The term "publisher" means different things in different contexts. In general in messaginga publisher (also called "producer") is an application (or application instance)that publishes (produces) messages. The same application can also consume messagesand thus be a consumer at the same time.
Messaging protocols also have the concept of a lasting subscription for message delivery.Subscription is one term commonly used to describe such entity. Consumer is another.Messaging protocols supported by RabbitMQ use both terms but RabbitMQ documentation tends toprefer the latter.
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Publishers publish to a destination that varies from protocol to protocol. In AMQP 0-9-1,publishers publish to exchanges. In AMQP 1.0, publishing happens on a link.In MQTT, publishers publish to topics. Finally, STOMP supportsa variety of destination types: topics, queues, AMQP 0-9-1 exchanges. This is coveredin more details in the protocol-specific differences section.
A publish message has to be routed to a queue (topic, etc). The queue (topic) may have onlineconsumers. When the message is successfully routed to a queue and there isa consumer online that can accept more deliveries, the message will besent to the consumer.
Publishers can be more dynamic and begin publishing in reaction to a system event, stoppingwhen they are no longer necessary. This is common with WebSocket clientsused via Web STOMP and Web MQTT plugins,mobile clients and so on.
The process of publishing messages is quite similar in every protocol RabbitMQ supports.All four protocols allow the user to publish a message which has a payload (body) andone or more message properties (headers).
All four protocols also support an acknowledgement mechanism for publisherswhich allows the publishing application to keep track of the messages that have or haven't beensuccessfully accepted by the broker, and continue publishing the next batch or retry publishingthe current one.
In AMQP 0-9-1, publishing happens on a channel to an exchange.The exchange uses a routing topology set up by defining bindings betweenone or more queues and the exchange, or source exchange and destination exchange.Successfully routed messages are stored in queues.
Publishers can provide a hint to the server that the published message on the topicmust be retained (stored for future delivery to new subscribers). Only the latest published message for each topiccan be retained.
STOMP provides a way for the server to communicate an error in message processing back to the publisher.Its variation of publisher acknowledgements is called receipts, which is a feature clients enable when publishing.
More exchange types can be provided by plugins.Consistent hashing exchange, random routing exchange,internal event exchange and delayed message exchange areexchange plugins that ship with RabbitMQ. Like all plugins they must be enabledbefore they can be used.
When a published message cannot be routed to any queue (e.g. because there are no bindings defined for thetarget exchange), and the publisher set the mandatory message property to false (this is the default),the message is discarded or republished to an alternate exchange, if any.
When a published message cannot be routed to any queue, and the publisher set the mandatorymessage property to true, the message will be returned to it. The publisher must have a returnedmessage handler set up in order to handle the return (e.g. by logging an error or retrying witha different exchange).
Alternate Exchanges is an AMQP 0-9-1 exchange feature that lets clients handle messagesthat an exchange was unable to route (i.e. either because there were no bound queues or no matching bindings).Typical examples of this are detecting when clients accidentally or maliciously publish messages thatcannot be routed "or else" routing semantics where some messages are handled specially andthe rest by a generic handler.
Publishing to a new topic would set up a queue for it. Different topic/QoS level combinations willuse different queues with different properties. Publishers and consumers therefore must use the same QoS level.
Every delivery combines message metadata and delivery information. Different clientlibraries use slightly different ways of providing access to those properties. Typicallydelivery handlers have access to a delivery data structure.
The type property on messages is an arbitrary string that helps applications communicate what kindof message that is. It is set by the publishers at the time of publishing.The value can be any domain-specific string that publishers and consumers agree on.
Networks can fail in less-than-obvious ways and detecting some failures takes time.Therefore a client that's written a protocol frame or a set of frames (e.g. a published message) toits socket cannot assume that the message has reached the server and was successfully processed.It could have been lost along the way or its delivery can be significantly delayed.
