Initial implementation

This is an initial implementation of ShareDB's [`MilestoneDB`][1]. The
bare bones of `MongoMilestoneDB` connection logic are based on the
[`sharedb-mongo`][2] database adapter.

This adapter is a slight departure from other ShareDB code, as it is
written in ES6. This is because this code is server-side-only, so we
don't need to support legacy browsers and all LTS Node versions support
ES6. The move should ease maintenance and understanding of asynchronous
code through the use of `Promise`s, which the [`mongodb`][3] library
supports out of the box.

This library notably uses v2 of `mongodb` instead of the newer v3. This
is to stay consistent with `sharedb-mongo`, so in theory the same config
(or style of config) can be used in both `sharedb-mongo` and in
`sharedb-milestone-mongo`.

We also introduce usage of [`eslint`][4] instead of [`jshint`][5], which
is used in other projects. `eslint` supports a much, much wider range of
style rules and helps to maintain a uniform coding style.

[1]: share/sharedb#236
[2]: share/sharedb-mongo
[3]: mongodb.github.io/node-mongodb-native
[4]: https://eslint.org/
[5]: http://jshint.com/

.eslintrc

@@ -0,0 +1,14 @@
+{
+  "extends": "./eslint.base.yaml",
+  "parserOptions": {
+    "sourceType": "module",
+    "ecmaFeatures": {
+      "modules": true
+    }
+  },
+  "env": {
+    "mocha": true,
+    "es6": true,
+    "node": true
+  }
+}

.gitignore

@@ -5,6 +5,9 @@ npm-debug.log*
 yarn-debug.log*
 yarn-error.log*
 
+# IDEs
+.vscode
+
 # Runtime data
 pids
 *.pid

.travis.yml

@@ -0,0 +1,27 @@
+language: node_js
+
+node_js:
+  - "6"
+  - "8"
+  - "10"
+
+services:
+  - docker
+
+env:
+  - MONGODB_VERSION="2.6"
+  - MONGODB_VERSION="3.6"
+  - MONGODB_VERSION="4.0"
+
+before_install:
+  - docker run -d -p 127.0.0.1:27017:27017 mongo:$MONGODB_VERSION
+
+before_script:
+  - until nc -z localhost 27017; do echo Waiting for MongoDB; sleep 1; done
+
+script:
+  - npm run test-cover
+
+# Send coverage data to Coveralls
+after_script:
+  - cat ./coverage/lcov.info | ./node_modules/coveralls/bin/coveralls.js

README.md

@@ -1,2 +1,128 @@
 # sharedb-milestone-mongo
-A Mongo implementation of the ShareDB Milestone Database API
+
+[![Build Status](https://travis-ci.com/share/sharedb-milestone-mongo.svg?branch=master)](https://travis-ci.com/share/sharedb-milestone-mongo.svg?branch=master)
+[![Coverage Status](https://coveralls.io/repos/github/share/sharedb-milestone-mongo/badge.svg?branch=master)](https://coveralls.io/github/share/sharedb-milestone-mongo?branch=master)
+
+MongoDB milestone snapshot database adapter for [`sharedb`][1]. Milestone snapshots can be used to speed up the results
+of ShareDB's `connection.fetchSnapshot` method by providing points in time on top of which a smaller number of ops can
+be applied to reach the requested version.
+
+Milestone snapshots will be stored in a collection called `m_COLLECTION` where `COLLECTION` is the name of your ShareDB
+collection.
+
+## Quick start
+
+```javascript
+const MongoMilestoneDB = require('sharedb-milestone-mongo');
+const ShareDB = require('sharedb');
+
+const milestoneDb = new MongoMilestoneDB('mongodb://localhost:27017/test');
+const shareDb = new ShareDB({ milestoneDb: milestoneDb });
+```
+
+## Configuration
+
+### Mongo
+
+The underlying Mongo database can be configured in a number of ways. This library wraps v2 of the [`mongodb`][2]
+library, so any configuration that can be used there can be used in this library.
+
+Mongo can be configured simply using a connection string and any desired [options][3]:
+
+```javascript
+const milestoneDb = new MongoMilestoneDB('mongodb://localhost:27017/test', { loggerLevel: 'debug' });
+```
+
+It can also be configured with a callback that provides an instance of a Mongo [`Db` object][4]:
+
+```javascript
+const mongodb = require('mongodb');
+
+const milestoneDb = new MongoMilestoneDB((callback) => {
+  mongodb.connect('mongodb://localhost:27017/test', callback);
+});
+```
+
+The Mongo connection string or function can be used as the first argument of the constructor, or as part of an options
+object:
+
+```javascript
+const milestoneDb = new MongoMilestoneDB({
+  mongo: 'mongodb://localhost:27017/test',
+  loggerLevel: 'debug',
+});
+```
+
+### Milestone snapshot saving
+
+#### Intervals
+
+By default, ShareDB will save a milestone snapshot with a given frequency. This library defaults to an interval of
+1,000, saving milestones when the 1,000th, 2,000th, etc. versions are committed. That default interval can be
+configured:
+
+```javascript
+const milestoneDb = new MongoMilestoneDB({
+  mongo: 'mongodb://localhost:27017/test',
+  interval: 500,
+});
+```
+
+#### Complex saving logic
+
+If you need more complex saving logic (eg different intervals depending on collection), this can be achieved using
+ShareDB middleware:
+
+```javascript
+const milestoneDb = new MongoMilestoneDB('mongodb://localhost:27017/test');
+const shareDb = new ShareDB({ milestoneDb: milestoneDb });
+
+shareDb.use('commit', (request, callback) => {
+  switch (request.collection) {
+    case 'foo':
+      // Save every 100 versions for collection 'foo'
+      request.saveMilestoneSnapshot = request.snapshot.v % 100 === 0;
+      break;
+    case 'bar':
+    case 'baz':
+      // Save every 500 versions for collections 'bar' and 'baz'
+      request.saveMilestoneSnapshot = request.snapshot.v % 500 === 0;
+      break;
+    default:
+      // Don't save any milestones for collections not named here.
+      // IMPORTANT: We have to set this to false to actively disable milestones
+      // If left to null, then the default interval will still apply
+      request.saveMilestoneSnapshot = false;
+  }
+
+  callback();
+});
+```
+
+Note that the default value of `request.saveMilestoneSnapshot` is `null`. If left to this value, it will use the default
+interval logic. If you want to actively disable snapshots, you must make sure to set it to `false`.
+
+### Indexing
+
+By default, indexing is enabled on the milestone collections in order to speed up fetching. This can have an adverse
+impact on performance if being enabled on an existing collection that is missing the index. The indexing can be
+disabled:
+
+```javascript
+const milestoneDb = new MongoMilestoneDB({
+  mongo: 'mongodb://localhost:27017/test',
+  disableIndexCreation: true,
+});
+```
+
+## Error codes
+
+### 5100 -- Internal error - DB
+
+* 5101 -- Mongo closed
+
+
+[1]: https://github.com/share/sharedb
+[2]: https://mongodb.github.io/node-mongodb-native/
+[3]: http://mongodb.github.io/node-mongodb-native/2.2/api/MongoClient.html#connect
+[4]: http://mongodb.github.io/node-mongodb-native/2.2/api/Db.html