*rqlite* is a lightweight, distributed relational database, which uses [SQLite](https://www.sqlite.org/) as its storage engine. Forming a cluster is very straightforward, it gracefully handles leader elections, and tolerates failures of machines, including the leader. rqlite is available for Linux, macOS, and Microsoft Windows.
rqlite gives you the functionality of a [rock solid](http://www.sqlite.org/testing.html), fault-tolerant, replicated relational database, but with very **easy installation, deployment, and operation**. With it you've got a **lightweight** and **reliable distributed relational data store**. Think [etcd](https://github.com/coreos/etcd/) or [Consul](https://github.com/hashicorp/consul), but with relational data modelling also available.
You could use rqlite as part of a larger system, as a central store for some critical relational data, without having to run larger, more complex distributed databases.
Finally, if you're interested in understanding how distributed systems actually work, **rqlite is a good example to study**. Much thought has gone into its [design](https://github.com/rqlite/rqlite/blob/master/DOC/DESIGN.md) and implementation, with clear separation between the various components, including storage, distributed consensus, and API.
rqlite uses [Raft](https://raft.github.io/) to achieve consensus across all the instances of the SQLite databases, ensuring that every change made to the system is made to a quorum of SQLite databases, or none at all. You can learn more about the design [here](https://github.com/rqlite/rqlite/blob/master/DOC/DESIGN.md).
- An easy-to-use [HTTP(S) API](https://github.com/rqlite/rqlite/blob/master/DOC/DATA_API.md). A [command-line interface is also available](https://github.com/rqlite/rqlite/tree/master/cmd/rqlite), as are various [client libraries](https://github.com/rqlite).
- Optional [read-only (non-voting) nodes](https://github.com/rqlite/rqlite/blob/master/DOC/READ_ONLY_NODES.md), which can add read scalability to the system.
_Detailed documentation [is available](https://github.com/rqlite/rqlite/tree/master/DOC). You may also wish to check out the [rqlite Google Group](https://groups.google.com/forum/#!forum/rqlite)._
The quickest way to get running on macOS and Linux is to download a pre-built release binary. You can find these binaries on the [Github releases page](https://github.com/rqlite/rqlite/releases). If you prefer Windows you can download the latest build [here](https://ci.appveyor.com/api/projects/otoolep/rqlite/artifacts/rqlite-latest-win64.zip?branch=master). Once installed, you can start a single rqlite node like so:
While not strictly necessary to run rqlite, running multiple nodes means you'll have a fault-tolerant cluster. Start two more nodes, allowing the cluster to tolerate failure of a single node, like so:
_This demonstration shows all 3 nodes running on the same host. In reality you probably wouldn't do this, and then you wouldn't need to select different -http-addr and -raft-addr ports for each rqlite node._
With just these few steps you've now got a fault-tolerant, distributed relational database. For full details on creating and managing real clusters, including running read-only nodes, check out [this documentation](https://github.com/rqlite/rqlite/blob/master/DOC/CLUSTER_MGMT.md).
There is also a rqlite _Discovery Service_, allowing nodes to automatically connect and form a cluster. This can be much more convenient, allowing clusters to be dynamically created. Check out [the documentation](https://github.com/rqlite/rqlite/blob/master/DOC/DISCOVERY.md) for more details.
Let's insert some records via the [rqlite CLI](https://github.com/rqlite/rqlite/blob/master/DOC/CLI.md), using standard SQLite commands. Once inserted, these records will be replicated across the cluster, in a durable and fault-tolerant manner. Your 3-node cluster can suffer the failure of a single node without any loss of functionality or data.
rqlite has a rich HTTP API, allowing full control over writing to, and querying from, rqlite. Check out [the documentation](https://github.com/rqlite/rqlite/blob/master/DOC/DATA_API.md) for full details. There are also [client libraries available](https://github.com/rqlite).
By default rqlite uses an [in-memory SQLite database](https://www.sqlite.org/inmemorydb.html) to maximise performance. In this mode no actual SQLite file is created and the entire database is stored in memory. If you wish rqlite to use an actual file-based SQLite database, pass `-on-disk` to rqlite on start-up.
Since the Raft log is the authoritative store for all data, and it is stored on disk by each node, an in-memory database can be fully recreated on start-up from the information stored in the Raft log. Using an in-memory database does not put your data at risk.
* In-memory databases are currently limited to 2GiB (2147483648 bytes) in size. You can learn more about possible ways to get around this limit in the [documentation](https://github.com/rqlite/rqlite/blob/master/DOC/PERFORMANCE.md#in-memory-database-limits).
* Only SQL statements that are [__deterministic__](https://www.sqlite.org/deterministic.html) are safe to use with rqlite, because statements are committed to the Raft log before they are sent to each node. In other words, rqlite performs _statement-based replication_. For example, the following statement could result in a different SQLite database under each node:
* This has not been extensively tested, but you can directly read the SQLite file under any node at anytime, assuming you run in "on-disk" mode. However there is no guarantee that the SQLite file reflects all the changes that have taken place on the cluster unless you are sure the host node itself has received and applied all changes.
* In case it isn't obvious, rqlite does not replicate any changes made directly to any underlying SQLite file, when run in "on disk" mode. **If you change the SQLite file directly, you may cause rqlite to fail**. Only modify the database via the HTTP API.
* SQLite dot-commands such as `.schema` or `.tables` are not directly supported by the API, but the [rqlite CLI](https://github.com/rqlite/rqlite/blob/master/DOC/CLI.md) supports some very similar functionality. This is because those commands are features of the `sqlite3` command, not SQLite itself.
Learn how to hot backup your rqlite cluster [here](https://github.com/rqlite/rqlite/blob/master/DOC/BACKUPS.md). You can also load data [directly from a SQLite dump file](https://github.com/rqlite/rqlite/blob/master/DOC/RESTORE_FROM_SQLITE.md).
How do I pronounce rqlite? For what it's worth I try to pronounce it "ree-qwell-lite". But it seems most people, including me, often pronouce it "R Q lite".
