Database / CouchDB Interview Questions
1. 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...
2. What data model does CouchDB use and how is a document structured?
CouchDB uses a document data model : all data is stored as discrete JSON objects grouped into databases. There is no enforced schema — documents in the same database can have entirely different fields. Every CouchDB document has two mandatory system fields: _id — the unique primary key. If omitte...
3. What is the CouchDB HTTP REST API and how do you perform basic CRUD operations?
CouchDB maps every database operation to a standard HTTP method and URL. The server root is typically http://localhost:5984 . No driver installation is required — curl or any HTTP client works directly. # Create a database curl -X PUT http://admin:pass@localhost:5984/inventory # {"ok":true} # Cre...
4. What is MVCC (Multi-Version Concurrency Control) in CouchDB and how does it handle write conflicts?
MVCC in CouchDB means every write produces a new immutable version of the document instead of modifying data in place. Readers always see a consistent snapshot from the moment they start reading; no read locks are acquired. The append-only B-tree storage engine keeps old revisions on disk until c...
5. What is the _rev field in CouchDB and why is it required for updates and deletes?
The _rev field is CouchDB's revision token — a unique identifier for a specific version of a document. Its format is {generation}-{hash} where generation is a monotonically increasing integer (starting at 1) and hash is an MD5 of the document body. Example: "1-967a00dff5e02add41819138abb3284d" . ...
6. What is the CouchDB storage engine (B-tree) and how does its append-only write work?
CouchDB stores each database as a single file on disk structured around an append-only B-tree . There are multiple B-trees per database file: one for documents and one for each view index. Every write — new document, updated revision, or index update — is appended to the end of the file. The exis...
7. What is database compaction in CouchDB and when should you run it?
Database compaction rewrites a CouchDB database file from scratch, retaining only the current (winning) revision of each document and discarding all stale revisions and orphaned B-tree nodes. Because CouchDB uses an append-only storage engine, every update grows the file. A database with millions...
8. What are CouchDB attachments and when would you use them?
Attachments in CouchDB are binary blobs stored directly alongside a document under the reserved _attachments key. Each attachment has a filename, a MIME content type, byte length, and an MD5 digest. They are stored in the same database file as the document but transferred separately — a GET on th...
9. What is the difference between CouchDB and Couchbase?
CouchDB and Couchbase are two distinct products. CouchDB is an Apache project. Couchbase emerged from a 2011 merger of CouchDB and Membase (a Memcached-compatible store). They diverged sharply afterward and now target different use cases with different architectures. Apache CouchDB vs Couchbase S...
10. 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...
11. What are CouchDB design documents and what do they contain?
Design documents are special CouchDB documents whose IDs begin with _design/ . They live in the same database as regular documents but hold server-side JavaScript code that CouchDB's query server executes. Updating a design document invalidates and rebuilds all its associated indexes. A design do...
12. What are MapReduce views in CouchDB and how do you define a map function?
MapReduce views are CouchDB's primary indexing mechanism. A view has a map phase (mandatory) and an optional reduce phase. The map function is a JavaScript function that CouchDB runs against every document in the database. For each document it emits zero or more key-value pairs. CouchDB stores th...
13. How does the reduce function work in CouchDB views and what are the built-in reduce functions?
The reduce function in a CouchDB MapReduce view aggregates the values emitted by the map function within a key range. CouchDB implements reduce using a rereduce mechanism: values are first reduced in small groups (reduce pass), then those partial results are reduced again (rereduce pass) until a ...
14. What are view indexes in CouchDB and how are they built and updated, including stale options?
A view index in CouchDB is a persistent B-tree file on disk that stores all the key-value pairs emitted by a view's map function across all documents in the database. It is stored separately from the main database file (with a .view extension in the views/ directory). The index is sorted by emitt...
15. What is the Mango query language in CouchDB and how does it differ from MapReduce views?
Mango is CouchDB's declarative, MongoDB-inspired query language introduced in CouchDB 2.0. Instead of writing JavaScript map functions, you POST a JSON selector document to the _find endpoint. CouchDB evaluates the selector against a Mango index (or falls back to a full scan) and returns matching...
16. How do you create and use a Mango index in CouchDB (json and text indexes)?
Mango supports two index types: json indexes (B-tree, for equality and range queries on specific fields) and text indexes (full-text Lucene-backed, for free-text search on string fields). Both are created via POST to /_index . # Create a JSON index on status + created_at for the orders collection...
