pubsure-core

Pub? Sure!

Pubsure is a set of Clojure protocols that together form a publish-subscribe framework. Some of its properties are:

• A central directory service is used for registering to publisher sources for a specific topic. Potential subscribers use this directory to find the publishers.

• The protocols do not deal with semantics like:

  • whether all subscribed clients receive all messages, or some load-balancing or work distribution takes place;
  • Starting, stopping and communication between the components;
  • How to deal with issues like the slow joiner problem;

• The directory service API uses core.async channels to inform clients about joined and left publishers.

API

The DirectoryWriter protocol

This protocol is for adding and removing URIs as a sources of messages on the given topic.

• add-source [this topic uri] - Given a topic, this function writes the given URI to the directory service.

• remove-source [this topic uri] - Given a topic, this function removes the given URI from the directory service.

The DirectoryReader protocol

This protocol is for retrieving a one-time batch of current topic sources, or for continuesly receiving updates on joined and left sources. Clojure clients can talk to a DirectoryReader instance directly. In all other cases, the Clojure protocol needs to be exposed in some other way.

• sources [this topic] - This function returns the currently known sources (URIs) of the given topic.

• watch-sources [this topic init] [this topic init chan] - This function starts a backgrould loop which reads the directory service and puts updates about sources for the given topic on a, possibly supplied, channel. The channel is returned. Whenever a channel is closed, all watches for that channel are removed. Updates on the channel are SourceUpdate records. The init parameter is one of the following:

  • :all - all currently known sources are put on the channel.

  • :last - the last joined source is put on the channel.

  • :random - a randomly chosen source is put on the channel.

  • :none - no current source is put on the channel.

• unwatch-sources [this identifier chan] - The given channel won't receive any updates on the directory service anymore regarding the given topic. Returns the channel.

The Source protocol

This protocol is for publishing messages. Starting, stopping and accepting connections to a Source is not part of this protocol.

• publish [this topic message] - Publish a message on the given topic. The Source needs to register the topic automatically in the directory service. I.e. the Source implementation needs a DirectoryWriter instance.

• done [this topic] - Notify the Publisher that the no more messages for the given topic will be published (for now). The Publisher will be unregistered from the directory service."

Current implementations

Zookeeper

The pubsure-zk project holds a DirectoryWriter and DirectoryReader implementation using a Zookeeper cluster as its store.

Websockets

The pubsure-ws project holds a Source implementation, using Websockets publishing messages to its subscribers. It also contains a Websocket server exposing a DirectoryReader implementation. Both use the WAMP specification for communication over the Websockets.

Memory

This project also contains a memory implementation of all three protocols, and two simple subscribe / unsubscribe functions for getting messages over core.async channels. This memory implementation can be used for testing other implementations or for mocking in other systems.

TODO

• Decide whether some more semantics should be formalised, in favor of interoperabillity between various implementations. E.g., support X last messages on subscribe by default, or have all published messages be send to all subscribers at that time.

License

Copyright © 2014

Distributed under the Eclipse Public License either version 1.0 or (at your option) any later version.