Add usage example

[curran]

No description provided.

[curran]

Here's a good reference example https://github.com/startupjs/startupjs/blob/e9ff2af2a4ff64e3628976845dd444e287fccd3e/packages/backend/index.js

[curran]

Here's a complete starter.

import redis from 'redis';
import redisPubSub from 'sharedb-redis-pubsub';

import ShareDB from 'sharedb'; 
import ShareDBMingoMemory from 'sharedb-mingo-memory';
import ShareDBMongo from 'sharedb-mongo';

// Singletons.
let shareDB;
let connection;

// Environment variables.
const mongoURI = process.env.MONGO_URI; 
const redisHost = process.env.REDIS_HOST;

export const getShareDB = () => {
  if (!shareDB) {
    const options = {
      db: mongoURI ? new ShareDBMongo(mongoURI) : new ShareDBMingoMemory(),
    };
    if (process.env.VIZHUB_REDIS_HOST) {
      options.pubsub = redisPubSub({
        client: redis.createClient({ host: redisHost }),
      });
    }
    shareDB = ShareDB(options);
  }
  return shareDB;
};

export const getConnection = () => {
  if (!connection) {
    connection = getShareDB().connect();
  }
  return connection;
};