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Obtaining of statistics about a replica-set oplog of a MongoDB instance.

If you have ever wondered why is the duration of your oplog so short, look no further. The tool queries the oplog of a MongoDB instance and prints an overview about which collections and operation types have the greatest impact on its duration. For each collection and operation type, it provides the number of operations, their total size, and their share among all the operations in the oplog.

In a greater detail, each MongoDB operation (e.g. insert or updated) is represented by a BSON document. Whenever you run a MongoDB operation, it is recorded in the oplog so that all the secondary nodes in your replicas set can reflect the operation in their storage. The bigger the size of the oplog, the longer your replicas can be down without a need for a complete resync. Sometimes, you may have a large oplog, but its duration is not as long as you would expect. In such cases, you can utilize the fact that the oplog is a MongoDB collection. The mongodb-oplog-stats tool queries the documents in the oplog and gives you its overview that you can use to see which collections and operation types are shortening your oplog duration the most.

For very basic info about the oplog, you can also use the rs.printReplicationInfo() function from the mongo shell:

> rs.printReplicationInfo()
configured oplog size:   1024MB
log length start to end: 65422secs (18.22hrs)
oplog first event time:  Mon Jun 23 2019 17:47:18 GMT+0200 (CEST)
oplog last event time:   Tue Jun 24 2019 11:57:40 GMT+0200 (CEST)
now:                     Thu Jun 26 2019 14:24:39 GMT+0200 (CEST)


A recent-enough version of Rust. The tool is written in the 2018 edition of Rust.

Build and run

git clone
cd mongodb-oplog-stats
cargo run --release --host --username myuser

For more information about the available parameters, run

cargo run --release -- --help

Connection and authentication parameters are identical to those used by standard MongoDB tools.


Example output

| Entry                                | Documents | Total size | Share (%) |
| moviedb.movies:i                     |      2634 |  675.12 MB |     71.04 |
| store.books:i                        |       196 |  146.23 MB |     17.68 |
| moviedb.actors:u                     |        35 |   10.22 MB |      3.64 |
| blog.comments:i                      |     10598 |    9.07 MB |      2.34 |
| moviedb.movies:i                     |        18 |    7.87 MB |      2.03 |
| store.books:u                        |        81 |    7.59 MB |      1.96 |
|                      |     14459 |    5.07 MB |      1.31 |
| moviedb.trailers:d                   |       205 |    3.17 MB |      0.82 |
| store.employees:d                    |         1 |      170 B |    < 0.01 |
| blog.articles:i                      |         1 |      158 B |    < 0.01 |

Output format

Each entry has the following format:



  • database is the name of the database,
  • collection is the name of the collection,
  • op is one of the following abbreviations:
    • i for inserts,
    • u for updates,
    • d for deletes,
    • c for commands that affect databases at a high level,
    • db which announces the presence of a database,
    • n for no-ops, used for operations which do not result in a change in the stored data. If you have a lot of these operations in your oplog, take a look here and here.

As for the other columns, Documents is the number of documents corresponding to the operations, Total size is the total size of the BSON representation of those documents, and Share gives you the percentage of the entry with regards to other entries in the oplog.


Here are few useful commands to use during development:

  • Check for errors without code generation (faster than cargo run):
    $ cargo check
  • Format sources files:
    $ rustup component add rustfmt
    $ cargo fmt
  • Run lints to catch common mistakes and improve the code:
    $ rustup component add clippy
    $ cargo clippy
  • Add a new dependency:
    $ cargo install cargo-edit
    $ cargo add PACKAGE_NAME
  • List outdated dependencies (-R is for the direct ones):
    $ cargo install cargo-outdated
    $ cargo outdated -R


Copyright (c) 2020 Avast Software, licensed under the MIT license. See the LICENSE file for more details.


A tool for obtaining statistics about a MongoDB replica-set oplog








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