PouchDB Interview Questions and Answers for 2 years experience

1. What is PouchDB?

   • Answer: PouchDB is a JavaScript database that mimics the functionality of CouchDB, enabling client-side storage of data. It's an open-source, offline-first database that syncs with CouchDB or other PouchDB instances.

2. What are the key advantages of using PouchDB?

   • Answer: Key advantages include offline capabilities, data persistence even without internet connection, synchronization with remote databases (CouchDB, Cloudant), ease of use with JavaScript, and scalability for large datasets (relatively speaking for client-side databases).

3. Explain the concept of "offline-first" in the context of PouchDB.

   • Answer: "Offline-first" means the application works seamlessly even without an internet connection. Data is stored locally using PouchDB, and when connectivity is restored, changes are synchronized with a remote database.

4. How does PouchDB handle conflicts during synchronization?

   • Answer: PouchDB uses a revision system to track changes. During synchronization, if conflicts arise (same document updated on both client and server), PouchDB will highlight the conflict and usually requires manual intervention to resolve the conflict, typically by choosing a winning revision.

5. What are the different ways to create a PouchDB database?

   • Answer: You can create a PouchDB database using the `new PouchDB('mydatabase')` method, specifying the database name. You can also specify options like location (e.g., in-memory, browser storage).

6. How do you add a document to a PouchDB database?

   • Answer: You use the `db.put(document)` method. The `document` is a JavaScript object. PouchDB automatically assigns a unique `_id` if not provided.

7. How do you retrieve a document from a PouchDB database?

   • Answer: Use `db.get(documentId)` where `documentId` is the unique identifier of the document.

8. How do you update a document in PouchDB?

   • Answer: You retrieve the document using `db.get()`, modify its properties, and then use `db.put()` again with the updated document, including the `_rev` property (revision number).

9. How do you delete a document from PouchDB?

   • Answer: Use `db.remove({ _id: documentId, _rev: revision })`. You need both the `_id` and the `_rev` to delete a document successfully.

10. Explain the concept of revisions in PouchDB.

    • Answer: Revisions track the history of a document. Each time a document is updated, a new revision is created with a unique revision ID (`_rev`). This enables conflict resolution and data recovery.

11. What are PouchDB's different storage adapters?

    • Answer: PouchDB supports various adapters, including IndexedDB, WebSQL (deprecated), and localStorage. The adapter choice depends on the browser's capabilities and desired persistence level.

12. How do you perform a query in PouchDB?

    • Answer: PouchDB uses MapReduce or Mango queries (a JSON query language). Mango queries are easier to use for simpler queries while MapReduce offers more flexibility for complex queries.

13. Explain the difference between `db.allDocs()` and `db.find()` in PouchDB.

    • Answer: `db.allDocs()` returns all documents in the database, while `db.find()` allows for querying documents based on specific criteria using selectors (similar to a SQL `WHERE` clause) – typically using Mango syntax.

14. What is replication in PouchDB?

    • Answer: Replication is the process of synchronizing data between a PouchDB database and a remote CouchDB database (or another PouchDB instance). It ensures data consistency across multiple locations.

15. How do you perform replication in PouchDB?

    • Answer: You use `PouchDB.replicate(source, target, options)` where `source` and `target` are database instances (or URLs), and `options` can specify settings like continuous replication and conflict resolution strategies.

16. Explain the concept of continuous replication in PouchDB.

    • Answer: Continuous replication keeps the local and remote databases synchronized in real-time. Changes made on either side are immediately replicated to the other.

17. How do you handle errors in PouchDB?

    • Answer: PouchDB operations usually return promises. You can use `.then()` for successful operations and `.catch()` to handle errors. Error objects provide information about the type of error encountered.

18. What are some common use cases for PouchDB?

    • Answer: Common use cases include mobile applications, offline-capable web applications, applications requiring data persistence across sessions, and applications needing synchronization with a central server.

