Background indexing

APPLIES TO: MongoDB vCore

Background indexing is a technique that enables a database system to perform indexing operations on a collection without blocking other queries or updates. Azure Cosmos DB for MongoDB vCore accepts the background indexing request and asynchronously performs it in background.

If working with smaller tiers or workloads with higher I/O needs, it's recommended to predefine indexes on empty collections and avoid relying on background indexing.

Important

It is advised to create unique indexes on an empty collection as those are created in foreground, which results in blocking of reads and writes.

It is advised to create indexes based on query predicates beforehand, while the collection is still empty. It prevents resource contention if pushed on read-write heavy large collection.

Monitor index build

We can learn about the progress of index build using command currentOp().

db.currentOp("db_name":"<db_name>", "collection_name":"<collection_name>")

• db_name is an optional parameter.
• collection_name is optional parameter.

// Output for reviewing build status
{
inprog: [
  {
    shard: 'defaultShard',
    active: true,
    type: 'op',
    opid: '10000003049:1701252500485346',
    op_prefix: Long("10000003049"),
    currentOpTime: ISODate("2024-06-24T10:08:20.000Z"),
    secs_running: Long("2"),
    command: {createIndexes: '' },
    op: 'command',
    waitingForLock: true
  },
  {
    shard: 'defaultShard',
    active: true,
    type: 'op',
    opid: '10000003050:1701252500499914',
    op_prefix: Long("10000003050"),
    currentOpTime: ISODate("2024-06-24T10:08:20.000Z"),
    secs_running: Long("2"),
    command: {
      createIndexes: 'BRInventory', },
      indexes: [
        {
          v:2,
          key: {vendorItemId: 1, vendorId: 1, itemType: 1},
          name: 'compound_idx'
        }
      ],
      '$db': 'test'
      op: 'command',
      waitingForLock: false,
      progress: {
         blocks_done: Long("12616"),
         blocks_done: Long("1276873"),
         documents_d: Long("0"),
         documents_to: Long("0")
      },
      msg: 'Building index.Progress 0.0098803875. Waiting on op_prefix: 10000000000.'
    }
  ],
  ok: 1
}

Limitations

• Unique indexes can't be created in the background. It's best to create them on an empty collection and then load the data.
• Background indexing is performed sequentially within a single collection. However, the number of simultaneous index builds on different collections is configurable (default: 2).

Next Steps

Best practices