.