CODEWITHSUNDEEP
mongodb
Ian Wang
Ian Wang

Reputation: 271

what does db.oplog.rs.stats().count mean in mongodb?

In mongo local db, you can check oplog related data by using db.oplog.rs.stats()

But what does the "count" field mean? And I see it's decreasing every second in my db server.

Upvotes: 0

Views: 1959

Answers (1)

Stennie
Stennie

Reputation: 65333

The replica set oplog (oplog.rs) is a capped collection, which means it has a maximum total size for data. The underlying implementation varies by storage engine (eg WiredTiger vs MMAPv1) but the conceptual outcome is the same: capped collections make room for new documents by overwriting or expiring the oldest documents in the collection in FIFO order (First In, First Out).

But what does the "count" field mean?

As with any collection, the count information in db.collection.stats() indicates the number of documents currently in the collection.

For an explanation of collection stats output, see collStats in the MongoDB documentation.

Note: The output will vary depending on your version of MongoDB server and storage engine used.

I see it's decreasing every second in my db server.

The count of documents in the oplog will vary over time based on the size of the write operations being applied, so this is expected to fluctuate for an active deployment. For example, single field updates will generally write smaller oplog entries than full document updates. Once your oplog reaches its maximum data size, the count may also decrease as the oldest oplog documents are removed to make room for new oplog entries.

Upvotes: 1

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