CouchDB Interview Questions and Answers

1. What is CouchDB?

   • Answer: CouchDB is a NoSQL, document-oriented database. It stores data in JSON documents and uses HTTP as its API. It's known for its simplicity, scalability, and ease of use.

2. What is a document in CouchDB?

   • Answer: A document in CouchDB is a JSON object that contains key-value pairs. Each document is uniquely identified by its ID.

3. How does CouchDB handle data replication?

   • Answer: CouchDB uses a pull replication model. A database can be configured to replicate from another database, pulling changes from the source.

4. Explain the concept of views in CouchDB.

   • Answer: Views in CouchDB are pre-defined queries that provide indexed access to your data. They are defined using MapReduce functions and allow for efficient querying of large datasets.

5. What are MapReduce functions in CouchDB?

   • Answer: MapReduce functions are used to create views. The map function processes each document and emits key-value pairs. The reduce function aggregates the emitted values.

6. What is a design document in CouchDB?

   • Answer: A design document stores views, shows, lists, and updates. It's a special type of document used to organize and manage query functions.

7. How does CouchDB handle concurrency?

   • Answer: CouchDB uses a multi-version concurrency control (MVCC) system. This allows multiple users to update documents concurrently without data loss or corruption.

8. Explain the concept of revisions in CouchDB.

   • Answer: Each document in CouchDB maintains a history of its revisions. This allows for easy tracking of changes and rollback capabilities.

9. What are attachments in CouchDB?

   • Answer: Attachments are binary files that can be associated with documents. They are stored within the document itself.

10. How does CouchDB handle data consistency?

    • Answer: CouchDB ensures eventual consistency. While data may temporarily be inconsistent across replicas, it eventually converges to a consistent state.

11. What is Fauxton?

    • Answer: Fauxton is the default web-based administration interface for CouchDB.

12. What are some common use cases for CouchDB?

    • Answer: Common use cases include content management systems, real-time collaboration tools, mobile applications, and applications requiring offline capabilities.

13. How does CouchDB handle indexing?

    • Answer: CouchDB uses inverted indexes to enable efficient querying. These indexes are automatically generated based on the map functions of views.

14. What are some advantages of using CouchDB?

    • Answer: Advantages include ease of use, scalability, built-in replication, and a simple HTTP-based API.

15. What are some limitations of CouchDB?

    • Answer: Limitations include the lack of advanced querying capabilities compared to relational databases and potential performance bottlenecks with complex queries.

16. Explain the concept of security in CouchDB.

    • Answer: CouchDB offers various security features including user authentication, authorization, and SSL encryption.

17. How can you perform backups and restores in CouchDB?

    • Answer: CouchDB can be backed up using various methods, including copying the database files or using replication to a separate database.

18. What are some tools that can be used with CouchDB?

    • Answer: Tools include Fauxton (web UI), Futon (older web UI), and various command-line tools and client libraries.

19. How does CouchDB handle schema design?

    • Answer: CouchDB is schema-less, meaning documents don't need to adhere to a predefined structure. However, a well-defined schema is often recommended for consistency.

20. Explain the difference between a PUT and a POST request in CouchDB.

    • Answer: PUT is used to create or update a document with a specified ID. POST is used to create a new document with an automatically generated ID.

21. What is the role of _design documents in CouchDB?

    • Answer: _design documents contain views, shows, lists, and update handlers, defining how the data in the database can be queried and manipulated.

22. How do you handle conflicts in CouchDB?

    • Answer: Conflicts arise when multiple updates occur to the same document without proper coordination. CouchDB highlights conflicts, and developers need to resolve them manually or through custom conflict resolution functions.

23. Describe the process of setting up a CouchDB cluster.

    • Answer: Setting up a CouchDB cluster involves deploying multiple CouchDB nodes and configuring them to replicate data among themselves to achieve high availability and scalability.

24. Explain how CouchDB handles document updates.

    • Answer: CouchDB tracks document revisions. Updates create new revisions, preserving the history of changes. This allows for conflict resolution and rollback capabilities.

25. What are some common performance tuning techniques for CouchDB?

    • Answer: Techniques include optimizing views, using appropriate indexing strategies, ensuring efficient data modeling, and scaling the cluster.

26. How do you query data in CouchDB?

    • Answer: Data is primarily queried using views defined in design documents. These views utilize MapReduce functions to index and filter data efficiently.

27. What is the purpose of the `_users` database in CouchDB?

    • Answer: The `_users` database is a built-in database for managing users and their authentication credentials.

28. How can you ensure data integrity in CouchDB?

    • Answer: Data integrity is maintained through revision tracking, conflict resolution mechanisms, and appropriate application-level validation.

29. Explain the concept of replication in CouchDB and its types.

    • Answer: Replication allows data synchronization between databases. Pull replication involves a database pulling changes from another, while push replication pushes changes to another database.

30. What is a "show" function in CouchDB?

    • Answer: A "show" function in a design document is used to format and present a document in a specific way, often for display in a web application.

31. What is a "list" function in CouchDB?

    • Answer: A "list" function processes the results of a view and transforms them into a custom format before returning them.

