This post was originally published on datanerds.io in March 2016. I’ve republished it here as part of consolidating my writing on this blog. The original content remains unchanged.

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.

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");
...
// 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.

To find some more resource take a look here: GitHub, Maven Central