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Database / CouchDB Interview Questions

What is CouchDB Cluster mode (CouchDB 2.x+) and how does it differ from single-node CouchDB 1.x?

CouchDB 2.0 (released 2016) absorbed the BigCouch clustering code from Cloudant and made clustered operation the default architecture. CouchDB 3.x continues this model. A CouchDB cluster consists of multiple Erlang nodes that cooperate via a distributed hash ring (using consistent hashing) to shard and replicate data automatically.

CouchDB 1.x Single Node vs 2.x/3.x Cluster
AspectCouchDB 1.x (single node)CouchDB 2.x/3.x (cluster)
Horizontal scalabilityNone — single process, single machineAdd nodes; data shards distributed automatically
Fault toleranceSingle point of failureConfigurable replica count (n) per database
Shard distributionNo sharding — one database fileConfigurable Q shards per database, each with n copies
Quorum reads/writesN/AConfigurable r (read quorum) and w (write quorum)
Admin interfaceFutonFauxton (modern React UI)
Single-node deploymentDefault modeSupported via single_node config option in 3.x
Database creationPUT /dbPUT /db?q=8&n=3 (control shards and replicas)

In a cluster, each database is split into Q shards (default 8). Each shard has n copies (default 3) stored on different nodes. When a node is added to the cluster, CouchDB uses the _cluster_setup API to join it to the ring and the rebalancing happens via standard replication. There is no external ZooKeeper or etcd dependency — cluster membership and topology are stored in the _dbs and _nodes internal databases.

What was the source of the CouchDB clustering code introduced in CouchDB 2.0?
In a CouchDB 3.x cluster, where is cluster topology (node membership and shard placement) stored?

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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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