MDP02

This is the implementation of the MDP written in ES6 and relying on bindings for node.js and io.js to ZeroMQ

The Majordomo Protocol (MDP) defines a reliable service-oriented request-reply dialog between a set of client applications, a broker and a set of worker applications.
MDP covers presence, heartbeating, and service-oriented request-reply processing.
This protocol is ideal for a multiprocess distributed microservice architecture.

The goals of MDP are to

• Allow requests to be routed to workers on the basis of abstract service names.
• Allow both peers to detect disconnection of the other peer, through the use of heartbeating.
• Allow the broker to implement a "least recently used" pattern for task distribution to workers for a given service.
• Allow the broker to recover from dead or disconnected workers by resending requests to other workers.

This library is ideal for a distributed micro-services architecture where scalability and performance are key features.

Broker, workers and clients, don't require a specific order to start and broker and workers can be disconnected without breaking the request-response process, if the request timeout is respected.

Broker

Broker is the central unit that has the task to dispatch messages between a Client originating a request and a Worker that exposes a service and returns a response.
A broker can be bound to more than a single socket. The service can be provided by workers connected to different end points.

const broker = makeBroker({
    addresses: [
        "tcp://127.0.0.1:4242",
        "tcp://192.168.100.1:4243",
        "ipc:///tmp/mdp02-01"
    ]
});
 
broker.start();

Worker

Worker is a unit that serves synchronous request-response pairs for a given service.

A service is identified by its name, and a service may be served by more than a worker, a worker can serve only a request a time.

const makeWorker = require('mdp02').makeWorker,
    fs = require('fs');
 
const worker = makeWorker({
    serviceName: "fileTransfer",
    address: "tcp://127.0.0.1:4242"
});
 
worker.on('request', function (req) {
    let response = req.response,
        readable = fs.createReadStream(req.request.toString()); //no exception handling
    readable.pipe(response);
});
 
worker.start();

Client

Client is the request originator.
Clients can execute a request at a time, requests sent are queued into Client internal buffer for sequential execution.
On send method a stream is returned.

const makeClient = require('mdp02').makeClient,
    fs = require('fs');
 
let helloClient = makeClient({address: "tcp://127.0.0.1:4242"}),
    fileName = path.resolve('remoteFile.txt'),
    outputFile = fs.createWriteStream('path-to-local-file.txt');
 
let response = helloClient.send("fileTransfer", fileName);
 
response.pipe(outputFile);
 
outputFile.on('finish', function() {
    helloClient.stop();
    doSomethingWithLocalFile();
});

API

Broker

Broker can be accessed with: const makeBroker = require('mdp02').makeBroker;

makeBroker(opts) -> Broker is a function that returns a Broker instance. A Broker is an instance of EventEmitter.

opts::Object{bindings, workerTimeout, clientTimeout}

• bindings(required): Array of addresses to which the Broker will be bound to. ex.: ['tcp://127.0.0.1:3333', 'ipc:///tmp/mdp02-01']
• workerTimeout: (default to 5000 msec) Timeout for which a connected worker is considered stale and unregistered. A disconnect message is sent to the worker, so that the worker will try to automatically disconnect and reconnect to the worker.
• clientTimeout: (default to 5000 msec) Timeout for which a client request is considered expired and removed form the request queue.
• hbFrequence: (default to 1/5 of workerTimeout) Frequence of heart beat, used to check connections with brokers and clients ealth.

Worker Methods

• start Binds the broker to the given addresses
• stop Disconnects the broker and removes all listeners

Broker Events

name: 'error', Event: Error
Emitted on erros (soket or protocol errors)

name: 'request', Event::Object{binding, service, command}
Emitted when a request comes from a client.

• Event.binding: the binding address that received the request
• Event.service: the service name
• Event.command: the command request

name: 'worker-connect', Event: Object{binding, service}
Emitted when a worker is connected to the service pool.

• Event.binding: the binding address that received the request
• Event.service: the service name

name: 'worker-ready', Event: Object{binding, service}
Emitted when a worker is ready to serve a request.

• Event.binding: the binding address that received the request
• Event.service: the service name

name: 'worker-busy', Event: Object{binding, service}
Emitted when a worker is actually serving a request

• Event.binding: the binding address that received the request
• Event.service: the service name

name: 'worker-disconnect', Event: Object{binding, service}
Emitted when a worker is going to be disconnected from the broker

• Event.binding: the binding address that received the request
• Event.service: the service name

Worker

Worker can be accessed with: const makeWorker = require('mdp02').makeWorker;

makeWorker(opts) -> Worker is a function that returns a Worker instance. A Worker is an instance of EventEmitter.

opts::Object{address, timeout, hbFrequence}

• address(required): The address to use to connect to the Broker. ex.: 'tcp://127.0.0.1:3333'
• timeout: (default to 5000 msec) Timeout for which a broker is considered in stale. A disconnect message is sent to the broker and a restart action is performed.
• hbFrequence: (default to 1/5 of timeout) Frequence of heart beat, used to check connection/broker ealth.

Worker Methods

• start Connects the worker to the Broker
• stop Disconnects the worker from the Broker

Worker Events

name: 'error', Event: Error
Emitted on erros (soket or protocol errors).

name: 'request', Event: Object{response::Stream, request::Buffer|Buffer[]}
Emitted when a worker receives a request from a Client through a broker

• Event.response: a stream.Writable used to send the response back
• Event.request: the request message originated from the client

name: 'close-request', Event: null
Emitted when the worker has finished sennding back its reply

Client

Client can be accessed with: const makeWorker = require('mdp02').makeClient;

makeClient(opts) -> Client is a function that returns a Client instance. A Client enqueue requests for the desired services and obtain responses in the give order.

function newClient(address) {
    return makeClient({
        address: 'tcp://127.0.0.1:3333'
    });
}
 
function makePromise() {
    return new Promise(function(resolve, reject) {
        let client = newClient(),
            response = client.send("now");
        response.on('data', function(data) {
            console.log('received', data);
        });
 
        response.on('end', function(data) {
            resolve(parseInt(data, 10));
        });
        response.on('error', function(err) {
            reject(err);
        });
    });
}
 
let p1 = makePromise(),
    p2 = makePromise(),
    p3 = makePromise();
 
Promise.all([p1, p2, p3]).then(function (result) {
    let [r1, r2, r3] = result;
 
    assert(r1 < r2 < r3, `Resuts not in the correct order ${r1} < ${r2} < ${r3} ===  ${r1 < r2 < r3}`);
});

• address(required): The address to use to connect to the Broker. ex.: 'tcp://127.0.0.1:3333'
• timeout: (default to 5000 msec) Timeout for which a client is considered in stale. A retry is performed in case.
• maxRetries Number of time ca client can try a request. Must be grater than 1 (default is 3). If the number of tries exceeds this limit an error event is emitted and the enqueued requests are discarded

Client Properties

Client Methods

send(service::String, message::String|Buffer|String[]|Buffer[]) -> stream.Readable Send a request for a service and returns a stream.Readeable.

• service(required): The service name to call
• message: The message to send to the desired service. Can be a String, Buffer or an array of strings or buffers.

stop Disconnects the client from the Broker and resets the request queue

1. workers, clients and broker timeouts should match to avoid false error handling