rqlite

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rqlite is a lightweight, distributed DBMS which uses SQLite as its storage engine. rqlite uses the Raft protocol to achieve consensus across multiple instances of the SQLite database, ensuring that every change made to the system is made to a quorum of SQLite databases, or none at all.

History

rqlite was originally a prototype application, showing how the Raft consensus algorithm could be applied to various software state machines -- in this particular instance a SQLite database.

It has since developed into a full relational database management system.

Storage Architecture

Hybrid

rqlite supports both in-memory and disk-oriented storage. The SQLite database can be stored on disk, or in memory. The Raft log is stored on disk.

Storage Model

N-ary Storage Model (Row/Record)

Since rqlite uses SQLite as its storage engine, it offers the same storage model as SQLite.

Stored Procedures

Not Supported

Since SQLite does not offer Stored Procedures, neither does rqlite.

Data Model

Relational

rqlite supports the relational data model, since it uses SQLite as its storage engine.

Foreign Keys

Supported

Since rqlite uses SQLite as its storage engine, it can make full use of SQLite Foreign Key support.

Indexes

B+Tree

Since rqlite uses SQLite as its storage engine, it can make full use of SQLite Indexes

Isolation Levels

Read Uncommitted Serializable Snapshot Isolation

Since rqlite uses SQLite as its storage engine, it offers the same isolation levels as SQLite.

Joins

Index Nested Loop Join

Since rqlite uses SQLite as its storage engine, it performs Joins in the same manner as SQLite, within a single rqlite node. It is important to note however, that even though rqlite supports clustering, it does not support Joins across rqlite nodes in that cluster.

Logging

Physical Logging

rqlite provides data durability via the Raft consensus system. Every change made to the SQLite database is written to the Raft log, and that log is persisted to disk. In the event of a restart or recovery operation, the SQLite database is completely rebuilt from the data contained in the Raft log.

Query Interface

SQL HTTP / REST

rqlite is primarily queried via its HTTP(S) API. A query is formed by encapsulating a SQL statement in a HTTP request. And when using the "on disk" mode of rqlite, the SQLite database can also be queried directly.

System Architecture

Embedded

rqlite's system architecture is comprised of 3 distinct subsystems: the HTTP serving layer, the Raft consensus layer, and the (embedded) SQLite database.

The HTTP layer is responsible for responding to queries, and returning results. The Raft consensus layer controls communication between nodes, implementing the distributed consensus system. Once a change to the database has been committed to the Raft log on each node, each node then makes the change to the last subsystem, which is the embedded SQLite database.