Introduction to Thread Pools
Some services work asynchronously, hence they require thread pools to perform their tasks. All thread pooling facilities are centralized behind the ExecutionService interface.
Let's start with a bit of theory.
What ExecutionService provides
ExecutionService is an interface providing:
ScheduledExecutorService to schedule tasks, i.e.: tasks that happen repeatedly after a configurable delay.
Unordered
ExecutorService to execute tasks as soon as a thread is available.
Ordered
ExecutorService to execute tasks as soon as a thread is available, with the guarantee that tasks are going to be executed in the order they were submitted.
Available ExecutionService implementations
There currently are two bundled implementations:
OnDemandExecutionService creates a new pool each time an executor service (scheduled or not) is requested. This implementation is the default one and requires no configuration at all.
PooledExecutionService keeps a configurable set of thread pools and divides them to handle all executor service requests. This implementation must be configured with a
PooledExecutionServiceConfiguration when used.
Configuring PooledExecutionService
When you want total control of the threads used by a cache manager and its caches, you have to use a PooledExecutionService that itself must be configured as it does not have any defaults.
The PooledExecutionServiceConfigurationBuilder can be used for this purpose, and the resulting configuration it builds can simply be added to a CacheManagerBuilder to switch the ExecutionService implementation to a PooledExecutionService.
The builder has two interesting methods:
defaultPool that is used to set the default pool. There can be only one default pool, its name does not matter, and if thread-using services do not specify a thread pool, this is the one that will be used.
pool that is used to add a thread pool. There can be as many pools as you wish but services must explicitly be configured to make use of them.
Using the configured thread pools
Following is the list of services making use of ExecutionService:
Disk store: disk writes are performed asynchronously.
OffHeapDiskStoreConfiguration is used to configure what thread pool to use at the cache level, while OffHeapDiskStoreProviderConfiguration is used to configure what thread pool to use at the cache manager level.
Write Behind:
CacheLoaderWriter write tasks happen asynchronously.
DefaultWriteBehindConfiguration is used to configure what thread pool to use at the cache level, while WriteBehindProviderConfiguration is used to configure what thread pool to use at the cache manager level.
Eventing: produced events are queued and sent to the listeners by a thread pool.
DefaultCacheEventDispatcherConfiguration is used to configure what thread pool to use at the cache level, while CacheEventDispatcherFactoryConfiguration is used to configure what thread pool to use at the cache manager level.
The different builders will make use of the right configuration class, you do not have to use those classes directly. For instance, calling CacheManagerBuilder.withDefaultDiskStoreThreadPool(String threadPoolAlias) actually is identical to calling CacheManagerBuilder.using(new OffHeapDiskStoreProviderConfiguration(threadPoolAlias)).
The thread pool to use can be configured on a service through the builders by using the methods carrying a ThreadPool related name. When a service is not told anything about which thread pool to use, the default thread pool is used.