The registry
As Pacemaker Community Edition (CE) is built to run in single and multithreaded/multiprocess environments, it needs a possibility to share the data between the components in a single process and between threads and processes.
Whenever data has to be shared, the registry will be the right place.
The default implementation only works in a single-threaded environment, where it does not need to take care of concurrency issues that will occur in multithreaded or multiprocess ones.
Further Thoughts regard multithreading and multiprocessing
As the registry implementation comes with the core components only works in a single process, probably the choice in most use cases. Depending on the environment Pacemaker Community Edition (CE) will be used, it will be necessary to implement a registry processor implementation that fits these environments' needs.
To give you an example of what kind of changes will be necessary for a multithreaded environment, e.g.
like appserver.io we will implement the method mergeAttributesRecursive().
A thread safe version of this method would look like this:
namespace TechDivision\Import\Services;
class RegistryProcessor implements RegistryProcessorInterface
{
// other methods still go here
/**
* This method merges the passed attributes with an array that
* has already been added under the given key.
*
* If no value will be found under the passed key, the attributes
* will be registered.
*
* @param mixed $key The key of the attributes that have to be merged with the passed ones
* @param array $attributes The attributes that have to be merged with the existing ones
*
* @return void
* @throws \Exception Is thrown, if the already registered value is no array
* @link http://php.net/array_replace_recursive
*/
public function mergeAttributesRecursive($key, array $attributes)
{
// merge the passed attributes in a thread-safe manner
return $this->synchronized(function ($k, $a) {
// if the key not exists, simply add the new attributes
if (!isset($this->attributes[$k])) {
$this->attributes[$k] = $a;
return;
}
// if the key exists and the value is an array, merge it with the passed array
if (isset($this->attributes[$k]) && is_array($this->attributes[$k])) {
$this->attributes[$k] = array_replace_recursive($this->attributes[$k], $a);
return;
}
}, $key, $attributes);
// throw an exception if the key exists, but the found value is not of type array
throw new \Exception(sprintf('Can\'t merge attributes, because value for key %s already exists, but is not of type array', $key));
}
}