jsonhub - A Pattern Matching Pub/Sub

A publish/subscribe message queue with JSON pattern matching based subscriptions.

Any JSON value is a valid message and clients can subscribe to an exchange using a description of the JSON messages they wish to match against. The client will then obtain messages that match the pattern. Subscription patterns are themselves JSON and can contain "match values" to specify more complex matching such as greater-than and less-than matches, geospatial matches and case insensitive matches.

Install

As jsonhub is a Go application, downloading and building it is pretty easy if you have the Go tools installed and setup. Just run

go get github.com/aidenbell/jsonhub/jsonhubd

and then

]$ cd $GOPATH/src/github.com/aidenbell/jsonhub/jsonhubd
]$ $GOPATH/bin/jsonhubd
2015/02/04 23:21:40 JSON Message Queue
2015/02/04 23:21:40 Running server on port 9977

You can then find a Javascript test client at http://localhost:9977/client/ , POSTing JSON and listening for messages happens on http://localhost:9977/ .

Example Messages and Subscriptions

Example Published Message

Imagine an RFID office entry system for a shared office building. Every time a person uses their entry card, a message detailing the access is sent to the message queue with the details of the person and the area of the building their card gives them access to.

{
  "name" : "Dave",
  "age" : 22,
  "occupation": "Programmer",
  "hardware" : [ "monitor", "workstation", "laptop", "chair" ],
  "employer" : {
    "name": "Big Co Inc",
    "address1" : "123 BigCo Lane",
    "address2" : "Businesston",
    "postcode" : "ABC XYZ"
  },
  "access_area": {
    "type": "Polygon",
    "coordinates": [
      [ [100.0, 0.0], [101.0, 0.0], [101.0, 1.0], [100.0, 1.0], [100.0, 0.0] ]
    ]
  },
  "door": "stair-22",
  "granted" : true
}

Sub 1: Value Comparison

An client could subscribe using the following pattern and obtain the details of any programmer aged 22 accessing the building

{
  "age" : 22,
  "occupation" : "Programmer"
}

Sub 2: Using Matcher Values

We can also use "matchers" to find all programmers older than 22 entering the building

{
  "age" : {
      "__match__" : "greater-than",
      "value" : 22
    },
  "occupation" : {
    "__match__" : "case-insensitive",
    "value" : "programmer"
  }
}

Sub 3: Match Everything

You can match against all messages using an empty subscription

{}

This is because subscriptions match unless a portion of the pattern does not match against the message. Each attribute of a subscription object must match the message. If you provide no attributes, there are no match constraints to fail. This allows you to only list your constraints when matching without having to describe an entire object.

Matchers

Aside from basic value comparison, subscriptions can contain special "matcher" objects that describe more complex matches in JSON format. The following matchers are supported (todo: link these to .md files)

• Case insensitive string matching
• GeoJSON based 'within'

Server Sent Events

Currently matching messages are polled via Server Sent Events. Websocket support and other protocols are planned.

Future

Future and needed features include

• Websocket support
• Multiple named exchanges
• Configurable message distribution for equivalent patterns (round-robin, random)
• Negations in patterns
• Various matchers
• Support for sending arrays of messages and providing arrays of subscriptions

Disclaimer

This is a little side project I use to learn Go. The source probably isn't very nice or idiomatic, but as I learn I refactor and improve it. It is also missing some important features like negation, tests, documentation etc. Pull requests welcome.