17. What are the query operators available in the Mango selector syntax?
Mango selectors are JSON objects where each key is a document field or a Mango operator. Operators begin with $ . They fall into four groups: comparison, logical, element, and array operators. POST /mydb/_find { "selector": { "$and": [ { "type": { "$eq": "product" } }, { "price": { "$gte": 10, "$...
18. What is the _all_docs endpoint in CouchDB and how does it differ from a custom view?
The _all_docs endpoint is a built-in view that CouchDB automatically maintains for every database. It returns all non-deleted documents sorted by their _id (ascending by default). Internally it is backed by the same document B-tree that stores the documents themselves, so it is always up to date ...
19. How do you paginate results in CouchDB views using startkey, endkey, and skip/limit?
CouchDB views are sorted B-trees, so efficient pagination uses key-based cursoring rather than offset-based skipping. Two approaches exist: offset pagination (simpler but slow at large offsets) and key-based pagination (efficient at any depth). # ── Approach 1: Offset-based (avoid for deep pages)...
20. What is a list function in CouchDB and when would you use it?
A list function is a server-side JavaScript function stored in a design document under the lists key. It acts as a streaming transformer for view query results — instead of returning raw JSON rows, it lets you produce any output format (HTML, XML, CSV, plain text) directly from CouchDB without an...
21. How does CouchDB replication work and what is the replication protocol?
CouchDB replication is a document-level sync protocol that copies documents from a source database to a target database using standard HTTP. Either or both of source and target can be local or remote CouchDB instances. Replication is initiated by posting a replication document to the _replicator ...
22. What is the difference between one-shot and continuous replication in CouchDB?
CouchDB supports two replication modes: one-shot (the default) and continuous . The mode is set by the continuous boolean in the replication document. One-shot replication syncs all documents changed since the last checkpoint, then completes. The replication job disappears once finished. It is ap...
23. What is filtered replication in CouchDB and how do you implement it?
Filtered replication allows you to replicate only a subset of documents from a source database, rather than copying every document. This reduces bandwidth, storage on the target, and replication lag. There are two ways to filter: using a filter function (server-side JavaScript) or using a Mango s...
24. 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 sha...
25. How does CouchDB cluster sharding work — what are the Q, n, r, and w parameters?
In a CouchDB cluster, each database is divided into Q shards (also called range partitions). The key space of document IDs is divided into Q equally-sized ranges using consistent hashing. Each shard is stored as an independent database file on a node. Each shard has n replicas — copies stored on ...
26. What is the _node and _cluster_setup API used for in CouchDB clustering?
The _node and _cluster_setup APIs are the two primary endpoints for managing a CouchDB cluster's topology. They are distinct in scope: _node operates on individual node configuration, while _cluster_setup orchestrates the multi-step process of forming or extending a cluster. The _node API ( /_nod...
27. How does CouchDB handle replication conflicts and what strategies exist to resolve them?
A replication conflict in CouchDB occurs when two nodes have independently updated the same document (same _id ) and neither update knew about the other. This is the normal result of multi-master or offline-sync workflows — it is not an error, it is an expected state that the application must han...
28. What is the CouchDB winning revision algorithm for conflict resolution?
When CouchDB has two or more conflicting revisions of the same document, it must deterministically pick one as the winning revision — the one returned by a normal GET request without ?conflicts=true . The algorithm is deterministic so that all cluster nodes independently arrive at the same winner...
29. What is PouchDB and how does it enable offline-first applications with CouchDB sync?
PouchDB is an open-source JavaScript database that runs entirely inside the browser (using IndexedDB or WebSQL as the local storage backend) or in Node.js (using LevelDB). It implements the CouchDB replication protocol, which means it can sync bidirectionally with any CouchDB-compatible server — ...
30. What is Couchbase Sync Gateway and how does it relate to CouchDB's replication model?
Couchbase Sync Gateway is the replication middleware layer in the Couchbase mobile stack. It sits between mobile clients running Couchbase Lite (the embedded mobile database) and a Couchbase Server cluster, handling authentication, authorization, and document routing. Historically it implemented ...
31. How does CouchDB implement authentication — cookie auth, JWT, and proxy auth?
CouchDB supports four authentication mechanisms, configurable simultaneously. Each request is checked against the enabled handlers in order. 1. Basic Authentication — HTTP Basic Auth over HTTPS. Credentials are sent with every request. Simple to implement but requires HTTPS in production to avoid...
