Concurrency Control

Multi-version Concurrency Control (MVCC)

The CouchDB uses Multi-version Concurrency Control (MVCC) as the concurrency control policy for read operations, each clients sees a consistent snapshot during the read operation.

Data Model

Document / XML

The data model in CouchDB is document using JSON format.

Indexes

B+Tree

The documents in CouchDB are indexed by their name and sequence id, these index are organized by B-trees.

Isolation Levels

Snapshot Isolation

In CouchDB, a read request will always see the most recent snapshot of the database at the time of the beginning of the request because of MVCC.

Checkpoints

Non-Blocking

In CouchDB, any changes to a document simply appends a new record to the database file, it is always non-blocking to take a snapshot of the file system to get the latest version of the database.

Compression

Naïve (Record-Level)

CouchDB does compaction operation to reduce the disk usage similar like the vacuum in SQLite. CouchDB support user-defined number of revisions. The compaction operations can either be manually triggered or automatically.

Joins

Not Supported

The data in CouchDB are store as documents, which is unnecessary for joins operations. The way to replace join operation is to do denormalization or stored with related data in documents.

Logging

Shadow Paging

CouchDB uses shadow paging as its logging method, it only does appending operations to the current database file, which provides the MVCC features.

Query Interface

HTTP / REST

CouchDB provide RESTful HTTP API as its query interface.

Storage Architecture

Disk-oriented

CouchDB store its data on disk and all update are synchronously flushed to disk.

Storage Model

Custom

The storage model of CouchDB is simply a large file, this database file contains variable-length data chunks, and a custom file critical header.

As the CouchDB is append-only, the header of the database file is in the tail of the file to be access or re-append by each append operation.

Storage Organization

Copy-on-Write / Shadow Paging

CouchDB implements append-only B+Tree and uses copy-on-write method to update the database file as well as the index.

Stored Procedures

Not Supported

System Architecture

Shared-Nothing

CouchDB can provide the same version of replica in different peer, therefore, woyeach peer can provide same data for user to access. They do not share anything.