This guide covers various topics related to channels, an AMQP 0-9-1-specific abstraction.Channels cannot exist without a connection, so getting familiar with the Connections guide firstis highly recommended.
Some applications need multiple logical connections to thebroker. However, it is undesirable to keep many TCPconnections open at the same time because doing so consumessystem resources and makes it more difficult to configurefirewalls. AMQP 0-9-1 connections are multiplexed withchannels that can be thought of as "lightweightconnections that share a single TCP connection".
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Every protocol operation performed by a client happens on a channel.Communication on a particular channel is completely separatefrom communication on another channel, therefore every protocolmethod also carries a channel ID (a.k.a. channel number), an integerthat both the broker and clients use to figure out which channel the method is for.
Much like connections, channels are meant to be long lived. That is, there is no need to opena channel per operation and doing so would be very inefficient, since opening a channel is anetwork roundtrip.
If a channel is closed immediately after a consumer acknowledged a number of deliveries on it,the acknowledgements may or may not reach their target queue before the channel is terminated.In this case the messages with a pending acknowledgement on the channel will be automatically requeuedfollowing the channel closure.
This scenario usually applies to workloads with short lived channels. Using long lived channels anddesigning consumers in a way that they can handle redeliveries potential surprises may beassociated with this edge case behavior. Note that redelivered messages will be explicitly marked as such.
Certain scenarios are assumed to be recoverable ("soft") errors in the protocol. They renderthe channel closed but applications can open another one and try to recover or retry a number oftimes. Most common examples are:
Any attempted operation on a closed channel will fail with an exception. Note that when RabbitMQcloses a channel, it notifies the client of that using an asynchronous protocol method. In other words,an operation that caused a channel exception won't fail immediately but a channel closure eventhandler will fire shortly after.
Some client libraries may use blocking operations that wait fora response. In this case they may communicate channel exceptions differently, e.g. usingruntime exceptions, an error type, or other means appropriate for the language.
Each channel consumes a relatively small amount of memory on the client. Depending on client library'simplementation detail it can also use a dedicated thread pool (or similar) where consumeroperations are dispatched, and therefore one or more threads (or similar).
Each channel also consumes a relatively small amount of memory on the node the client is connected to,plus a few Erlang processes. Since a node usually serves multiple channel connections, the effectsof excessive channel usage or channel leaks will primarily be reflected in RabbitMQ nodes' metricsand not those of clients.
Given both of these factors, limiting the number of channels used per connection is highly recommended.As a guideline, most applications can use a single digit number of channels per connection.Those with particularly high concurrency rates (usually such applications are consumers)can start with one channel per thread/process/coroutine and switch to channel poolingwhen metrics suggest that the original model is no longer sustainable, e.g. because it consumestoo much memory.
The lower value of the two is used: the client cannotbe configured to allow for more channels than the server configured maximum.Clients that attempt that will run into an error that looks like this in the logs:
Because they affect node resource use,the number of currently open channels and channel opening/closure rates are important metricsof the system that should be monitored. Monitoring them will help detect a number ofcommon problems:
The number of channels is displayed in the management UI on the Overview tab,as is the number of connections.By dividing the number of channels by the number of connectionsthe operator can determine an average number of channels per connection.
Overview and individual node pages provide a chart of channel churn rate as of RabbitMQ 3.7.9.If the rate of channel open operations is consistently higher than that of channel close operations,this is evidence of a channel leak in one of the applications:
A system is said to have high channel churn when its rate of newly opened channels is consistently high andits rate of closed channels is consistently high. This usually means that an applicationuses short lived channels or channels are often closed due to channel-level exceptions.
Management UI provides a chart of channel churn rate.Below is a chart that demonstrates a fairly low channel churn with a virtually identical number of channel open and closedin the given period of time:
While connection and disconnection rates are system-specific, rates consistently above 100/second likely indicate a suboptimalconnection management by one or more applications and usually are worth investigating.
rabbitmqctl list_connections and rabbitmqctl list_channels are theprimary commands for inspecting per-connection channel count and channel details such as the number ofconsumers, unacknowledged messages, prefetch and so on.
Channels that publish messages can outpace other parts of the system, most likely busy queues and queuesthat perform replication. When that happens, flow control is applied topublishing channels and, in turn, connections. Channels and connections that only consume messagesare not affected.
Monitoring systems can collect metrics on the number of connections in flow state.Applications that experience flow control regularly may consider to use separate connectionsto publish and consume to avoid flow control effects on non-publishing operations (e.g. queue management).
All snaps have a default track. Without the snap developer specifying otherwise, the default track is called latest. Similarly, when no track is specified, a snap will implicitly install from the latest track. The track can also be specified explicitly:
Developers can optionally choose whether to supplement the default latest track with additional tracks. The developer is also free to designate one of these as the default track to be installed when no further preference is given.
Developers must currently make a request for tracks to be added to their snap via the #store-requests forum category. Releases are verified and checked to ensure that reasonable user expectations are being met. For example, only 3.2.* versions are accepted into a 3.2 track.
Candidate releases are considered almost ready for going into stable, but need some additional real world experimentation before they move forward. Software reaching this stage will typically have passed all available QA and review processes, since users following it expect a high stability level. Should almost never break.
Beta is the first level towards the stabilisation of what was before a fast moving stream of changes. Specific projects may have slightly different terminology for such releases (alpha, beta, etc) but all of these are welcome on this risk level. These releases will almost certainly have passed some sort of review and QA, but may still have unfinished parts. Breaking changes are still relatively common here.
Edge releases often include a moving stream of changes without QA or review promises and are typically built automatically by a CI process from an arbitrary source code snapshot. Often the CI will only publish after some sort of automatic QA passed, and code reviews remain a good practice, but these are project specific. Assume edge releases may break often.
This option will not automatically refresh the snap to force the installationof a new snap. To switch channels and update the snap with a single command, add the --channel option to the refresh command:
A branch is an optional, fine-grained subdivision of a channel for a published snap that allows for the creation of short-lived sequences of snaps that can be pushed on demand by snap developers to help with fixes or temporary experimentation.
After 30 days with no further updates, a branch will be closed automatically. The replacement snap will then be chosen as it would be with closed channels (see below). For example, beta/fix-for-bug123 will fall back to beta after the fix-for-bug123 branch is closed.
For example, when a specific risk-level channel is closed, the snap store will select a snap from a more stable risk-level of the same track. If the original channel is re-opened, snaps will once again be selected from the original channel.
This approach is commonly used for beta testing. If a snap is following a beta channel that is then closed, the store will offer the snap from the candidate channel. If the candidate channel is not available, the snap from the stable channel will be selected. If the beta channel re-opens, the snap will once again be selected from that channel.
First, clients connect to the server using some transport, like WebSocket. Once connected, they join one or more topics. For example, to interact with a public chat room clients may join a topic called public_chat, and to receive updates from a product with ID 7, they may need to join a topic called product_updates:7.
Clients can push messages to the topics they've joined, and can also receive messages from them. The other way around, Channel servers receive messages from their connected clients, and can push messages to them too.
Broadcasts work even if the application runs on several nodes/computers. That is, if two clients have their socket connected to different application nodes and are subscribed to the same topic T, both of them will receive messages broadcasted to T. That is possible thanks to an internal PubSub mechanism. 152ee80cbc
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