32. What is CouchDB's permission model — admin party, database admins, and database readers?
CouchDB has a two-tier permission hierarchy: server-level admins and database-level members . Understanding each tier and the dangerous default state ("admin party") is essential before deploying any CouchDB instance. Admin Party — when CouchDB is first installed, there are no server admins confi...
33. How do you implement document-level security in CouchDB using validate_doc_update functions?
The validate_doc_update (VDU) function is a JavaScript function stored in a design document that CouchDB calls before every document write to that database. If the function throws an error, the write is rejected with the specified HTTP status and message. This is the primary mechanism for enforci...
34. What is a CouchDB _security object and how do you configure roles and members?
The _security object is a special document stored at /db/_security . It defines which users and roles can act as admins (write design documents, change security) or members (read and write regular documents) for that specific database. Every database has one. { "admins": { "names": ["alice", "bob...
35. How do you enable SSL/TLS in CouchDB and what configuration is required?
CouchDB has a built-in HTTPS listener that can be enabled by adding a [ssl] section to the CouchDB configuration ( local.ini or local.d/*.ini ). No reverse proxy is required for basic TLS, though using nginx in front is common in production for certificate management and connection pooling. [ssl]...
36. How do you monitor CouchDB performance using the _stats and _active_tasks endpoints?
CouchDB exposes two key monitoring endpoints out of the box — /_stats and /_active_tasks — which together give a real-time snapshot of server health and ongoing operations. GET /_stats returns a JSON object of cumulative performance counters organized by category. Key metrics to watch: httpd.requ...
37. What are the key CouchDB configuration parameters to tune for production (max_dbs_open, os_process_limit, etc.)?
CouchDB's default configuration targets a single-developer workstation. Production deployments require tuning several parameters across different configuration sections: Key CouchDB Production Configuration Parameters Section / Key Default What it controls [couchdb] max_dbs_open 500 Maximum numbe...
38. How does CouchDB handle large document sets — what are the performance trade-offs of large vs many small documents?
CouchDB does not have a hard document size limit (the default max_document_size is 4GB), but the performance trade-offs between storing data as a small number of large documents versus many small documents are significant. Large documents (e.g., one document per entity with thousands of nested it...
39. What is the CouchDB _changes feed and how do you use it for real-time event streaming?
The _changes feed is CouchDB's built-in event stream. It reports every document change (create, update, delete) in a database as a sequence of events, each with a sequence number ( seq ), document ID ( id ), list of changed revisions ( changes ), and optionally the full document body. It is the m...
40. What are CouchDB update handlers and how do they differ from direct PUT operations?
Update handlers are server-side JavaScript functions stored in a design document under the updates key. They allow you to perform document transformations atomically on the server without a client round-trip — the client sends a POST request, and the update handler reads the current document, app...
41. What are CouchDB show functions and when were they deprecated?
Show functions are server-side JavaScript functions stored in a design document under the shows key. They transform a single document into any output format (HTML, XML, plain text) directly from CouchDB, without requiring a separate application server. When a client calls GET /db/_design/ddoc/_sh...
42. How do you back up and restore a CouchDB database?
CouchDB does not have a dedicated backup command like mysqldump . The recommended backup approaches depend on your deployment type and RPO requirements: 1. Replication-based backup (recommended for live systems) — replicate the database to a dedicated backup CouchDB instance (local or remote). Be...
43. How does CouchDB compare to MongoDB for document storage use cases?
Both CouchDB and MongoDB are JSON document databases, but they make fundamentally different architectural choices that determine where each excels. CouchDB vs MongoDB for Document Storage Aspect CouchDB MongoDB Query interface HTTP REST — any HTTP client; Mango JSON queries MongoDB wire protocol;...
44. What are common CouchDB anti-patterns and how do you avoid them?
Several CouchDB anti-patterns cause performance degradation, excessive conflicts, or runaway disk usage. Understanding them helps you design applications that work with CouchDB's architecture rather than against it. High-frequency counter documents — updating a single document hundreds of times p...
45. How do you migrate data between CouchDB versions or instances?
CouchDB provides several migration paths depending on whether you are upgrading in place, moving to a new cluster, or changing data structure during migration. 1. Replication-based migration (zero-downtime, recommended) # Step 1: Replicate from old instance to new curl -X POST http://admin:pass@n...
