PouchDB Interview Questions and Answers for 5 years experience

1. What is PouchDB?

   • Answer: PouchDB is a JavaScript database that is an open-source, client-side database that API mirrors CouchDB's. It allows you to create and manage databases within a web browser or Node.js environment without needing a server.

2. How does PouchDB handle data synchronization with CouchDB?

   • Answer: PouchDB uses a replication mechanism to synchronize data with a remote CouchDB or Cloudant database. Changes made locally are replicated to the remote database, and changes made remotely are replicated locally, ensuring data consistency across both environments. It uses HTTP for communication.

3. Explain the concept of replication in PouchDB.

   • Answer: Replication is the core feature of PouchDB, allowing bi-directional synchronization between local and remote databases. It involves pushing and pulling changes between databases. You can configure replication to be continuous or one-time. Conflict resolution mechanisms handle discrepancies between data in the local and remote databases.

4. What are the advantages of using PouchDB?

   • Answer: Advantages include offline capabilities, allowing functionality even without network connectivity; data persistence within the browser; ease of use with its CouchDB-like API; efficient handling of large datasets; and the ability to synchronize data with remote CouchDB/Cloudant databases.

5. What are the limitations of PouchDB?

   • Answer: Limitations include potential performance issues with extremely large datasets compared to server-side databases; being client-side only, it relies on the client's resources; and complex query capabilities are less powerful than those of many server-side databases. Security is also a consideration as all data resides on the client.

6. How does PouchDB handle conflicts during replication?

   • Answer: PouchDB uses a revision system to track changes. During replication, if conflicts occur (both local and remote databases have changed the same document), PouchDB will mark the document as conflicted and provide tools to resolve them. The developer needs to implement logic to handle these conflicts, often by choosing which revision to keep or creating a new one that combines information from both.

7. Explain the difference between `put()` and `post()` methods in PouchDB.

   • Answer: `put()` is used to update or create a document with a predefined _id. If the _id already exists, it updates; otherwise, it creates a new document. `post()` creates a new document, assigning a unique _id automatically. Use `put()` when you know the document's ID and `post()` when you want PouchDB to manage it.

8. How do you query data in PouchDB?

   • Answer: PouchDB uses `find()` method for querying documents using JSON-like queries. Simple queries can be performed with the `allDocs` method to retrieve all documents or a subset of documents. Advanced querying capabilities are available using MapReduce and views (requires design documents).

9. What are design documents in PouchDB?

   • Answer: Design documents contain views, which are pre-defined queries that improve the performance of data retrieval. They allow efficient querying of large datasets by creating indexes on the data. They are saved into the database and used to perform more complex queries beyond simple `find()` statements.

10. How can you handle schema changes in PouchDB?

    • Answer: PouchDB doesn't enforce a schema. You can handle schema changes by updating your application logic to accommodate new fields or changes to existing fields. Migration scripts or functions could be created to update existing documents to reflect the schema changes. Data validation can be added in application logic.

11. Explain the concept of revisions in PouchDB.

    • Answer: PouchDB uses a revision system to track all changes made to a document. Each change generates a new revision, allowing for rollback and conflict resolution. Revisions are identified by their unique revision IDs, forming a history of the document's changes.

12. How do you handle bulk document insertion in PouchDB?

    • Answer: PouchDB's `bulkDocs()` method allows for efficient insertion of multiple documents simultaneously. This improves performance compared to inserting each document individually. It handles document creation and update requests in a batch.

13. What are some best practices for using PouchDB?

    • Answer: Best practices include using appropriate data validation to ensure data integrity, employing design documents to optimize query performance, handling conflict resolution strategically, employing error handling and logging, and testing thoroughly across various network conditions.

14. How can you optimize PouchDB performance?

    • Answer: Optimization involves using efficient query strategies (design documents), minimizing the number of replication requests, batching operations (e.g., `bulkDocs()`), using appropriate indexes, and compressing data before storage (if appropriate for your data format).