19. How does PouchDB handle large datasets?

    • Answer: While PouchDB is efficient for client-side storage, performance can degrade with extremely large datasets. Strategies like proper indexing, efficient querying, and data filtering are crucial for managing performance.

20. What are the limitations of PouchDB?

    • Answer: PouchDB is a client-side database; it relies on the browser's capabilities. Complex queries might be less performant compared to server-side databases. Data security is dependent on the browser's security mechanisms.

21. How does PouchDB compare to other JavaScript databases like IndexedDB?

    • Answer: PouchDB builds on top of IndexedDB (among other adapters) providing a simpler, higher-level API. IndexedDB is more low-level and requires more manual coding for common database operations.

22. Describe your experience working with PouchDB in a real-world project.

    • Answer: [This requires a personalized response based on your actual experience. Describe a specific project, the challenges encountered, the solutions implemented, and the outcome.]

23. Explain how you would design a data model for a specific application using PouchDB. (e.g., a to-do list app)

    • Answer: [This requires a personalized response. Describe the data structure (e.g., tasks with title, description, completion status), how it would be represented as JSON documents, and how queries would be performed.]

24. How would you handle data migration when upgrading a PouchDB database schema?

    • Answer: Data migration involves creating a new schema, extracting data from the old database, transforming it to fit the new schema, and then importing it into the new database. Techniques like using a separate migration script or leveraging PouchDB's bulk document insertion are useful.

25. How would you optimize PouchDB performance in a high-volume application?

    • Answer: Optimizations include proper indexing, using efficient queries (Mango or MapReduce depending on complexity), batching operations (bulkDocs), data compression (where applicable), and potentially caching frequently accessed data.

26. What are some security considerations when using PouchDB?

    • Answer: Security depends heavily on the application's architecture and the overall security measures implemented. Sensitive data shouldn't be stored directly in PouchDB without proper encryption. HTTPS should be used for communication with remote servers.

27. How would you debug PouchDB related issues in your application?

    • Answer: Debugging involves using browser's developer tools to inspect network requests, checking the console for errors, utilizing PouchDB's logging capabilities, and potentially using a network proxy to monitor communication between the client and server.

28. What are some best practices for using PouchDB in a team environment?

    • Answer: Best practices include version control for database schema and code, using a consistent coding style, writing clear and well-documented code, and using a robust testing strategy to ensure data integrity.

29. Explain your understanding of the PouchDB ecosystem and its community support.

    • Answer: PouchDB has a reasonably active community, although it's not as large as some other JavaScript frameworks. The documentation is relatively well-maintained, and community forums and issue trackers are available for assistance.

30. How would you design a PouchDB application that supports multiple users?

    • Answer: This would likely involve using a unique identifier for each user, potentially stored within the documents themselves (e.g., a `userId` field) or using separate databases for each user to maintain data isolation. Authentication and authorization mechanisms would be essential.

31. How do you handle attachments in PouchDB?

    • Answer: Attachments are handled by including a base64 encoded representation of the file within the document. This can be cumbersome for large files; alternative approaches like using external storage and only storing references in PouchDB are preferable for large files.

32. What is the role of the `_id` and `_rev` fields in a PouchDB document?

    • Answer: `_id` is the unique identifier for the document, and `_rev` is the revision number that tracks changes made to the document. Both are crucial for managing document versions and resolving conflicts during replication.

33. What are the different types of PouchDB queries?

    • Answer: PouchDB primarily uses `db.allDocs()` for simple retrieval and `db.find()` with Mango selectors for more complex queries; `db.query()` with MapReduce functions is available for advanced scenarios but Mango is generally preferred for its simplicity.

34. How do you filter documents in PouchDB using `db.find()`?

    • Answer: Use Mango selectors within the `selector` property of the `db.find()` options. The selector uses JSON-like syntax to specify criteria for filtering, such as equality, comparison, and logical operators.

35. How do you sort documents retrieved using `db.find()`?

    • Answer: Use the `sort` property within the `db.find()` options. This takes an array of field names (or objects for specifying ascending/descending order) to specify the sort order.

