How can you use MongoDB's change streams for real-time data processing?
Instruction: Explain how change streams can be used in MongoDB for real-time data capture and processing.
Updated
Official answer
Change streams expose database changes on supported replica-set and sharded deployments through driver watch APIs. Choose collection, database or deployment scope and filter for relevant operations. They are not supported by an ordinary standalone server.
Persist the resume token only after the corresponding effect is durably handled. A crash between effect and checkpoint can cause replay, so the consumer needs idempotent processing or deduplication. Multiple consumers do not automatically form a work-sharing queue, and a stream does not make downstream side effects exactly once.
Resume requires sufficient retained oplog history. Monitor lag and checkpoint age; if history is lost, stop and run an explicit resnapshot/reconciliation procedure rather than silently skipping ahead. Handle invalidation, permission changes and resumable versus terminal errors according to the driver/version.
For updates, fullDocument with updateLookup can return a later current version, or no document after deletion; it is not always the exact document at the event instant. Use supported pre/post-image features when exact images are required, with their enablement and retention costs.
Bound queues and concurrency, preserve required ordering, and test duplicates, reconnects, slow consumers and history loss. Measure end-to-end processing delay and failures against the application's freshness objective; “real-time” does not imply zero latency.