What is the Cosmos DB Change Feed and what are its main use cases?

The Change Feed is a persistent, ordered log of every insert and update that occurs in a Cosmos DB container. It records changes in the order they occur within each logical partition, and this log is available for consumption indefinitely (it is not ephemeral like a message queue). Deletes are not captured by default, but you can enable a soft-delete pattern by marking items with a deleted flag and relying on TTL to purge them.

The Change Feed exposes two primary consumption models:

• Change Feed Processor (CFP) — A library (available in all Cosmos DB SDKs) that manages distributed consumption across multiple worker instances. It uses a separate lease container to track each worker's read position per logical partition. This is the recommended approach for most applications — it handles partition splits, lease management, and worker scaling automatically.
• Azure Functions Cosmos DB Trigger — A serverless binding that invokes a function for each batch of changes. Internally it uses the Change Feed Processor library. Zero infrastructure to manage.

Common use cases:

• Real-time event streaming — Forward Cosmos DB mutations to Azure Event Hubs or Kafka for downstream analytics pipelines.
• Materialized views — Maintain a denormalized view in another container that is optimized for a different query pattern (e.g., store orders by customerId in the source, maintain a separate container keyed by productId for product-based reporting).
• Cache invalidation — When an item in Cosmos DB changes, push the updated version to Redis or a CDN edge cache.
• Auditing — Write every change to an immutable audit log container or Azure Blob storage.
• Event-driven microservices — Use Cosmos DB as an event store and the Change Feed as the outbox, replacing traditional message broker coupling.

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