Mongoose released v5.2.0 to support MongoDB 4.0 and transactions this week. In addition to transaction support, Mongoose 5.2.0 introduces several important new features. The syncIndexes() function represents Mongoose's first steps toward providing a real index management solution, something that Mongoose historically has not done.

Motivation: Indexes in Mongoose

MongoDB indexes, like relational database indexes, are a construct for speeding up queries. If you want to find a document in the User collection where name = 'Val' , you would normally have to look through every document in the User collection. If you have millions of documents in the User collection, searching through every one can be too slow. To avoid a full collection scan, you might store a hash map that maps names to users with that name. At a high level, an index is a sophisticated implementation of this sort of hash map.

Mongoose supports 2 syntaxes for declaring an index on a user's name.

const userSchema = new Schema({ name: { type: String , index: true } }); const userSchema = new Schema({ name: String }); userSchema.index({ name: 1 });

In Mongoose, you declare indexes in your schemas. When you compile a model from your schema, Mongoose will build indexes for you in the background. You can use Model.init() to get back a promise that fulfills when your index build is done. Under the hood, when you call mongoose.model() , Mongoose calls the Model.ensureIndexes() function, which calls the MongoDB driver's ensureIndex() function for every index in your schema. This ensures that every index in your schema exists in MongoDB.

However, building an index may be very slow for collections with millions of documents, so Mongoose supports an autoIndex option that tells Mongoose to not build indexes by default.

Unfortunately, there is one major gap with the above index functionality: how do you remove or change an index? Suppose you decide you no longer need an index on a user's name . You would remove the index from your schema definition, but that would not drop the index from your production database. Before 5.2.0, Mongoose had no mechanism for dropping indexes that were not defined in your schema.

const userSchema = new Schema({ name: String });

Introducing Model.syncIndexes()

The Model.syncIndexes() function ensures that the indexes defined in your model's schema line up with the indexes in your MongoDB collection. Here's what syncIndexes() does:

Get all the indexes defined on the collection in MongoDB. For each index in MongoDB, check to see if an index with the same keys and options is defined in your schema. If the index is not in the schema, drop the index. Call Model.ensureIndexes() to make sure every index in the schema exists in MongoDB.

Mongoose does not call syncIndexes() for you, you're responsible for calling syncIndexes() on your own. There are several reasons for this, most notably that syncIndexes() doesn't do any sort of distributed locking. If you have multiple servers that call syncIndexes() when they start, you might get errors due to trying to drop an index that no longer exists.

If you have only one process that uses Mongoose and aren't concerned about the potential performance overhead, you can do await Model.syncIndexes() on server startup. For more distributed applications, you're better off running Model.syncIndexes() in a separate process.

Moving On