json-rpc-core

json-rpc-core

Transport agnostic JSON RPC message handling API meant for streams and promises.

Install

npm install --save json-rpc-core

Usage

This library is meant to be used with some transport layer, it can be any type, Redis, TCP, UDP, SocketIO, etc. All this library does is create a node stream compatible interface to pipe in incoming messages and pipe out outgoing messages. As long as your server and client classes are syncronous or return promises then you can wrap them with JSON-RPC-Core add a transport layer and you have a working RPC interface.

//Simple example using an event emitter as our transport layer
var RPC = require('json-rpc-core')
var Emitter = require('events')

//stub for an actual class with functions
//these functions can return promises as well.
//any possible errors will be caught and returned to the caller.
var rpcMethods = {
  echo:function(msg){
    return msg
  }
}

//pretend this emitter is a socket or some transport layer
var emitter = new Emitter()

//this will wire up the emitter into the rpc-core
function makeEmitterRPC(localid,remoteid,methods){

  //instantiate the rpc stream. A node compatible stream, with extra functions
  //thanks to the highland library. See http://highlandjs.org.
  var rpc = RPC(methods)
  
  //emitter will fire on the localid when it receives a message
  emitter.on(localid,function(msg){
    //write that message to the rpc stream
    rpc.write(msg)
  })

  //rpc will fire when its emitting a message to the remote, such as a response
  //or a notification
  rpc.on('data',function(msg){
    //emit that message to the remote id
    emitter.emit(remoteid,msg)
  })

  return rpc
}

//these rpc clients can talk to each other using the rpcMethods class
var client = makeEmitterRPC('client','server',rpcMethods)
var server = makeEmitterRPC('server','client',rpcMethods)

//the client calls the echo function and the server responds
client.call('echo','message to echo').then(function(result){
  //result == 'message to echo'
})

//client sends a 'knock knock' event to the server. Notifications dont expect a response.
client.notify('knock knock')

//discover your remote server commands
client.discover().then(function(result){
  //result == ['echo']
})

//server would be able to call these same functions to the client, since the client
//was created with the rpcMethods class as well. 

API

Initialization

Create the rpc core stream.

  var RPC = require('json-rpc-core')

  var rpc = RPC(methods,timeoutMS)

Parameters

• methods (optional) - methods which are available to be called over RPC. Functions must be syncronous or use promises.
• timeoutMS (optional) - defaults to 10000, 10 seconds. Milliseconds that messages should timeout when waiting for response. Can be disabled if set to 0, but this is not recommended as requests will pile up waiting for responses from remote.

Returns

A highland stream. This stream conforms to node streams, but adds additional useful functions.

Call

Call a remote function by name. Allows any number of parameters. Returns a promise which can resolve or reject. Nested functions are now supported using lodash's "get" syntax. Seperate nested paths as an array or using "dot" notation. Be aware that nested calls get serialized to a string with '.'. Call can also access non functions, and will try to return them as is. This will allow you to remotely access to the prototype or other parts of your class.

  rpc.call(remoteFunction,param1,param2, etc...).then(function(result){
    //result is the result of the remote function call
  })

  //ways to call nested functions, these will call a function on the object
  //{
  //  deeply:{
  //    nested:{
  //      method:function(){ return true}
  //    }
  //  }
  //}
  rpc.call(['deeply','nested','method']).then(function(result)...
  rpc.call('deeply.nested.method').then(function(result)...


  //call will work on non functions, and will just try to return them as is
  rpc.call('plainObject').then(function(result)...
  //can also use nested notation

Parameters

Parameters are variable, they will just be passed through to remote function

• remoteFunction (required) - the name of the remote function you wish to call
• params_n (optional) - N number of parameters which the remote functions requires

Returns

Promise with the result from the remote call

Notify

Notify the remote, essentially cause a specific event to fire. Certain events names are reserved since they conflict with the node stream api: 'data', 'close', 'drain', 'error', 'finish', 'pipe', 'unpipe', 'end', 'readable'. The library does not prevent you from emitting these events but the resulting behavior is undefined.

  rpc.notify(eventName,param1,param2, etc...)

Parameters

Parameters are variable, they will just be passed through to remote function

• eventName (required) - the name of the event you want to emit on the remote
• params_n (optional) - N number of parameters which get passed into the event callback.

Returns

Nothing

Discover

Utility function to discover remote method names. This is implemented as a custom RPC message "rpc_discover". The JSON RPC protocol allows for custom methods signified by the "rpc" prefix. If a conflict is found with a local method the local method will override the extension. Discover will not return nested methods.

  rpc.discover().then(function(result){
    //result are the available remote methods names
  })

Returns

A promise which resolves to an array of callable remote method names. Use in conjuction with rpc.createRemoteCalls.

Create Remote Calls

Produces a new object with functions keyed by the function names you pass into it. The functions are just shortcuts to the rpc.call function. Use in conjuction with rpc.discover to create an object which mirrors the remote API calls.

  rpc.discover().then(function(methods){
    //server is an object with remote method calls
    var server = rpc.createRemoteCalls(methods)
  })

Parameters

This takes an array of strings, it just creates an object keyed by the strings

• methods (required) - an array of strings which you want to turn into function calls

Returns

An object keyed by the string names which act as a shortcut to rpc.call

Echo

Utility function which allows you to "ping" the remote with a message, like a connectivity test. This is implemented as a custom RPC message "rpc_echo". The JSON RPC protocol allows for custom methods signified by the "rpc" prefix. If a conflict is found with a local method the local method will override the extension.

  rpc.echo(message)

Parameters

Function takes single paramter

• message (optional) - a message you want echoed back to you.

Returns

A promise which resolves to your original message.

Errors

Errors can be thrown from the remote service syncronously or through an asycnronous promise. The RPC library will attempt to translate a JSON RPC error into a standard node Error with a message and stack trace and reject it through the promise. The stacktrace, if available, will be the stack from the server side error.

//the type of error thrown when the remote call throws an error
rpc.call('functionThatThrowsError').catch(function(err){
  //err is a javascript Error class
  err == {
    message:'your error message',
    stack:'stacktrace...'
  }
})

//the type of error when no such method exists on remote server
rpc.call('functionThatDoesNotExist').catch(function(err){
  //err is a javascript Error class
  err == {
    message:'The JSON-RPC method does not exist, or is an invalid one.',
    stack:null
  }
})

You should always add a catch block at some point to your function calls.
The most recent version of json-rpc-core adds stack trace data when available.

Previous Versions

Anyone using a version before 1.2 should upgrade, The previous message stream implementation messages only allowed messages to resolve in order. This would be an issue if there are major delays in responding to requests, causing essentially a traffic jam in serving requests.