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Database / CouchDB Interview Questions

What are the CAP theorem trade-offs for CouchDB — is it CP or AP?

CouchDB is an AP system — it prioritizes Availability and Partition tolerance over strict Consistency. When a network partition occurs, CouchDB nodes on either side continue accepting reads and writes rather than refusing requests to maintain linearizability. The result is that two nodes can hold diverged versions of the same document (called open conflicts) until replication heals the partition.

CouchDB's consistency model is eventual consistency: after a partition heals and replication runs, all nodes converge. The conflict resolution mechanism — the deterministic winning-revision algorithm plus application-level merge — is how convergence is achieved without a global coordinator.

The AP versus CP distinction surfaces in these specific scenarios:

  • Multi-master replication — both nodes independently accept writes to the same document. The replication protocol syncs them and surfaces the conflict for application resolution.
  • CouchDB 3.x cluster quorum settings — the write quorum w and read quorum r default to a majority of n replicas. Raising both to n makes the cluster refuse writes when a node is down, shifting behavior toward CP at the cost of availability.

This AP design is the reason CouchDB excels in offline-first and mobile applications via PouchDB: the mobile client writes locally (always available) and syncs to the server when connectivity returns, with conflicts resolved deterministically.

Under the CAP theorem, which guarantees does CouchDB prioritize in a multi-master distributed setup?
How can you shift a CouchDB 3.x cluster toward CP behavior during node failures?

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What is Apache CouchDB and what makes it different from relational databases? What data model does CouchDB use and how is a document structured? What is the CouchDB HTTP REST API and how do you perform basic CRUD operations? What is MVCC (Multi-Version Concurrency Control) in CouchDB and how does it handle write conflicts? What is the _rev field in CouchDB and why is it required for updates and deletes? What is the CouchDB storage engine (B-tree) and how does its append-only write work? What is database compaction in CouchDB and when should you run it? What are CouchDB attachments and when would you use them? What is the difference between CouchDB and Couchbase? What are the CAP theorem trade-offs for CouchDB — is it CP or AP? What are CouchDB design documents and what do they contain? What are MapReduce views in CouchDB and how do you define a map function? How does the reduce function work in CouchDB views and what are the built-in reduce functions? What are view indexes in CouchDB and how are they built and updated, including stale options? What is the Mango query language in CouchDB and how does it differ from MapReduce views? How do you create and use a Mango index in CouchDB (json and text indexes)? What are the query operators available in the Mango selector syntax? What is the _all_docs endpoint in CouchDB and how does it differ from a custom view? How do you paginate results in CouchDB views using startkey, endkey, and skip/limit? What is a list function in CouchDB and when would you use it? How does CouchDB replication work and what is the replication protocol? What is the difference between one-shot and continuous replication in CouchDB? What is filtered replication in CouchDB and how do you implement it? What is CouchDB Cluster mode (CouchDB 2.x+) and how does it differ from single-node CouchDB 1.x? How does CouchDB cluster sharding work — what are the Q, n, r, and w parameters? What is the _node and _cluster_setup API used for in CouchDB clustering? How does CouchDB handle replication conflicts and what strategies exist to resolve them? What is the CouchDB winning revision algorithm for conflict resolution? What is PouchDB and how does it enable offline-first applications with CouchDB sync? What is Couchbase Sync Gateway and how does it relate to CouchDB's replication model? How does CouchDB implement authentication — cookie auth, JWT, and proxy auth? What is CouchDB's permission model — admin party, database admins, and database readers? How do you implement document-level security in CouchDB using validate_doc_update functions? What is a CouchDB _security object and how do you configure roles and members? How do you enable SSL/TLS in CouchDB and what configuration is required? How do you monitor CouchDB performance using the _stats and _active_tasks endpoints? What are the key CouchDB configuration parameters to tune for production (max_dbs_open, os_process_limit, etc.)? How does CouchDB handle large document sets — what are the performance trade-offs of large vs many small documents? What is the CouchDB _changes feed and how do you use it for real-time event streaming? What are CouchDB update handlers and how do they differ from direct PUT operations? What are CouchDB show functions and when were they deprecated? How do you back up and restore a CouchDB database? How does CouchDB compare to MongoDB for document storage use cases? What are common CouchDB anti-patterns and how do you avoid them? How do you migrate data between CouchDB versions or instances?
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