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mongosync

Overview

mongosync is MongoDB's official continuous-replication tool for migrating data between MongoDB clusters — notably from a self-managed MongoDB Community Edition deployment into MongoDB Atlas, or between Atlas clusters. It streams data from source to target while the source keeps serving production traffic, so a cutover can happen with a short (or zero) downtime window instead of requiring a full offline export/import. Official docs: https://www.mongodb.com/docs/mongosync/current/reference/mongosync/.

Role in managed-service migrations

In the five-phase managed-service migration playbook, mongosync is the tool of the Migrate phase: the vendor-provided continuous-replication primitive that handles the terabyte-scale bulk transition without an application-layer copy. Paired with MongoDB's encryption functionality it provides in-transit encryption for regulated-industry moves.

The source-cluster-stays-authoritative property is what lets the next-phase Validate step run automated parity scripts without having already cut over irreversibly — making the migration match patterns/nondisruptive-migration.

Named capabilities

  • Continuous replication — source keeps receiving traffic while target catches up and stays in sync.
  • Sharded → sharded moves — BharatPE's 3 × (1-primary + 2-secondary) topology was migrated to Atlas via mongosync.
  • Encryption in-transit — explicitly called out as the mechanism for protecting "sensitive financial information" during a fintech migration.
  • CLI-driven — runs as an external process between source and target.

What the case study does not report: mongosync throughput, lag bounds during sync, specific write-path semantics during the cutover window, or resume-from-checkpoint behaviour. These are documented in MongoDB's own docs but not published by BharatPE.

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