CouchDB MVCC: Concepts and Basics Quiz Quiz

Explore key principles of CouchDB's Multi-Version Concurrency Control (MVCC) with this quiz, designed to check your understanding of document versioning, conflicts, and concurrent updates in document-oriented databases. Perfect for developers and database enthusiasts seeking a solid foundation in MVCC mechanisms for safe and reliable data handling.

1. MVCC Purpose

   What is the primary purpose of Multi-Version Concurrency Control (MVCC) in a document-oriented database?

   1. To automatically back up all versions of the database
   2. To allow multiple users to read and write data simultaneously without conflicts
   3. To ensure data is always encrypted
   4. To prevent any user from updating documents

   Explanation: MVCC enables multiple users to read and write to the database at the same time by managing different versions of documents, thereby preventing direct conflicts. It is not primarily responsible for encryption, backups, or restricting users from making updates. Encryption protects data privacy, backups prevent data loss, and preventing updates entirely would not support collaboration.

2. Understanding _rev Field

   What role does the '_rev' field play in document updates within a CouchDB MVCC system?

   1. It monitors database uptime
   2. It tracks the document's current revision version for conflict detection
   3. It controls user access permissions
   4. It stores all document attachments

   Explanation: The '_rev' field is used to keep track of a document’s current revision, ensuring that updates are only applied if the revision matches. This mechanism helps detect and resolve update conflicts. It does not manage user permissions, monitor system status, or store attachments—which are handled by other fields and processes.

3. Handling Simultaneous Updates

   If two users attempt to update the same document at the same time, how does MVCC handle this situation?

   1. It merges the updates automatically without issue
   2. It deletes both changes and keeps the original document
   3. It creates a conflict that must be resolved before changes are accepted
   4. It alerts users and prevents any updates

   Explanation: MVCC detects simultaneous updates and marks them as conflicting, requiring user or application intervention to resolve the conflict. MVCC does not automatically merge changes or delete both updates. Simply alerting users without allowing resolution is not standard behavior.

4. Reading Documents

   How does MVCC ensure a consistent view of a document when it is being read while another process is updating it?

   1. By immediately returning the updated version
   2. By forcing the reader to wait for the update to finish
   3. By serving the version that existed at the start of the read operation
   4. By locking the document to prevent updates

   Explanation: MVCC serves the version of the document that was present when the read began, ensuring data consistency for the reader. It does not lock the document, as this would reduce performance. Returning the updated version or forcing the reader to wait would risk inconsistent reads and degrade user experience.

5. Conflict Resolution

   Which of the following best describes how update conflicts are typically resolved in an MVCC database?

   1. A user or application picks which version to keep
   2. The oldest version is always chosen
   3. The database deletes both conflicting versions
   4. Conflicts are ignored and both versions are kept forever

   Explanation: Resolving conflicts usually involves users or applications selecting which version should become the winner, and incorporating necessary changes. MVCC does not always choose the oldest version, ignore conflicts indefinitely, or delete all versions, as these actions could lead to data loss.

6. Write Operations

   What must a client provide when submitting an update to a document in MVCC systems?

   1. The user’s email address
   2. The creation date of the document
   3. The document’s access log
   4. The current revision ID (_rev) of the document

   Explanation: When updating a document, the client must include the current _rev value so MVCC can verify that the update is based on the latest version. The creation date, user’s email, and access log are unrelated to revision control and are not required for concurrency checks.

7. Old Versions

   Which statement describes what happens to outdated document versions in an MVCC database after updates?

   1. They are made publicly available for review
   2. They are instantly deleted from the database
   3. They are backed up to a separate file automatically
   4. They may be kept for conflict resolution or removed by compaction

   Explanation: Outdated versions can be kept temporarily for conflict resolution and then removed by automated processes like compaction. Immediate deletion would risk losing necessary data, automatic backup to external files is not a standard MVCC feature, and making old versions public could breach privacy standards.

8. Consistency in Reads

   MVCC helps provide which type of consistency for database read operations?

   1. Inconsistent snapshots
   2. Snapshot consistency
   3. No consistency
   4. Eventual consistency

   Explanation: MVCC provides snapshot consistency, meaning that reads see a stable view of the data as it existed at a given point. Eventual consistency refers to a different model often used in distributed systems. No consistency and inconsistent snapshots do not accurately describe MVCC.

9. Benefits of No Locking

   Why does MVCC avoid locking documents during reads or updates?

   1. To improve database performance and allow high concurrency
   2. To enforce stricter validation on new entries
   3. To hide document changes from some users
   4. To reduce server disk usage

   Explanation: Avoiding locks allows MVCC to support high levels of concurrent operations, making the database more responsive. Reducing disk usage, enforcing validation, or hiding changes are not directly achieved by skipping locks. Locking would decrease performance and make the database less scalable.

10. Document Deletion

    When a document is deleted in an MVCC-based database, what typically happens?

    1. A special deletion marker (tombstone) is created instead of immediate removal
    2. The deletion is rolled back after a short time
    3. All revisions are merged into the newest version
    4. The database erases all traces of the document right away

    Explanation: When a document is deleted, MVCC systems usually insert a special marker or tombstone to indicate deletion without removing the document instantly. Immediate removal would hinder synchronization and recovery. Rollbacks are not a typical result of deletions, and merging all revisions ignores the document's deletion state.