You might remember locking from your undergrad systems programming class. Locks help when multiple threads that can be interrupted at any time access a shared resource. In distributed programming (like building a Node.js server that talks to a database) you have a similar problem: parallel operations can mutate documents in the database in conflicting ways. In particular, enforcing uniqueness, such as making sure only one user has a given email, is tricky.

For example, consider mongoose-unique-validator, a mongoose plugin which has a well-known race condition. The goal of mongoose-unique-validator is simple enough: enforce that a given path, say a user's email property, is unique across the collection. The way this plugin enforces uniqueness is simple, do a find() for documents that have the same email and throw an error if there are any. The below insertUser function is a drastically simplified example of mongoose-unique-validator's approach to enforcing uniqueness:

const { MongoClient } = require('mongodb');
const assert = require('assert');

async function insertUser(db, user) {
  const count = await db.collection('User').count({ email: user.email });
  assert.equal(count, 0, `There is already a user with email ${user.email}`);

  return db.collection('User').insertOne(user);
}

async function run() {
  const db = await MongoClient.connect('mongodb://localhost:27017');
  await db.dropDatabase();

  await insertUser(db, { email: 'test@test.co' });
  // Throws an error, because there is already a user with this email
  await insertUser(db, { email: 'test@test.co' });
}

run().catch(error => console.error(error));

In most practical cases this works, however, there's always that nasty edge case of multiple requests in parallel (imagine a user double-clicking on a button that triggers a request). I don't mean to disparage mongoose-unique-validator, it is a useful plugin in many cases, and I'm well aware that enabling developers to provide a pristine user experience often requires trade-offs. Time and skill are tragically scarce resources, so think carefully about what you really need before choosing a tool.

const { MongoClient } = require('mongodb');
const assert = require('assert');

async function insertUser(db, user) {
  const count = await db.collection('User').count({ email: user.email });
  assert.equal(count, 0, `There is already a user with email ${user.email}`);

  // Let's assume we do some other async stuff
  await new Promise(resolve => setTimeout(() => resolve(), 100));

  return db.collection('User').insertOne(user);
}

async function run() {
  const db = await MongoClient.connect('mongodb://localhost:27017');
  await db.dropDatabase();

  // Will probably not throw an error (not strictly guaranteed),
  // because the `insertUser()` calls run in parallel, and both
  // `count()` checks will pass before either starts to insert
  await Promise.all([
    insertUser(db, { email: 'test@test.co' }),
    insertUser(db, { email: 'test@test.co' })
  ]);
}

run().catch(error => console.error(error));

Introducing Partial Unique Indexes

You can solve the unique email problem with a MongoDB unique index. MongoDB then enforces internally that the user's email is unique, so you'll never get duplicates.

However, unique constraints can be more sophisticated. For example, how about if some users don't have email addresses, but those that do should be unique? MongoDB has sparse indexes for this case. A sparse unique index will enforce email uniqueness for documents where the email property exists (including null values).

const { MongoClient } = require('mongodb');
const assert = require('assert');

async function insertUser(db, user) {
  return db.collection('User').insertOne(user);
}

async function run() {
  const db = await MongoClient.connect('mongodb://localhost:27017');
  await db.dropDatabase();

  // Create a sparse unique index
  await db.collection('User').createIndex({ email: 1 }, { sparse: true, unique: true });

  await Promise.all([
    insertUser(db, {}),
    insertUser(db, {})
  ]);

  // Will always throw an error
  await Promise.all([
    insertUser(db, { email: 'test@test.co' }),
    insertUser(db, { email: 'test@test.co' })
  ]);
}

run().catch(error => console.error(error));

You can get even more fancy with unique constraints in MongoDB 3.2. For example, Booster has a restriction that a given vehicle can have at most one active fuel request. MongoDB 3.2 introduced the notion of partial indexes. A partial index is a much more general sparse index, it can be defined for all documents that match a given query, subject to restrictions I'll discuss in the last section. Here's how you create a partial index to enforce vehicleId uniqueness for requests that have status 'ACTIVE'.

const { MongoClient } = require('mongodb');

async function run() {
  const db = await MongoClient.connect('mongodb://localhost:27017');
  await db.dropDatabase();

  await db.collection('FuelRequest').createIndex({ vehicleId: 1 }, {
    unique: true,
    partialFilterExpression: {
      status: { $eq: 'ACTIVE' }
    }
  });

  // Succeeds, because uniqueness only enforced for 'ACTIVE' requests
  await Promise.all([
    db.collection('FuelRequest').insertOne({ vehicleId: 1, status: 'COMPLETE' }),
    db.collection('FuelRequest').insertOne({ vehicleId: 1, status: 'COMPLETE' })
  ]);

  // Throws an error!
  await Promise.all([
    db.collection('FuelRequest').insertOne({ vehicleId: 1, status: 'ACTIVE' }),
    db.collection('FuelRequest').insertOne({ vehicleId: 1, status: 'ACTIVE' })
  ]);
}

run().catch(error => console.error(error));

A partial unique index with $exists: true is equivalent to a sparse unique index.

Limitations

Internally, Booster does not actually use partial unique indexes to enforce the one active request per vehicle constraint. We instead use a distributed locking mechanism around creating a request to prevent race conditions. Why? Because there's more request statuses under the hood that count as 'active' for the purposes of this constraint, and the query passed to partialFilterExpression cannot contain $in. In other words, the below code will throw an error:

const { MongoClient } = require('mongodb');

async function run() {
  const db = await MongoClient.connect('mongodb://localhost:27017');
  await db.dropDatabase();

  // MongoError: unsupported expression in partial index: status $in [ "ACTIVE" "ANOTHER_STATUS" ]
  await db.collection('FuelRequest').createIndex({ vehicleId: 1 }, {
    unique: true,
    partialFilterExpression: {
      status: { $in: ['ACTIVE', 'ANOTHER_STATUS'] }
    }
  });
}

run().catch(error => console.error(error));

Unfortunately, partial indexes only support an extremely limited subset of MongoDB query operators. As of MongoDB 3.4, you can only use:

  • $exists: true
  • $eq
  • $gte, $gt, $lt, $lte
  • $type
  • $and of the above operators

This means no $in, no $regex, and in general no way of matching multiple values. It also means no $nin, $ne, or $exists: false. Here's a few things that you can't do with partial unique indexes:

  • Enforce that email is unique with the exception of administrators
  • Enforce that a property is unique if it exists and is not null
  • Enforce that a property is unique unless an isArchived property is true, unless you disallow isArchived being not defined

Moving On

MongoDB partial indexes give you flexibility in specifying unique constraints, but they're very limited. Tools like mongoose-unique-validator give you more flexibility, but sacrifice strict uniqueness guarantees. In order to enforce sophisticated unique constraints, you need some notion of distributed locking, which I'll talk about in a future blog post.

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