251 lines
12 KiB
Markdown
251 lines
12 KiB
Markdown
# Flow Control
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Flow control is used to limit the flow of data between a client and
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server, or a server and another server in order to prevent the client or
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server being overwhelmed with data.
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## Consumer Flow Control
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This controls the flow of data between the server and the client as the
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client consumes messages. For performance reasons clients normally
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buffer messages before delivering to the consumer via the `receive()`
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method or asynchronously via a message listener. If the consumer cannot
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process messages as fast as they are being delivered and stored in the
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internal buffer, then you could end up with a situation where messages
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would keep building up possibly causing out of memory on the client if
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they cannot be processed in time.
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## Window-Based Flow Control
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By default, Apache ActiveMQ Artemis consumers buffer messages from the server in a
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client side buffer before the client consumes them. This improves
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performance: otherwise every time the client consumes a message,
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Apache ActiveMQ Artemis would have to go the server to request the next message. In
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turn, this message would then get sent to the client side, if one was
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available.
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A network round trip would be involved for *every* message and
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considerably reduce performance.
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To prevent this, Apache ActiveMQ Artemis pre-fetches messages into a buffer on each
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consumer. The total maximum size of messages (in bytes) that will be
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buffered on each consumer is determined by the `consumerWindowSize`
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parameter.
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By default, the `consumerWindowSize` is set to 1 MiB (1024 \* 1024
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bytes).
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The value can be:
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- `-1` for an *unbounded* buffer
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- `0` to not buffer any messages.
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- `>0` for a buffer with the given maximum size in bytes.
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Setting the consumer window size can considerably improve performance
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depending on the messaging use case. As an example, let's consider the
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two extremes:
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### Fast consumers
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Fast consumers can process messages as fast as they consume them (or
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even faster)
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To allow fast consumers, set the `consumerWindowSize` to -1. This
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will allow *unbounded* message buffering on the client side.
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Use this setting with caution: it can overflow the client memory if
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the consumer is not able to process messages as fast as it receives
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them.
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### Slow consumers
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Slow consumers takes significant time to process each message and it
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is desirable to prevent buffering messages on the client side so
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that they can be delivered to another consumer instead.
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Consider a situation where a queue has 2 consumers; 1 of which is
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very slow. Messages are delivered in a round robin fashion to both
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consumers, the fast consumer processes all of its messages very
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quickly until its buffer is empty. At this point there are still
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messages awaiting to be processed in the buffer of the slow consumer
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thus preventing them being processed by the fast consumer. The fast
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consumer is therefore sitting idle when it could be processing the
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other messages.
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To allow slow consumers, set `consumerWindowSize` on the URI to 0
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(for no buffer at all). This will prevent the slow consumer from
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buffering any messages on the client side. Messages will remain on
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the server side ready to be consumed by other consumers.
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Setting this to 0 can give deterministic distribution between
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multiple consumers on a queue.
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Most of the consumers cannot be clearly identified as fast or slow
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consumers but are in-between. In that case, setting the value of
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`consumerWindowSize` to optimize performance depends on the messaging
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use case and requires benchmarks to find the optimal value, but a value
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of 1MiB is fine in most cases.
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Please see [the examples chapter](examples.md) for an example which shows
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how to configure ActiveMQ Artemis to prevent consumer buffering when dealing
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with slow consumers.
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## Rate limited flow control
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It is also possible to control the *rate* at which a consumer can
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consume messages. This is a form of throttling and can be used to make
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sure that a consumer never consumes messages at a rate faster than the
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rate specified. This is configured using the `consumerMaxRate` URI
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parameter.
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The rate must be a positive integer to enable this functionality and is
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the maximum desired message consumption rate specified in units of
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messages per second. Setting this to `-1` disables rate limited flow
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control. The default value is `-1`.
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Please see [the examples chapter](examples.md) for a working example of
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limiting consumer rate.
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> **Note**
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>
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> Rate limited flow control can be used in conjunction with window based
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> flow control. Rate limited flow control only effects how many messages
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> a client can consume in a second and not how many messages are in its
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> buffer. So if you had a slow rate limit and a high window based limit
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> the clients internal buffer would soon fill up with messages.
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## Producer flow control
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Apache ActiveMQ Artemis also can limit the amount of data sent from a client to a
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server to prevent the server being overwhelmed.
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### Window based flow control
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In a similar way to consumer window based flow control, Apache ActiveMQ Artemis
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producers, by default, can only send messages to an address as long as
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they have sufficient credits to do so. The amount of credits required to
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send a message is given by the size of the message.
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As producers run low on credits they request more from the server, when
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the server sends them more credits they can send more messages.
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The amount of credits a producer requests in one go is known as the
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*window size* and it is controlled by the `producerWindowSize` URI
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parameter.
