In late 2014 DataStax published a blog post in which they explain how Cassandra can be leveraged in order to reach consensus in a distributed environment. A detailed explanation can be found in mentioned article. In essence, a distributed lock or mutex mechanism to enforce that only one resource in a distributed and concurrent environment can be changed at the same time. Inspired by said article LACK [luhk] was implemented. It is a very thin Java API on top of a few Cassandra queries. It is not meant as a consensus library such as libraft or atomix. We just needed something implemented fast and on top of Cassandra.

How to use it

Creating a keyspace

In order to run LACK you have to point it to a keyspace in which it can create necessary table.

CREATE KEYSPACE lack
    WITH REPLICATION = { 'class' : 'SimpleStrategy', 'replication_factor' : 1 };

Locking

You can import LACK from Maven Central Repository:

<dependency>
    <groupId>io.datanerds</groupId>
    <artifactId>lack</artifactId>
    <version>0.1.1</version>
</dependency>

The configuration of LACK is done via the LackConfig POJO, e.g. using this constructor: LackConfig(String username, String password, String[] nodes, String keyspace, int ttlInSeconds).

Connecting to a a local cluster using the previously created keyspace and a TTL of 42 seconds:

LackConfig config = new LackConfig("user", "pass", new String[]{"127.0.0.1"}, "lack", 42);
Lack lack = new Lack(config, "owner-1");

// Locking the resource:
lack.acquire("user-1");
...
// Renewing the lock in case we need longer than 42 seconds:
lack.renew("user-1");
...
// Claiming the lock results in renewal in case we're already owning it and acquiring otherwise
lack.claim("user-1");
...
// Releasing the lock in case we have finished our operation:
lack.release("user-1");

In case one of the above mentioned methods can not be executed since e.g. a resource might already be locked, a LackException will be thrown. To make sure you implement a logic for such cases, it is a checked exception.

Build

A gradle build will build the project. The test will start an embedded cassandra server. If the used ports are already in use the tests will fail.