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Database / CouchDB Interview Questions

What is Apache CouchDB and what makes it different from relational databases?

Apache CouchDB is an open-source NoSQL document database that stores all data as self-contained JSON documents and exposes its entire API over plain HTTP/HTTPS. No proprietary wire protocol or special client driver is required. It was created by Damien Katz, open-sourced in 2005, and graduated to an Apache Software Foundation top-level project in 2008. The 3.x line introduced a native clustered mode built on the existing Erlang OTP foundation.

Three properties set CouchDB apart from relational databases:

  • Schema-free JSON documents — there are no tables or fixed column definitions. Each document in a database can have a completely different structure, and adding a field to one document has zero effect on any other.
  • HTTP as the primary interface — every operation (CRUD, querying, replication, admin) is a plain HTTP request. You can interact with CouchDB using curl, a browser, or any HTTP library without installing a database driver.
  • Built-in, protocol-level replication — CouchDB's replication is a peer-to-peer HTTP protocol where any node can replicate to or from any other node, supporting master-master setups, offline sync, and mobile clients natively.

Relational databases enforce ACID across multi-row, multi-table transactions using row-level locks and a shared transaction log. CouchDB provides ACID at the single-document level through MVCC and an append-only B-tree engine. There are no JOINs; relationships are denormalized or referenced by document ID.

Which primary interface does CouchDB use for all database operations, including replication?
At what granularity does CouchDB guarantee ACID properties?

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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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