What are change streams in MongoDB?
Why Interviewers Ask This
This is a classic screening question for MongoDB roles. Hiring managers ask it early in interviews to gauge your baseline understanding and determine if you can communicate technical concepts clearly.
Answer
Change streams allow applications to access real-time data changes in MongoDB collections, databases, or entire deployments without the complexity of tailing the oplog. Introduced in MongoDB 3.6, they provide a higher-level, resumable, filtered stream of change events. Opening a change stream: const changeStream = db.orders.watch(); changeStream.on("change", (change) => { console.log(change); });. Change event structure: each event includes: _id (resume token — allows resuming from a specific point after reconnection), operationType (insert/update/delete/replace/drop/rename), fullDocument (the affected document), updateDescription (what fields changed for update operations), ns (namespace: database + collection). Filtering: pass an aggregation pipeline to filter events: db.orders.watch([{ $match: { operationType: "insert" } }]). Resuming: store the last processed _id (resume token); on reconnect: db.orders.watch([], { resumeAfter: lastToken }) — continues from where you left off. Requirements: change streams require a replica set or sharded cluster (not standalone). Use cases: real-time notifications, syncing data to cache/search index, event-driven microservices (trigger workflow on data change), audit logging, replicating data to other systems (CDC). Fullness: the fullDocument: "updateLookup" option returns the full document after an update (not just the changed fields) — note this is slightly stale if concurrent updates occur.
Pro Tip
Before answering, structure your response: one-line definition → real-world analogy → concrete example from a project. This makes even complex MongoDB answers easy to follow.