Problem Statement
What are Change Streams in MongoDB and what are practical use cases for them?
Explanation
Change Streams allow applications to access real-time data changes without the complexity of tailing the oplog. They provide a high-level API for subscribing to data changes across collections, databases, or entire clusters. Change Streams are built on top of the oplog and are available on replica sets and sharded clusters starting from MongoDB 3.6.
Change Streams capture insert, update, replace, and delete operations as they occur. When you open a change stream, you receive a cursor that emits change events as documents are modified. Each event includes the operation type, the affected document or document key, and the timestamp.
Practical use cases are numerous. First, cache invalidation where you update application caches when data changes in MongoDB. When a product price changes, you can immediately invalidate the cached price and fetch the new value. This keeps caches synchronized without polling.
Second, real-time notifications where you push updates to users. For example, in a social media application, notify users when someone likes their post or sends a message. Change Streams detect these inserts and trigger notifications immediately.
Third, data synchronization across systems where you replicate changes to other databases, search engines, or data warehouses. When an order is placed in MongoDB, you can push the order to an analytics database or update a search index in Elasticsearch.
Fourth, audit logging where you record all changes to sensitive collections for compliance. Change Streams capture who changed what and when, creating an audit trail. Fifth, microservices communication where services react to data changes in other services, enabling event-driven architectures.
Change Streams support filtering using aggregation pipeline syntax, allowing you to subscribe only to relevant changes. You can resume from a specific point using resume tokens, ensuring no events are missed during application restarts.