15. Describe the different types of replication in PouchDB.

    • Answer: PouchDB supports one-way and bi-directional replication. One-way replication copies data in a single direction (e.g., from local to remote). Bi-directional replication synchronizes changes in both directions, ensuring data consistency between local and remote databases. Continuous replication keeps the databases synchronized in real time, whereas one-time replication synchronizes only once.

16. How can you use PouchDB in a web application that needs to work offline?

    • Answer: PouchDB is inherently designed for offline usage. You create a local database and work with it even without a network connection. When connectivity is restored, use replication to synchronize data with a remote database. Proper error handling should be in place to handle cases when the network is unavailable.

17. Explain how to use transactions in PouchDB.

    • Answer: PouchDB itself doesn't have explicit transactions like some SQL databases. To simulate transactions, you'd need to use the `bulkDocs()` method to ensure that a set of operations either succeed completely or fail together. If any operation in the `bulkDocs` fails, the entire set fails and the database remains unchanged.

18. How do you delete a document in PouchDB?

    • Answer: You delete a document using the `remove()` method, providing the document's ID and revision. It's important to specify the correct revision ID to ensure successful deletion.

19. What is the role of the `_id` and `_rev` fields in PouchDB documents?

    • Answer: `_id` is the unique identifier for a document. `_rev` is a unique identifier for a specific revision of the document. Each time a document is changed, the `_rev` changes, allowing PouchDB to track the document's history and handle conflicts.

20. How can you secure data stored in PouchDB?

    • Answer: Client-side security measures should be implemented. Encryption of data before storage is a key aspect. Using HTTPS to communicate with remote databases is essential. Implementing strong authentication and authorization mechanisms within the application is vital. Remember that client-side databases are susceptible to client-side attacks.

21. What are some common errors encountered while using PouchDB and how to debug them?

    • Answer: Common errors include replication conflicts, database corruption, network errors (during replication), and incorrect usage of API methods. Debugging involves checking console logs for errors, using PouchDB's debugging tools, examining the database's contents using the browser's developer tools, carefully reviewing the code for API usage mistakes and handling potential error conditions.

22. How do you handle large attachments in PouchDB?

    • Answer: Large attachments can impact performance. Consider strategies like compressing attachments before storing them, storing attachments externally (e.g., using a cloud storage service), and using efficient handling methods to minimize the impact on the database's performance. PouchDB's attachment handling needs consideration for these larger files.

23. Compare and contrast PouchDB with other JavaScript databases like IndexedDB.

    • Answer: PouchDB simplifies data handling and replication with its CouchDB-like API, offering offline capabilities. IndexedDB, on the other hand, is a lower-level API, requiring more manual handling of data management. PouchDB builds on top of IndexedDB for persistence. PouchDB is focused on ease of replication and synchronisation. IndexedDB is lower-level and doesn't have those features built in.

24. Explain how to use PouchDB with React.

    • Answer: Use PouchDB within React components to interact with the database. State management (Redux, Context API, or others) is used to update the UI based on the data changes from the database. The lifecycle methods of React components might be leveraged to handle database operations. Asynchronous handling using Promises or async/await is essential for database interactions.

25. How would you integrate PouchDB with a Node.js backend?

    • Answer: Use PouchDB in the browser (client-side) for offline capabilities, and use a server-side database (CouchDB, Cloudant, or another solution) on the backend. Replication is used to synchronize data between the client-side PouchDB and the server-side database. API calls would be made between the client-side and server-side components to facilitate data sharing and synchronization.

26. How do you test your PouchDB applications?

    • Answer: Implement unit tests to verify individual functions and components using a mocking approach for database interactions. Integration tests can confirm that the data interactions with PouchDB work correctly. End-to-end tests are used to test the whole application flow, including database interaction. Tools like Jest, Mocha, or similar can be used.

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