Overview

This library simply replicates a MongoDb collection from an existing deployment into memory, keeping it up to date via a change stream, and providing basic querying and change-alerting functionality.

Note that the target MongoDb deployment must be a replica set (though a cluster of size 1 is fine.) Change streams aren't provided for non-replica-set MongoDb deployments. Couldn't tell you why.

const CollectionMirror = require('@shieldsbetter/local-mongodb-collection-mirror');
const { MongoClient } = require('mongodb');

async main() {
    const client = MongoClient.connect(process.env.MONGO_CONNECT_STRING);
    const fooMirror = new CollectionMirror(
            client.db('fooDb').collection('fooCollection'));
    
    await fooMirror.waitUntilReady();
    
    fooMirror.on('changed',
            (mirror, key) => console.log(`Document "${key}" changed.`));
    
    // No need for `await`!  This is a synchronous scan of docs in memory.
    for (let doc of fooMirror.find({ bar: { $in: ['plugh', 'waldo'] } }) {
        console.log(doc);
    }
}

main().catch(e => console.log(e));

Motivation

Frequently, in light production situations a cloud configuration service would be beneficial, but a full deployment of consul, etcd or their ilk would be overkill.

Since a database service is likely already available, it can serve as a clearinghouse for shared configuration data.

API

Methods

<constructor>(collection)

Takes a collection from the MongoDb node drive (as derived perhaps from mongoClient.db('fooDb').collection('fooCollection')) and immediately kicks off the process of populating an in-memory mirror of the documents in that collection.

async close()

Blocks until the underlying ChangeStream is closed and associated resources are released. Future calls to query methods will throw MirrorClosed errors.

find(query)

Takes a MongoDb-like query (specifically, something acceptable to sift) and returns an array containing the subset of documents that match. If no documents match, an empty array will be returned.

Returned documents will be copies and modifying them will not modify underlying data.

get(key)

Takes a MongoDb document id and returns the corresponding document. If a document with that key does not exist, throws a NoSuchKey error.

has(key)

Takes a MongoDb document id and returns true iff a document with that key is present in the mirror.

isClosed()

Returns true iff the mirror has been gracefully closed.

isReady()

Returns true iff the following three criteria are met:

1. The mirror has not yet been closed
2. The underlying collection has not become invalidated
3. The mirror has completed the initial process of seeding itself with documents currently present in the collection and begun watching the collection for future changes

isValid()

Returns true iff the underlying collection has not become invalidated. Collections can become invalidated by being dropped, renamed, or being part of a database that is dropped.

async waitUntilReady()

Blocks until isReady() would return true. (Or, if isReady() would already return true, returns immediately.)

Events

invalidated

Fired when the underlying collection becomes invalidated, which happens when it is dropped, renamed, or is part of a database that is dropped. Following this event, calls to query methods will throw CollectionNoLongerValid errors.

Callback will receive the mirror instance as its first argument and the MongoDb change stream event as its second argument, which may contain details about why the collection has become invalidated.

ready

Fired when the mirror transitions from its initial process of seeding itself with documents currently present in the collection into the steady state of watching the change stream for additional changes.

Callback will receive the mirror instance as its first and only argument.

changed

Fired when a document is locally inserted, removed, or modified.

Callback will receive the mirror instance as its first argument and the MongoDb document id of the changed document as its second argument.