36. How can you limit the number of documents returned by a `db.find()` query?

    • Answer: Use the `limit` property in the `db.find()` options to specify the maximum number of documents to return.

37. How do you perform pagination with PouchDB queries?

    • Answer: Combine the `skip` and `limit` properties in `db.find()` options. The `skip` property specifies the number of documents to skip before starting the result set, simulating pagination.

38. What is the purpose of the `_deleted` field in PouchDB?

    • Answer: When a document is deleted, the `_deleted` field is set to `true`. It's not physically removed until replication or compaction, preserving the deletion record for synchronization.

39. How do you compact a PouchDB database?

    • Answer: Use the `db.compact()` method. This removes old revisions and optimizes the database size and performance.

40. What is the role of PouchDB's change feed?

    • Answer: The change feed allows you to subscribe to changes in the database. It provides real-time notifications when documents are added, updated, or deleted.

41. How would you implement a search functionality using PouchDB?

    • Answer: Combine `db.find()` with appropriate selectors for full-text or partial string matching on relevant fields. For complex searches, consider using a dedicated search engine or library integrated with PouchDB.

42. How do you handle authentication with a remote CouchDB server when replicating with PouchDB?

    • Answer: Typically, you'd use authentication mechanisms like Basic Authentication or OAuth 2.0 provided by the CouchDB server, passing credentials in the replication options.

43. What are some common errors you encountered while working with PouchDB and how did you troubleshoot them?

    • Answer: [Describe specific errors, their root causes, and the steps taken to resolve them. Examples include revision conflicts, network errors, database schema inconsistencies.]

44. Describe your experience using PouchDB with different JavaScript frameworks like React, Angular, or Vue.js.

    • Answer: [Describe your experience, highlighting integration approaches, challenges overcome, and best practices.]

45. How would you test your PouchDB application thoroughly?

    • Answer: Use unit tests to verify individual functions, integration tests to check the interactions between different components, and end-to-end tests to ensure the overall functionality works correctly. Mocking the PouchDB API is often useful in testing.

46. Discuss your experience working with PouchDB and other technologies in a collaborative development environment.

    • Answer: [Describe your collaborative experience, highlighting teamwork aspects and problem-solving approaches.]

47. Explain how you'd approach designing a system using PouchDB that needs to support real-time updates to many clients.

    • Answer: This would require a combination of continuous replication, change feed listeners, and a real-time communication mechanism like WebSockets to efficiently push updates to multiple clients. Careful design of data synchronization and conflict resolution is crucial.

48. How would you improve the performance of a PouchDB application that's experiencing slow query response times?

    • Answer: Analyze slow queries to pinpoint bottlenecks. Optimize queries by refining selectors and using indexes appropriately. Consider using batch operations for bulk data insertions and updates. Examine network conditions to rule out connectivity issues.

49. What are your preferred tools and techniques for managing and monitoring PouchDB databases?

    • Answer: [Describe the tools and techniques you would use; examples could include browser developer tools, network monitoring tools, and potentially custom monitoring solutions to track database size, replication performance, and error rates.]

50. How would you secure a PouchDB application deployed to a production environment?

    • Answer: This involves securing the server-side components, using HTTPS for all communication, implementing robust authentication and authorization mechanisms, encrypting sensitive data both in transit and at rest, and regularly patching vulnerabilities.

51. What are some potential challenges you anticipate when migrating from another database system to PouchDB?

    • Answer: Challenges include data schema transformation, adapting to PouchDB's query language, handling potential data loss or corruption during migration, and ensuring data consistency across the transition.

52. How familiar are you with the different PouchDB plugins and extensions available?

    • Answer: [Describe your familiarity with relevant plugins. Examples might include plugins extending functionality like search, data validation, or specialized adapters.]

53. Describe a time you had to debug a complex issue involving PouchDB and other technologies.

    • Answer: [Describe a specific scenario, outlining the problem, the debugging process, and the solution. This demonstrates problem-solving skills.]

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