Indexes

B+Tree Skip List Hash Table Inverted Index (Full Text)

A few of indexes are available in Couchbase Server:

• Primary Index is based on the unique key of every item in a specified bucket. The bucket contains a full set of keys in a given key space.

• Global Secondary Index (GSI) is the most frequently used index in Couchbase Server, for queries performed with the N1Ql query-language. The indexer for GSI creates a B+ tree for fast scans on the index key. Also, there is a Memory-Optimized GSI index which uses the skip-list structure as opposed to B-tree indexes.

• Full Text is provided by the Search Service and it contains targets derived from the contents of documents within one or more specified buckets.

• Views are supported with fields and information extracted from documents.

Storage Architecture

Disk-oriented

Couchbase Server maintains recent accessed data in the memory to achieve high performance and scalability. The data which is not recently used will be swapped on to disks.

Foreign Keys

Not Supported

As a key-value database, there is no foreign key in the Couchbase Server.

Stored Procedures

Supported

A similar feature called "Prepared queries" is supported in Couchbase Server.

Data Model

Document / XML

The documents stored in Couchbase Server are in JSON or binary format.

Checkpoints

Consistent

The checkpoint inside the Couchbase Server is a record of last sequenceID. The Sequence ID is an internal unique number for every change happened in Couchbase Server with chronologically increasing order. The replicator of the Couchbase Server sends the checkpoint of last change to the target at the end of every replication cycle. The target here could be any remote database where the changes are replicated.

Isolation Levels

Read Committed Serializable

Couchbase Server provides read committed isolation by default at the single document level. However, by using the provided optimistic locking and pessimistic locking, user can obtain serialized isolation of an individual document.

Views

Materialized Views

Three views are supported by Couchbase Server: MapReduce views, Spatial views and Global Secondary Indexes (GSI) views. They could be accessed by customized APIs or N1QL queries.

Joins

Nested Loop Join

Couchbase Server supports lookup join, index join and ANSI join in its N1QL language.

Storage Model

Custom

Couchbase Server stores items in the Buckets. There are two kinds of Buckets in the Couchbase. One is Couchbase Buckets which could reside both in memory and disks. Another is Ephemeral bucket which only stays in the memory.

Concurrency Control

Optimistic Concurrency Control (OCC)

Couchbase Server offers both optimistic and pessimistic locking to guarantee concurrency. The optimistic locking is implemented on the Compare-and-Swap (CAS) Value, which is a unique and atomically incrementing identifier inside the metadata of document. Couchbase Server verifies the CAS Value before a document is modified. Couchbase Server also supports pessimistic locking but it is less commonly used.

Query Interface

Custom API

Couchbase Server presents a new language called N1QL which is a SQL extension for JSON as the query interface.

Storage Organization

Sorted Files

Couchbase Server uses Couchstore to store core data operations. Each Bucket (the smallest unit of Couchbase's storage model) is stored as a separate Couchstore file in the file system. Couchstore uses a B+tree structure to quickly access items through their keys. Append-only write model is used for efficient and safe writes.

System Architecture

Shared-Nothing

Each machine running Couchbase server is a single node, and multiple nodes could join together as a cluster. Every node of the cluster has a Cluster Manager module for communication. The data inside the cluster is evenly distributed and replicated over the nodes.

Compression

Bit Packing / Mostly Encoding

Couchbase Server (Enterprise Edition) applies data compression to documents with a 3rd party open-source library Snappy. It provides three modes of compression.

• Off Mode. The Couchbase Server decompresses the document if it is compressed, and stores the uncompressed document in memory but recompresses it when storing on disk. Also, Couchbase server sends the document in uncompressed form.

• Passive Mode. When Couchbase Server receives the compressed document, it stores the compressed document in memory and disk. Also, the documents are sent in compressed form.

• Active Mode. Couchbase Server stores and sends compressed document even if the document is uncompressed.