How often does MongoDB check its index 'expireAfterSeconds'?

Question

I use a single instance MongoDB locally and in a Spring Boot Repository, I annotate a document-class with this annotation:

@Indexed(name = "deleteAt", expireAfterSeconds = 0)
private LocalDateTime deleteAt;

As I read from the docs here, documents should automatically get deleted at the specified DateTime. It works mostly but I see a delay in deletion when I poll the same document frequently. Most documents were deleted immediately, yet some remain in the DB for up to 30 seconds after the specified deletion time.

That makes me wonder whether MongoDB runs a scheduler to clean up such documents and if so, how often does it run?

Answer 1

Spring Data's expireAfterSeconds attribute engages a MongoDB TTL index and the behaviour of a TTL index's expireAfterSeconds attribute is described in the docs as follows:

When you build a TTL index in the background, the TTL thread can begin deleting documents while the index is building. If you build a TTL index in the foreground, MongoDB begins removing expired documents as soon as the index finishes building.

The TTL index does not guarantee that expired data will be deleted immediately upon expiration. There may be a delay between the time a document expires and the time that MongoDB removes the document from the database.

The background task that removes expired documents runs every 60 seconds. As a result, documents may remain in a collection during the period between the expiration of the document and the running of the background task.

Because the duration of the removal operation depends on the workload of your mongod instance, expired data may exist for some time beyond the 60 second period between runs of the background task.

So, the answer is: 60 seconds though the expired data may live for longer than 60s because "the duration of the removal operation depends on the workload of your mongod instance ...".