Locators: a simple service discovery library

Locators is a library to wrap various service discovery mechanisms into a unified promise based format. At it's core, it revolves around the concept of Locations which are represented as a simple object with a host and port property, and Locators, which are a function which return a Promise<Location>. Additionally, a locator implementation may optionally expose a LocatorEmitter instead of a Locator, which is a Locator + EventEmitter.

There are currently three different locators, packaged in this library: SimpleLocator, RequestLocator and ZookeeperLocator. All locators expose a single static method getLocatorFactory which may or may not take a configuration depending on the locator type, and returns a function which can produce Locators that can be resolved in the normal promise flow.

SimpleLocator

SimpleLocator takes no configuration to it's factory creation method. It's locator takes a list of known servers and randomly returns one of the servers.

example

import { SimpleLocator } from 'locators';
const simpleLocator = SimpleLocator.getLocatorFactory();
const locator = simpleLocator({
    resource: "localhost;koalastothemax.com:80",
    defaultPort: 8181
});
locator.then((location) => {
    console.log(location); // either { host: "localhost", port: 8181 } or { host: "koalastothemax.com", port: "80" }
});

RequestLocator

RequestLocator takes no configuration to it's factory creation method. It's locator method takes a remote http/https endpoint that returns a list of servers, and retrieves a list of servers and randomly returns one of the servers, and optionally a data extractor to properly translate the response into a Location. The RequestLocator has a default data extractor which expects the server to respond with an object which has a servers property which is an array of objects which have address and port properties, since it was originally built to work with the exhibitor api call exhibitor/v1/cluster/listdocs.

example

import { RequestLocator } from 'locators'
const requestLocator = RequestLocator.getLocatorFactory();
const locator = requestLocator({
    url: "http://www.test-endpoint.com:8080/list" // returns {"blah": [{"address": "localhost", "port": 8080}, {"address": "localhost", "port": 1234}]}
    dataExtractor: (data) => {
        location = JSON.parse(data).blah[1],
        return {
            host: location.address,
            port: location.port
        }
});
locator.then((location) => { 
    console.log(location); //returns { host: 'localhost', port: 1234 }
})

ZookeeperLocator

ZookeeperLocator uses zookeeper to find other services. For maximum dog-fooding, it's factory creation method takes a Locator for the zookeeper cluster. If you are using exhibitor, you can make use of its list api with the RequestLocator, or if the servers exist in dns, a list of hosts, or ip addresses, then with a SimpleLocator. It is built on top of node-zookeeper-client

example

import { SimpleLocator, ZookeeperLocator } from 'locators';
const simpleLocator = SimpleLocator.getLocatorFactory();
const zookeeperLocator = Zookeeper.getLocatorFactory({
    serverLocator: simpleLocator('localhost:2181')
    path: '/discovery'
    locatorTimeout: 2000
});
const locator = zookeeperLocator('my:service');
locator.then((location) => {
    console.log(location); // returns host and port from zookeeper localhost:2181 
});

The ZookeeperLocator implements the LocatorEmitter pattern

  ZK_LOCATOR_ERROR: "failed to find zookeeper",
  CONNECTED: "connected",
  DISCONNECTED: "disconnected",
  STATE_CHANGE: "state",
  EXPIRED: "expired",
  CONNECTING: "connecting",
  FAILED_TO_GET_CHILDREN: "failed to get child list",
  EMPTY_POOL: "child pool is empty",
  FAILED_TO_GET_CHILD_INFO: "failed to get individual child's info",
  NEW_POOL: "got a new child pool",
  CHILDREN_CHANGED: "child list has changed",
  PATH_FOUND: "zookeeper path is found",
  PATH_NOT_FOUND: "zookeeper path is not found"

The codes emitted are exported as EXCEPTION_CODE from the library, i.e.

import { EXCEPTION_CODE } from 'locators';
import { ZookeeperLocator } from 'locators';

const zookeeperLocator = Zookeeper.getLocatorFactory({
    serverLocator: simpleLocatorFactory()('localhost:2181')
    path: '/discovery'
    locatorTimeout: 2000
});
const locator = zookeeperLocator('my:service');
locator.then((location) => {
    console.log(location); // returns host and port from zookeeper localhost:2181 
});
locator.on(EXCEPTION_CODE.EXPIRED, () => {
    console.log('expired');
});

shorthand syntax

A shorthand syntax exists for using the locators provide by this library for legacy reasons, and can be used as such.

SimpleLocator

example

simpleLocatorFactory = require('locators').simple

locator = simpleLocatorFactory()({
    resource: "localhost;koalastothemax.com:80"
    defaultPort: 8181
})
locator.then((location) -> 
    console.log location #either { host: "localhost", port: 8181 } or { host: "koalastothemax.com", port: "80" }
)

RequestLocator

example

requestLocatorFactory = require('locators').request

locator = requestLocatorFactory()({
    url: "http://www.test-endpoint.com:8080/list" # returns {"blah": [{"address": "localhost", "port": 8080}, {"address": "localhost", "port": 1234}]}
    dataExtractor: (data) ->
        location = JSON.parse(data).blah[1]
        return {
            host: location.address
            port: location.port
        }
})
locator.then((location) -> 
    console.log location #returns { host: 'localhost', port: 1234 }
)

ZookeeperLocator

example

zookeeperLocatorFactory = require('locators').zookeeper

zookeeperLocator = zookeeperLocatorFactory({
    serverLocator: simpleLocatorFactory()('localhost:2181')
    path: '/discovery'
    locatorTimeout: 2000
})
myServiceLocator = zookeeperLocator('my:service')
myServiceLocator.then((location) ->
    console.log location #returns host and port from zookeeper localhost:2181 
)

Development

npm run build to compile.

The ZookeeperLocator test currently require a local installation of zookeeper to run successfully, specified by the zkServerCommandPath variable which is defined near the beginning of the tests.