Detailed background on rqlite can be found on this blog post.

rqlite is a distributed system that provides a replicated SQLite database. rqlite is written in Go and uses Raft to achieve consensus across all the instances of the SQLite databases. rqlite ensures that every change made to the database is made to a majority of underlying SQLite files, or none-at-all.

Building rqlite requires Go 1.3 or later. gvm is a great tool for managing your version of Go.

Download, test (optional), and run rqlite like so (tested on 64-bit Kubuntu 14.04):

mkdir rqlite # Or any directory of your choice.
cd rqlite/
export GOPATH=$PWD
go get github.com/otoolep/rqlite
go get gopkg.in/check.v1; go test github.com/otoolep/rqlite/... # Optional testing
$GOPATH/bin/rqlite ~/node.1

This starts a rqlite server listening on localhost, port 4001. This single node automatically becomes the leader. To see all available command-line options, execute:

$GOPATH/bin/rqlite -h

Alternatively you can use a Vagrant environment. To do so, simply install Vagrant on your machine, a virtualization system such as VirtualBox, and execute the following commands:

$ cd $GOPATH/src/github.com/otoolep/rqlite
$ CLUSTER_SIZE=3 vagrant up rqlite

This will start a Vagrant box and install rqlite with all required dependencies. This will form a cluster with CLUSTER_SIZE nodes.

To execute queries against the cluster you can either ssh directly to the Vagrant box via vagrant ssh rqlite or execute the commands directly from your local box, accessing the cluster at 192.168.200.10 IP and any port within a range [4001, 4001 + CLUSTER_SIZE -1].

To terminate the Vagrant box simply execute:

$ vagrant destroy rqlite

While not strictly necessary to run rqlite, running multiple nodes means the SQLite database is replicated.

Start a second and third node (so a majority can still form in the event of a single node failure) like so:

$GOPATH/bin/rqlite -join localhost:4001 -p 4002 ~/node.2
$GOPATH/bin/rqlite -join localhost:4001 -p 4003 ~/node.3

Under each node will be an SQLite file, which should remain in consensus.

If a node needs to be restarted, perhaps because of failure, don't pass the -join option. Using the example nodes above, if node 2 needed to be restarted, do so as follows:

$GOPATH/bin/rqlite -p 4002 ~/node.2

On restart it will rejoin the cluster and apply any changes to its local sqlite database that took place while it was down. Depending on your snapshot threshold, restarts may take a little time. Check out the section below on Log Compaction.

rqlite exposes an HTTP API allowing the database to be modified such that the changes are replicated. Queries are also executed using the HTTP API,though the SQLite database could be queried directly. Modifications go through the Raft log, ensuring only changes committed by a quorum of Raft servers are actually executed against the SQLite database. Queries do not go through the Raft log, however, since they do not change the state of the database, and therefore do not need to be captured in the log.

All responses from rqlite are in the form of JSON.

To write data successfully to the database, you must create at least 1 table. To do this, perform a HTTP POST, with a CREATE TABLE SQL command in the body of the request. For example:

curl -L -XPOST localhost:4001/db?pretty -d '
CREATE TABLE foo (id integer not null primary key, name text)
'

where curl is the well known command-line tool. Passing -L to curl ensures the command will follow any redirect (HTTP status code 307) to the leader, if the node running on port 4001 is not the leader.

To insert an entry into the database, execute a second SQL command:

curl -L -XPOST 'localhost:4001/db?pretty&explain' -d '
INSERT INTO foo(name) VALUES("fiona")
'

The use of the URL param pretty is optional, and results in pretty-printed JSON responses. explain is also optional. If included, the response will include some basic information about the processing that took place -- how long it took, for example.

You can confirm that the data has been writen to the database by accessing the SQLite database directly.

 $ sqlite3 ~/node.3/db.sqlite
SQLite version 3.7.15.2 2013-01-09 11:53:05
Enter ".help" for instructions
Enter SQL statements terminated with a ";"
sqlite> select * from foo;
1|fiona

Note that this is the SQLite file that is under node 3, which is not the node that accepted the INSERT operation.

Bulk updates are supported. To execute multipe statements in one HTTP call, separate each statement with a semicolon. An example of inserting two records is shown below:

curl -L -XPOST 'localhost:4001/db?pretty' -d '
INSERT INTO foo(name) VALUES("fiona");INSERT INTO foo(name) VALUES("sinead")
'

Transactions are supported. To execute statements within a transaction, add transaction to the URL. An example of the above operation executed within a transaction is shown below.

curl -L -XPOST 'localhost:4001/db?pretty&transaction' -d '
INSERT INTO foo(name) VALUES("fiona");INSERT INTO foo(name) VALUES("sinead")
'

When a transaction takes place either both statements will succeed, or neither. Performance is much, much better if multiple SQL INSERTs or UPDATEs are executed via a transaction.

Qeurying data is easy. Simply perform a HTTP GET, setting the query statement as the query parameter q:

curl -L -G localhost:4001/db?pretty --data-urlencode 'q=SELECT * from foo'

An alternative approach is to read the database via sqlite3, the command-line tool that comes with SQLite. As long as you can be sure the file you access is under the leader, the records returned will be accurate and up-to-date.

rqlite replicates SQLite for fault-tolerance. It does not replicate it for performance. In fact performance is reduced somewhat due to the network round-trips.

Depending on your machine, individual INSERT performance could be anything from 1 operation per second to more than 10 operations per second. However, by using transactions, throughput will increase significantly, often by 2 orders of magnitude. This speed-up is due to the way SQLite works. So for high throughput, execute as many operations as possible within a single transaction.

An Administration API exists, which dumps some basic diagnostic and statistical information, as well as basic information about the underlying Raft node. Assuming rqlite is started with default settings, the endpoints are available like so:

curl localhost:4001/raft?pretty
curl localhost:4001/diagnostics?pretty
curl localhost:4001/statistics?pretty

The use of the URL param pretty is optional, and results in pretty-printed JSON responses.

rqlite does perform log compaction. After a configurable number of changes to the log, rqlite snapshots the SQLite database. And at start-up rqlite loads any existing snapshot.

Review issue #14 to learn more about how snapshots affect node restart time.

• SQLite commands such as .schema are not handled.
• Using PRAGMA directives has not been tested either.
• The supported types are those supported by go-sqlite3.

This is new software, so it goes without saying it has bugs. It's by no means finished -- issues are now being tracked, and I plan to develop this project further. Pull requests are also welcome.

rqlite reports a small amount anonymous data to Loggly, each time it is launched. This data is just the host operating system and system architecture and is only used to track the number of rqlite deployments. Reporting can be disabled by passing -noreport=true to rqlite at launch time.

This project uses the go-raft implementation of the Raft consensus protocol, and was inspired by the raftd reference implementation. rqlite also borrows some ideas from etcd, and uses go-sqlite3 to talk to the SQLite database.