This article overviews implementation behind interactions between consumers and queue.
The main players are
ConsumerTarget is the broker-side representation of a consuming client. Due to multi-queue consumers a
ConsumerTarget has one or more
Consumers associated with one
Queue each. It is this
Consumer that interacts with the
Queue is responsible for notification of at least one interested
Consumer when there is work to be done (message to consume).
Consumer is responsible for notification of its
Queue when it is ready to do some work (for example, consume messages). When notified by a
Queue of available work, a
Consumer MUST try to pull messages of said
Queue until either it notifies the
Queue that it is no longer interested OR there are no more messages available on the
Queue (i.e., the Queue does not return a message).
Messagearrives on the
Queuenotifies some of interested
Consumersthat there is work to be done
ConsumerTargetthat they would like to do work
Sessionsthat they would like to do work
Connectionsthat they would like to do work
Connectionsschedule themselves. This is the switch from the incoming
Schedulerkicks off a IO-Thread to process the work of a
Connectioniterates over its
Sessionsthat want to do work
Sessionsiterate over its
ConsumerTargetsthat want to do work
ConsumerTargetsiterate over its
Consumersthat want to do work
The consuming part is always invoked from the consuming connection‘s IO-Thread whereas the publishing part might be invoked from different threads (producing connection’s IO-Thread, Housekeeping thread for held or TTLed messages, a consuming connection's IO-Thread in case for message reject).
Therefore, the interfaces between
Consumers and the
Queue MUST be thread-safe and SHOULD be lock free.
These are the interfaces between
Queues and the scenarios when they are called.
AbstractQueue#setNotifyWorkDesiredCalled by the
Consumerto notify the
Queuewhether it is interested in doing work or not.
QueueConsumer#notifyWorkCalled by the
Queueto notify the
Consumerthat there is potentially work available.
AbstractQueue#deliverSingleMessageCalled by the
Consumerto get a message from the Queue.
QueueConsumerManager (QCM for short) keeps track of the state of
Consumers from the perspective of the
Queue. It shares and decides which
Consumer to notify of work with the
Queue. To do this in a performant way it maintains a number of lists and moves
Consumers between those lists to indicate state change. The lists it maintains are:
Typically we want these lists to be thread-safe and give us O(1) access/deletion if we know the element and O(1) size information. Unfortunately there is no data structure in the Java standard library with those characteristics which is why they are based on our own data structure
QueueConsumerNodeList is the underlying data structure of all of QCM‘s lists. It is thread-safe and allows O(1) appending and given you have a pointer to an entry O(1) deletion. It is essentially a singly linked list. To achieve O(1) deletion entries are marked for deletion but only actually removed upon the next iteration. The rationale being that, to delete an entry you would need to update the previous entry’s “next” pointer but to get to the previous element you would need a doubly linked list which it impossible to maintain in a thread-safe way without locking. Special care must be taken when removing elements from the tail since we keep an explicit reference to it in the
QueueConsumerNodeList to achieve O(1) appending. The data structure in the
QueueConsumerNodeList are called
QueueConsumerNodeListEntries which themselves have a reference to a
QueueConsumerNode which is the persistent entity that moves between QCM's lists and has a reference to the
QueueConsumer itself also has a reference to the
QueueConsumerNode to enable O(1) deletion given a Consumer. This tightly couples the
QueueConsumer and QCM classes.
All list contains all
Consumers registered with the
Consumers are added to this list when they are created and only removed when the
Consumer is closed. This list is necessary to be able to iterate over all consumers in a thread-safe way without locking. The danger of using several lists instead of a single
All list is that you might miss a
Consumer if it moves between lists during iteration.
This is a list of
Consumers that do not acquire messages for example
Queue Browsers. These need to be handled separately because they should always be notified about new messages. Where they kept in the same list as the acquiring consumers we would have to iterate of the entire list to make sure we did not miss a non-acquiring consumer. Non-acquiring consumers can only move between the “NonAcquiring” and “NotInterested” lists.
This list contains all
Consumers that indicated to the
Queue that they currently are not interested in doing any work (i.e., taking messages). This typically happens when a Consumer/Connection is suspended due to FlowControl/TCP backpressure. The main purpose of this list is to avoid spurious wake-ups of
Consumers which we know are not going to do any work.
This is the default list for acquiring
Consumers. It signifies that they are ready to process messages. When a message becomes available, the
Queue will notify
Consumers from this list and move them to the “Notified” list. It will only notify a single interested
Consumer to avoid spurious wake-ups.
Once an acquiring
Consumer is notified that there is work to do it is moved from the “Interested” list to the “Notified” list. The QCM expects such a
Consumer to either indicate that it is no longer interested (e.g., it became suspended in the meantime and therefore will not do the work we expected it to) or call
Consumer should remain in the “Notified” list and continue to call
deliverSingleMessage cannot deliver a message to it any more, in which case it is moved to the back of the “Interested” list. This is to decrease latency due to wake-ups when there continues to be work available (i.e., there is a steady stream of messages). Appending it to the end of the “Interested” list ensures some level of fairness. Note that this is not perfect. It is possible that a consumer is notified but at the time it tries to pull a message of the Queue there no longer is a message available and the Consumer is returned to the end of the “Interested” list without having done work. The assumption is that while this may happen it is unlikely to always happen to the same consumer leading to a kind of “asymptotic fairness”.
When deciding which
Consumer to notify the QCM should take consumer priorities into account. To do this in a performant way it maintains a
QueueConsumerNodeList per consumer priority in a list of
PriorityConsumerListPairs. This ensures that iteration of the Interested list happens in the right order and lookup of consumers with higher priority can be performed efficiently.