[[clustered_lock]]
== Clustered Lock

A _clustered lock_ is a lock which is distributed and shared among all nodes in the {brandname} cluster and currently provides a way to execute
code that will be synchronized between the nodes in a given cluster.

=== Installation

In order to start using the clustered locks, you needs to add the dependency in your Maven `pom.xml` file:

.pom.xml
include::topics/dependencies/clustered_locks.adoc[]

Currently no extra configuration needs to be provided.

=== The ClusteredLockConfiguration

Currently there is a single type of `ClusteredLock` supported : non reentrant, NODE ownership lock.

[[clustered_lock_ownership]]
==== Ownership

* `NODE`
When a `ClusteredLock` is defined, this lock can be used from all the nodes in the {brandname} cluster.
When the ownership is NODE type, this means that the owner of the lock is the {brandname} node that acquired the lock
at a given time.
This means that each time we get a `ClusteredLock` instance with the `ClusteredCacheManager`, this instance will be the
same instance for each {brandname} node.
This lock can be used to synchronize code between {brandname} nodes. The advantage of this lock is that any thread in the
node can release the lock at a given time.

* `INSTANCE` - not yet supported

When a `ClusteredLock` is defined, this lock can be used from all the nodes in the {brandname} cluster.
When the ownership is INSTANCE type, this means that the owner of the lock is the actual instance we acquired when
`ClusteredLockManager.get("lockName")` is called.

This means that each time we get a `ClusteredLock` instance with the `ClusteredCacheManager`, this instance will be
a new instance.
This lock can be used to synchronize code between {brandname} nodes and inside each {brandname} node.
The advantage of this lock is that only the instance that called 'lock' can release the lock.

==== Reentrancy

When a `ClusteredLock` is configured reentrant, the owner of the lock can reacquire the lock as many consecutive
times as it wants while holding the lock.

Currently, only non reentrant locks are supported. This means that when two consecutive `lock` calls are sent for the same
owner, the first call will acquire the lock if it's available, and the second call will block.

=== The ClusteredLockManager interface.

The `ClusteredLockManager` interface, *marked as experimental*, is the entry point to define, retrieve and remove a lock.
It automatically listen to the creation of `EmbeddedCacheManager` and proceeds with the registration  of an
instance of it per `EmbeddedCacheManager`.
It starts the internal caches needed to store the lock state.

Retrieving the `ClusteredLockManager` is as simple as invoking the `EmbeddedClusteredLockManagerFactory.from(EmbeddedCacheManager)`
as shown in the example below:

[source,java]
----
// create or obtain your EmbeddedCacheManager
EmbeddedCacheManager manager = ...;

// retrieve the ClusteredLockManager
ClusteredLockManager clusteredLockManager = ClusteredLockManagerFactory.from(manager);
----

[source,java]
----
@Experimental
public interface ClusteredLockManager {

   boolean defineLock(String name);

   boolean defineLock(String name, ClusteredLockConfiguration configuration);

   ClusteredLock get(String name);

   ClusteredLockConfiguration getConfiguration(String name);

   boolean isDefined(String name);

   CompletableFuture<Boolean> remove(String name);

   CompletableFuture<Boolean> forceRelease(String name);
}
----

* `defineLock` :
Defines a lock with the specified name and the default `ClusteredLockConfiguration`. It does not overwrite
existing configurations.

* `defineLock(String name, ClusteredLockConfiguration configuration)` :
Defines a lock with the specified name and `ClusteredLockConfiguration`. It does not overwrite existing
configurations.

* `ClusteredLock get(String name)` :
Get’s a `ClusteredLock` by it’s name. A call of `defineLock` must be done at least once in the cluster.
See link:#clustered_lock_ownership[ownership] level section to understand the implications of `get` method call.

Currently, the only ownership level supported is *NODE*.

* `ClusteredLockConfiguration getConfiguration(String name)` :

Returns the configuration of a `ClusteredLock`, if such exists.

* `boolean isDefined(String name)` :
Checks if a lock is already defined.

* `CompletableFuture<Boolean> remove(String name)` :
Removes a `ClusteredLock` if such exists.

* `CompletableFuture<Boolean> forceRelease(String name)` :
Releases - or unlocks - a `ClusteredLock`, if such exists, no matter who is holding it at a given time.
Calling this method may cause concurrency issues and has to be used in *exceptional situations*.

[[clustered_lock_interface]]
=== The Clustered Lock

`ClusteredLock` interface, *marked as experimental*, is the interface that implements the clustered locks.

[source,java]
----
@Experimental
public interface ClusteredLock {

   CompletableFuture<Void> lock();

   CompletableFuture<Boolean> tryLock();

   CompletableFuture<Boolean> tryLock(long time, TimeUnit unit);

   CompletableFuture<Void> unlock();

   CompletableFuture<Boolean> isLocked();

   CompletableFuture<Boolean> isLockedByMe();
}

----

* `lock` :
Acquires the lock. If the lock is not available then call blocks until the lock is acquired.
Currently, *there is no maximum time specified for a lock request to fail*, so this could cause thread starvation.

* `tryLock`
Acquires the lock only if it is free at the time of invocation, and returns `true` in that case. This method does not
block (or wait) for any lock acquisition.

* `tryLock(long time, TimeUnit unit)`
If the lock is available this method returns immediately with `true`.
If the lock is not available then the call waits until :
   - The lock is acquired
   - The specified waiting time elapses

If the time is less than or equal to zero, the method will not wait at all.

* `unlock`

Releases the lock. Only the holder of the lock may release the lock.

* `isLocked`
Returns `true` when the lock is locked and `false` when the lock is released.

* `isLockedByMe`
Returns `true` when the lock is owned by the caller and `false` when the lock is owned by someone else or it's released.

[[clustered_lock_usage]]
==== Usage Examples

[source,java]
----
  EmbeddedCache cm = ...;
  ClusteredLockManager cclm = ClusteredLockManagerFactory.from(cm);

  lock.tryLock()
    .thenCompose(result -> {
       if (result) {
        try {
            // manipulate protected state
            } finally {
               return lock.unlock();
            }
       } else {
          // Do something else
       }
    });
 }
----