32. What is an "update" function in CouchDB?

    • Answer: An "update" function defines how documents are modified. It's used for custom document update logic.

33. How do you handle large attachments in CouchDB?

    • Answer: For large attachments, consider using external storage services like Amazon S3 and referencing them in the document, rather than storing them directly in the database.

34. What are some strategies for optimizing view performance in CouchDB?

    • Answer: Strategies include careful selection of keys in the map function, reducing the amount of data emitted, and using efficient reduce functions.

35. How does CouchDB handle data validation?

    • Answer: Data validation can be implemented using update handlers, which allow you to perform custom validation logic before a document is updated.

36. Explain the importance of document IDs in CouchDB.

    • Answer: Document IDs uniquely identify documents within a database. They are used to retrieve and update specific documents.

37. What is the role of the `_rev` field in a CouchDB document?

    • Answer: The `_rev` field stores the revision ID of the document, tracking its changes over time.

38. How can you use CouchDB for offline applications?

    • Answer: CouchDB's replication capabilities enable offline functionality. Data can be replicated locally, allowing for updates even without network connectivity.

39. What are some alternatives to CouchDB?

    • Answer: Alternatives include MongoDB, Cassandra, and other NoSQL databases, as well as cloud-based document databases.

40. How do you troubleshoot common CouchDB errors?

    • Answer: Troubleshooting involves checking logs, examining the database's status, and verifying network connectivity. Using tools like Fauxton can help diagnose issues.

41. Explain the concept of eventual consistency in CouchDB.

    • Answer: Eventual consistency means that data will eventually be consistent across all replicas, but there may be temporary inconsistencies during replication.

42. How does CouchDB handle deletes?

    • Answer: Deletes in CouchDB are marked as special revisions. The document remains in the database, but is marked as deleted.

43. What are some best practices for designing CouchDB databases?

    • Answer: Best practices include careful schema design (even though it's schema-less), efficient view design, and considering scaling needs.

44. Explain the difference between a view and an index in CouchDB.

    • Answer: In CouchDB, a view is a pre-defined query that uses a map function to create an index. The index itself is an internal structure used to efficiently access data based on the view's definition.

45. How can you secure a CouchDB instance?

    • Answer: Security measures include enabling SSL/TLS encryption, using strong passwords, configuring user roles and permissions, and restricting network access.

46. What is the purpose of the `_changes` feed in CouchDB?

    • Answer: The `_changes` feed provides a continuous stream of events indicating changes in the database, enabling real-time updates and replication.

47. How do you monitor the performance of a CouchDB instance?

    • Answer: Monitoring involves using tools and metrics provided by CouchDB to track CPU usage, memory consumption, disk I/O, and replication performance.

48. Explain the concept of continuous replication in CouchDB.

    • Answer: Continuous replication involves constantly replicating changes between databases as they occur, ensuring near real-time data synchronization.

49. How do you manage users and permissions in CouchDB?

    • Answer: Users and permissions are managed using the `_users` database and roles, allowing for fine-grained control over database access.

50. What is the role of the `_local` documents in CouchDB?

    • Answer: `_local` documents are specific to a single database and are not replicated. They are often used for configuration or metadata within a database.

51. How does CouchDB handle different data types?

    • Answer: CouchDB stores data in JSON format, supporting various data types like strings, numbers, booleans, arrays, and nested objects.

52. Explain the use of filters in CouchDB views.

    • Answer: Filters allow for more complex selection criteria in views, reducing the amount of data processed and improving query performance.

53. How can you use CouchDB with other technologies?

    • Answer: CouchDB can be integrated with various programming languages using client libraries, and with other systems via its HTTP API.

54. Describe the process of migrating data to CouchDB from a relational database.

    • Answer: Migration involves extracting data from the relational database, transforming it into JSON format, and importing it into CouchDB using tools or scripts.

55. What are some considerations for scaling CouchDB?

    • Answer: Scaling considerations include adding more nodes to the cluster, optimizing views, and distributing data across multiple databases.

56. How can you debug CouchDB applications?

    • Answer: Debugging involves using logging, examining the database's internal state, and using debugging tools provided by the programming language and CouchDB itself.

57. What is the role of the `_id` field in a CouchDB document?

    • Answer: The `_id` field is the unique identifier for each document in CouchDB.

58. Explain the concept of optimistic locking in CouchDB.

    • Answer: CouchDB uses optimistic locking through revision IDs. Updates fail if the revision ID doesn't match the current version, preventing accidental overwrites.

59. How can you perform bulk operations in CouchDB?

    • Answer: Bulk operations can be achieved using the _bulk_docs API endpoint, allowing for efficient insertion or update of multiple documents.

60. What are some common performance bottlenecks in CouchDB?

    • Answer: Common bottlenecks include inefficient view design, slow replication, inadequate hardware resources, and poorly structured data.

61. How do you manage and monitor CouchDB replication?

    • Answer: Replication is managed through configuration and monitoring the replication logs. Tools like Fauxton provide visual representation of replication status.

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