Distributed locking - tarunchhabra/parakalo GitHub Wiki

**The purpose of a lock is to ensure that among several nodes that might try to do the same piece of work, only one actually does it (at least only one at a time). **

That work might be- to write some data to a shared storage system, to perform some computation, to call some external API, or suchlike.

At a high level, there are two reasons why you might want a lock in a distributed application:

a) for efficiency or b) for correctness

To distinguish these cases, you can ask what would happen if the lock failed:

**Efficiency: ** - Taking a lock saves you from unnecessarily doing the same work twice (e.g. some expensive computation). If the lock fails and two nodes end up doing the same piece of work, the result is a minor increase in cost (you end up paying 5 cents more to AWS than you otherwise would have) or a minor inconvenience (e.g. a user ends up getting the same email notification twice).

**Correctness: ** - Taking a lock prevents concurrent processes from stepping on each others’ toes and messing up the state of your system. If the lock fails and two nodes concurrently work on the same piece of data, the result is a corrupted file, data loss, permanent inconsistency, the wrong dose of a drug administered to a patient, or some other serious problem.

=> Both are valid cases for wanting a lock, but you need to be very clear about which one of the two you are dealing with.

Locks are important for correctness, and that it is a serious bug if two different nodes concurrently believe that they are holding the same lock.

Protecting a resource with a lock

It’s important to remember that a lock in a distributed system is not like a mutex in a multi-threaded application. It’s a more complicated beast, due to the problem that different nodes and the network can all fail independently in various ways.

https://martin.kleppmann.com/2016/02/08/how-to-do-distributed-locking.html

https://docs.hazelcast.org/docs/3.2/manual/html/lock.html

https://docs.hazelcast.org/docs/3.6/manual/html-single/index.html#hazelcast-clusters