Why MongoDB ObjectIds are only “almost” monotonic (and why it matters)

MongoDB ObjectIds are often treated as strictly increasing, but they are only “almost” monotonic. In distributed systems, this can lead to subtle bugs when relying on _id > lastProcessedId for incremental processing.

This post explains how ObjectIds are structured, why ordering can break within the same second, and how this can lead to missed events. It also covers a practical approach to handling this using partitioned ingestion and ordered processing.

Link to the blog-

https://www.planosys.com/mongodb-objectid-monotonic.html