otter

otter

Real-time pubsub for websocket clients.

This is ALPHA software and is under active development, the API may change at any time.

Features

• Signature-based authentication
• Publish/Subscribe to arbitrarily named channels
• Presence information (i.e. named clients)
• Per-channel list of clients subscribed
• Alerts to backend on client subscribing, unsubscribing, or disconnecting
• Redis or redis cluster used as backend, multiple otter instances can run independently and be used interchangeably

Model

Otter is modeled to be a very simple, if somewhat restrictive communication layer between a backend application and a large set of clients. The backend application is completely arbitrary and must be written by you. When the backend publishes to a channel all clients on that channel will receive the publish. When a client publishes to a channel, ONLY backend applications subscribed to that channel will receive it.

The interaction on the client-side is fairly straightforward:

• (Possibly optional, depending on config) Authenticate connection with signature previously retrieved from backend. Authentication also includes presence information.
• Subscribe to desired channels.
• Read publishes from these channels.
• Publish to any channel to communicate with backend. A channel does not need to be subscribed to to be published to.

The interaction for the backend is fairly similar:

• Authenticate to prove that connection is backend.
• Subscribe to desired channels.
• Receive publishes from clients on these channels.
• Send publishes to clients on any channel. A channel does not need to be subscribed to to be published to.
• Provide authentication signatures to clients out-of-band, if necessary.

Authentication

Connections to otter can optionally identify themselves using a "presence" string, which is a string containing any arbitrary text that the connection wants. This string, if given, must be accompanied by a signature. Signatures are generated by the backend application and subsequently checked by otter when it sees them, using the secret that both the backend and otter share.

Presence strings are always optional.

A signature of some presence string arbitrary_string is generated like so:

// hex encoded result of
SHA256HMAC(current_timestamp + "_" + arbitrary_string, secret)

where current_timestamp is the unix timestamp in string form.

For backend applications which are connecting to otter, the presence string must be the string "backend".

Subscribing

Channels are subscribed to by making a websocket connection to an endpoint like so:

GET ws://otterhost/subs/<channel1>,<channel2>?presence=arbitrary&sig=sig

Once subscribed otter will push publishes directed at any of the channels to the connection. Remember that clients only receive publishes from backend applications, and vice-versa. Publishes look like this:

{
    "type":"pub",
    "channel":"channel name",
    "message":{"foo":"bar"},
    "connection":{
        "id":"connection id",
        "presence":"some string"
    }
}

The connection field gives the connection ID and the presence information of the publishing client (if they did sent any, otherwise the field is omitted). If the client is receiving a publish it can assume it came from a backend application, and vice-versa.

Additionally, there are two special push messages a backend application can receive:

{
    "type":"sub",
    "channel":"channel name",
    "connection":{
        "id":"connection id",
        "presence":"some string"
    }
}

and

{
    "type":"unsub",
    "channel":"channel name",
    "connection":{
        "id":"connection id",
        "presence":"some string"
    }
}

These indicate when a client has subscribed or unsubscribed from a channel the backend application has subscribed to. A client is considered unsubscribed when its connection is closed. Backend application connections do not generate sub and unsub messages to other backend applications.

Publishing

Publishes are accomplished by POSTing to a channel's (or multiple channels') endpoint:

POST http://otterhost/subs/<channel1>,<channel2>?presence=arbitrary&sig=sig

{"foo":"bar"}

The POST body can be any arbitrary json, and will appear as the message field in the publishes that subscribed clients receive.

Listing

It's possible for backend (and only backend!) applications to retrieve a list of currently subscribed clients for one or more channels:

GET http://otterhost/subs/<channel1>,<channel2>/subbed?presence=backend&sig=sig

Which will return a json object like so:

{
    "conns":[
        {"id":"adfasdfasdf", "presence":"maybe something"},
        {"id":"adfasdjkljs", "presence":"maybe something else"},
        {"id":"adsfjakdsf"}
    ]
}

If more than one channel is given, the returned set of connection objects will be the union of all the subbed connections for those two channels.