CouchDB Interview Questions & Answers: Mastering NoSQL Document Databases

This comprehensive guide prepares you for CouchDB interviews by covering its architecture, data model, and key features. We explore CouchDB's strengths as a NoSQL, document-oriented database, emphasizing its ease of use, scalability, and fault tolerance through multi-master replication. This resource provides detailed answers to frequently asked CouchDB interview questions, including comparisons with other NoSQL databases (MongoDB) and relational database management systems (RDBMS). Learn about CouchDB's JSON document storage, RESTful API, MapReduce functionality, and efficient data handling techniques. Prepare for in-depth discussions on CouchDB's architecture, its unique features, and its suitability for various applications.

Top CouchDB Interview Questions and Answers

What is CouchDB?

CouchDB is a NoSQL, document-oriented database system that's known for its ease of use and scalability. It stores data as JSON documents and provides a RESTful HTTP API for interacting with the database. CouchDB's design emphasizes simplicity, making it a good choice for applications where schema flexibility and rapid development are important. It's open-source and uses Erlang for its core functionality, promoting high availability and fault tolerance.

CouchDB Programming Language

CouchDB is primarily written in Erlang, a functional programming language known for its concurrency and fault tolerance features. Parts of CouchDB are also written in C (specifically, the JavaScript engine, SpiderMonkey, is used for handling views).

CouchDB's Early Development

CouchDB's initial development involved C++, but Erlang was later adopted as the primary language due to its better suitability for building a highly scalable and fault-tolerant database system.

CouchDB vs. SQL Databases

CouchDB in the Software Industry

CouchDB's ease of use, scalability, and replication capabilities make it a popular choice for various software applications. It's used by numerous companies for tasks like content management, data storage, and application development.

CouchDB vs. MongoDB

Similarities Between CouchDB and MongoDB

• Both are open-source, document-oriented NoSQL databases.
• Both use JSON-like formats for data storage.
• Both support JavaScript for queries (MapReduce).
• Both support a range of programming languages.

Key Features of CouchDB

• JSON Document Storage: Data is stored as JSON documents.
• RESTful HTTP API: Simplifies interaction with the database.
• Multi-Master Replication: Enables high availability and scalability.
• Offline Support: Designed to work in offline environments.
• Replication Filters: Control which data is replicated.
• ACID Properties (to an extent): Ensures data integrity in many update operations.
• Eventual Consistency: Prioritizes availability over immediate consistency.
• Authentication and Session Management: Supports user authentication.
• Security: Provides database-level access control.
• Validation: Allows for data validation rules.
• Map/Reduce: Supports querying and data transformation using MapReduce.

Why CouchDB Doesn't Use Mnesia

CouchDB doesn't use Mnesia (an Erlang database) due to Mnesia's limitations in scalability and its design not being fully suited for the needs of a large-scale, general-purpose database like CouchDB.

Transactions in CouchDB

CouchDB uses optimistic concurrency control. Updates must include the document's revision ID (_rev) to prevent conflicts. If a conflict occurs, you need to retrieve the latest version and retry the update.

Example Scenario: Managing Inventory

Imagine a "master product" document and individual "inventory-ticket" documents. To update inventory, you fetch the relevant inventory ticket, reduce the quantity (if above zero), and update it using the _rev property. If there's a conflict (another change happened in the meantime), you fetch the latest document and retry.

function(doc) {
    if(doc.type === 'inventory-ticket' && doc.product_key === 'hammer-1'){
        emit(doc.product_key, doc.quantity_available)
    }
}
        

CouchDB: Transactions and Concurrency

Handling Transactions in CouchDB

CouchDB uses an optimistic concurrency control model. When updating a document, you provide its revision ID (_rev). If the revision ID doesn't match the current version, the update is rejected, indicating a conflict. This mechanism minimizes the need for complex locking schemes and enables efficient handling of concurrent updates.

Example: Managing Inventory

Imagine an inventory system where each product has a "master product" document and individual "inventory-ticket" documents representing available items. When a customer wants to buy an item, a view identifies available tickets (claimed_by === null). The system then attempts to claim a ticket, updating it's claimed_by field. If a conflict occurs (another user claimed it first), it tries again with a different ticket. The view only includes available items, reducing the likelihood of multiple users trying to access the same item.

function(doc) {
  if (doc.type === 'inventory-ticket' && doc.claimed_by === null) {
    emit(doc.product_key, { 'inventory-ticket': doc.id, '_rev': doc._rev });
  }
}
        

function(keys, values, rereduce) {
  return values.length;
}
        

Unicode Support in CouchDB

CouchDB handles Unicode using UTF-8 encoding internally, negating any potential issues related to Erlang's (CouchDB's core language) earlier limitations with Unicode support.

CouchDB Usage and Advantages

CouchDB is well-suited for applications requiring:

• Simplified Development: Direct client-side access; no server-side middle layer.
• High Performance: Local data storage minimizes latency.
• Easy Data Modeling: Flexible, schema-free JSON documents.
• Efficient Data Access: Simple RESTful HTTP API.
• Robust Replication: Easy data synchronization.

Couchdbkit

Couchdbkit is a Python library providing a framework for interacting with CouchDB. It simplifies database management, document handling, and view creation in Python applications.

Views and Data Modification

CouchDB views are read-only; they cannot modify documents or the database. Views are used for querying, filtering, sorting, and performing calculations on data.

CouchDB Platform Support

CouchDB primarily supports POSIX-compliant systems (Linux, macOS). While not officially supported, it might work on Windows with some effort.

Sequences in CouchDB

Sequences (for generating unique IDs) aren't typically needed in CouchDB. CouchDB automatically assigns unique IDs to documents, and you can define your own custom unique identifiers if necessary.

Replication in CouchDB

Replication in CouchDB synchronizes data between databases. It uses HTTP to send changes from a source database to a target database. This is a crucial mechanism for high availability and data distribution.

{
  "source": "$source_database",
  "target": "$target_database"
}
        

Accessing CouchDB Without HTTP

While CouchDB's primary interface is the RESTful HTTP API, there are plans to provide a more direct Erlang API for internal interactions.