Publisher confirms provide a mechanism for application developers to keep track of what messages have been successfully accepted by RabbitMQ. There are several commonly used strategiesfor using publisher confirms:
Most client libraries usually provide a way for developers to handle individual confirmationsas they arrive from the server. The confirms will arrive asynchronously. Since publishing isalso inherently asynchronous in AMQP 0-9-1, this option allows for safe publishing withvery little overhead. The algorithm is usually similar to this:
This strategy can be considered an anti-pattern and is documented primarily for completeness.It involves publishing a message and immediately waiting for the outstanding acknowledgement to arrive.It can be thought of as the above strategy with batch publishing where batch size equals to one.
Note that "exception" here means an error in the general sense; some programming languagesdo not have exceptions at all so clients there would communicate the error differently.The discussion and recommendations in this section should apply equally to most client librariesand programming languages.
The former type of exception can occur immediately during a write or with a certain delay.This is because certain types of I/O failures (e.g. to high network congestion or packet drop rate)can take time to detect. Publishing can continue after the connection recovers but if the connection is blocked due to an alarm, all further attemptswill fail until the alarm clears. This is covered in more details below in the Effects of Resource Alarms section.
The latter type of exception can only happen when the application developer provides a timeout.What timeout value is reasonable for a given application is decided by the developer.It should not be lower than the effective heartbeat timeout.
Metric collection and monitoring are as important for publishers as theyare for any other application or component in an application. Several metrics collectedby RabbitMQ are of particular interest when it comes to publishers:
The publishing and confirmation rates are mostly self-explanatory. The churn rates are so importantbecause they help detect applications that do not use connections or channels optimally and thusoffer sub-optimal publishing rates and waste resources.
Client libraries may also collect metrics. RabbitMQ Java client is oneexample. These metrics can provide insight into application-specific architecture (e.g. what publishingcomponent publishes unroutable messages) that RabbitMQ nodes cannot infer.
Concurrency topics are all about client library implementation specifics but somegeneral recommendations can be provided. In general, publishing on a shared "publishing context"(channel in AMQP 0-9-1, connection in STOMP, session in AMQP 1.0 and so on) should be avoidedand considered unsafe.
With a small number of concurrent publishers in a single application using one thread (or similar)per publisher is the optimal solution. With a large number (say, hundreds or thousands),use a thread pool.
Some applications open a new connection for every message published. This is highly inefficientand not how messaging protocols were designed to be used. Such condition can bedetected using connection metrics.
When connection is down, no publishes will go through or be internally enqueued (delayed)by clients. In addition, messages that were previously serialised and written to the socketare not guaranteed to reach the target node. It is therefore critically important for publishersthat need reliable publishing and data safety to use Publisher Confirms to keep track of whatpublishes were confirmed by RabbitMQ. Messages that were not confirmed should be considered undeliveredafter a period of time. Those messages can be republished if it's safe to do so for the application.This is covered in tutorial 7 and the Data Safety sectionin this guide.
A publisher can be successfully connected, authenticated and granted the permissions to publishto an exchange (topic, destination). However, it is possible that such messages would not berouted to any queues or consumers. This can be due to
Inspecting the topology and metrics usually helps narrow the problem quickly. For example,the individual exchange page in management UI can be used to confirmthat there is inbound message activity (ingress rate above zero) and what the bindings are.
When a resource alarm is in effect, all connections that publish will be blockeduntil the alarm clears. Clients can opt-in to receive a notification when theyare blocked. Learn more in the Resource Alarms guide.
With some protocols, such as AMQP 0-9-1 and STOMP, publishers can run into a condition knownas a protocol error (exception). For example, publishing to a non-existent exchange or bindingan exchange to a non-existent exchange will result in a channel exceptionand will render the channel closed. Publishing is not possible on a closed channel. Such eventsare logged by the RabbitMQ node the publisher was connected to. Failed publishing attemptswill also result in client-side exceptions or errors returned, depending on the client library used. 152ee80cbc
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