Queue items to allow later processing.
The queue system allows placing items in a queue and processing them later. The system tries to ensure that only one consumer can process an item.
Before a queue can be used it needs to be created by DrupalQueueInterface::createQueue().
Items can be added to the queue by passing an arbitrary data object to DrupalQueueInterface::createItem().
To process an item, call DrupalQueueInterface::claimItem() and specify how long you want to have a lease for working on that item. When finished processing, the item needs to be deleted by calling DrupalQueueInterface::deleteItem(). If the consumer dies, the item will be made available again by the DrupalQueueInterface implementation once the lease expires. Another consumer will then be able to receive it when calling DrupalQueueInterface::claimItem(). Due to this, the processing code should be aware that an item might be handed over for processing more than once.
The $item object used by the DrupalQueueInterface can contain arbitrary metadata depending on the implementation. Systems using the interface should only rely on the data property which will contain the information passed to DrupalQueueInterface::createItem(). The full queue item returned by DrupalQueueInterface::claimItem() needs to be passed to DrupalQueueInterface::deleteItem() once processing is completed.
There are two kinds of queue backends available: reliable, which preserves the order of messages and guarantees that every item will be executed at least once. The non-reliable kind only does a best effort to preserve order in messages and to execute them at least once but there is a small chance that some items get lost. For example, some distributed back-ends like Amazon SQS will be managing jobs for a large set of producers and consumers where a strict FIFO ordering will likely not be preserved. Another example would be an in-memory queue backend which might lose items if it crashes. However, such a backend would be able to deal with significantly more writes than a reliable queue and for many tasks this is more important. See aggregator_cron() for an example of how can this not be a problem. Another example is doing Twitter statistics -- the small possibility of losing a few items is insignificant next to power of the queue being able to keep up with writes. As described in the processing section, regardless of the queue being reliable or not, the processing code should be aware that an item might be handed over for processing more than once (because the processing code might time out before it finishes).
|DrupalQueue||Factory class for interacting with queues.|
|MemoryQueue||Static queue implementation.|
|SystemQueue||Default queue implementation.|
|DrupalReliableQueueInterface||Reliable queue interface.|
modules/ system/ system.queue.inc, line 8
- Queue functionality.