MongoDb background indexing and unique index

Question

When you create an index in MongoDb. There are 2 options:

• Do foreground indexing and lock all write operations while doing so
• Do background indexing a

Answer 1

@mltsy

If, from another client, you're doing something like adding a duplicate document while the index is being built, it will insert the document without an error.

I am not sure this is correct,as Mongodb Doc described as below:

When building an index on a collection, the database that holds the collection is unavailable for read or write operations until the index build completes.

I used the mongoose to test this :

var uniqueUsernameSchema = new Schema({
    username: {
        index: { unique: true, background: false },
        type: String,
        required: true
    }
})
var U = mongoose.model('U1', uniqueUsernameSchema)
var dup = [{ username: 'Val' }, { username: 'Val' }]
U.create(dup, function (e, d) {
    console.log(e, d)
})

The unique index failed to build. This result showed the foreground option didnot block the write operation in MongoDB.