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The window size therefore determines the amount of bytes that can be
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in-flight at any one time before more need to be requested - this
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prevents the remoting connection from getting overloaded.
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#### Blocking producer window based flow control using CORE protocol
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When using the CORE protocol (used by both the Artemis Core Client and Artemis JMS Client)
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the server will always aim give the same number of credits as have been requested.
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However, it is also possible to set a maximum size on any address, and the server
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will never send more credits to any one producer than what is available according to
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the address's upper memory limit. Although a single producer will be issued more
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credits than available (at the time of issue) it is possible that more than 1
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producer be associated with the same address and so it is theoretically possible
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that more credits are allocated across total producers than what is available.
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It is therefore possible to go over the address limit by approximately:
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'''total number of producers on address * producer window size'''
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For example, if I have a queue called "myqueue", I could set the
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maximum memory size to 10MiB, and the the server will control the number
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of credits sent to any producers which are sending any messages to
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myqueue such that the total messages in the queue never exceeds 10MiB.
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When the address gets full, producers will block on the client side
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until more space frees up on the address, i.e. until messages are
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consumed from the queue thus freeing up space for more messages to be
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sent.
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We call this blocking producer flow control, and it's an efficient way
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to prevent the server running out of memory due to producers sending
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more messages than can be handled at any time.
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It is an alternative approach to paging, which does not block producers
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but instead pages messages to storage.
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To configure an address with a maximum size and tell the server that you
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want to block producers for this address if it becomes full, you need to
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define an AddressSettings ([Configuring Queues Via Address Settings](queue-attributes.md)) block for the address and specify
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`max-size-bytes` and `address-full-policy`
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The address block applies to all queues registered to that address. I.e.
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the total memory for all queues bound to that address will not exceed
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`max-size-bytes`. In the case of JMS topics this means the *total*
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memory of all subscriptions in the topic won't exceed max-size-bytes.
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Here's an example:
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<address-settings>
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<address-setting match="exampleQueue">
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<max-size-bytes>100000</max-size-bytes>
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<address-full-policy>BLOCK</address-full-policy>
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</address-setting>
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</address-settings>
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The above example would set the max size of the queue "exampleQueue"
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to be 100000 bytes and would block any producers sending to that address
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to prevent that max size being exceeded.
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Note the policy must be set to `BLOCK` to enable blocking producer flow
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control.
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> **Note**
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>
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> Note that in the default configuration all addresses are set to block
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> producers after 10 MiB of message data is in the address. This means
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> you cannot send more than 10MiB of message data to an address without
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> it being consumed before the producers will be blocked. If you do not
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> want this behaviour increase the `max-size-bytes` parameter or change
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> the address full message policy.
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> **Note**
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>
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> Producer credits are allocated from the broker to the client. Flow control
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> credit checking (i.e. checking a producer has enough credit) is done on the
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> client side only. It is possible for the broker to over allocate credits, like
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> in the multiple producer scenario outlined above. It is also possible for
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> a misbehaving client to ignore the flow control credits issued by the broker
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> and continue sending with out sufficient credit.
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#### Blocking producer window based flow control using AMQP
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Apache ActiveMQ Artemis ships with out of the box with 2 protocols that support flow control. Artemis CORE protocol and
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AMQP. Both protocols implement flow control slightly differently and therefore address full BLOCK policy behaves slightly
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different for clients that use each protocol respectively.
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As explained earlier in this chapter the CORE protocol uses a producer window size flow control system. Where credits
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(representing bytes) are allocated to producers, if a producer wants to send a message it should wait until it has
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enough byte credits available for it to send. AMQP flow control credits are not representative of bytes but instead
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represent the number of messages a producer is permitted to send (regardless of the message size).
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BLOCK for AMQP works mostly in the same way as the producer window size mechanism above. Artemis will issue 100 credits
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to a client at a time and refresh them when the clients credits reaches 30. The broker will stop issuing credits once an
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address is full. However, since AMQP credits represent whole messages and not bytes, it would be possible in some
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scenarios for an AMQP client to significantly exceed an address upper bound should the broker continue accepting
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messages until the clients credits are exhausted. For this reason there is an additional parameter available on address
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settings that specifies an upper bound on an address size in bytes. Once this upper bound is reach Artemis will start
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rejecting AMQP messages. This limit is the max-size-bytes-reject-threshold and is by default set to -1 (or no limit).
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This is additional parameter allows a kind of soft and hard limit, in normal circumstances the broker will utilize the
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max-size-bytes parameter using using flow control to put back pressure on the client, but will protect the broker by
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rejecting messages once the address size is reached.
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### Rate limited flow control
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Apache ActiveMQ Artemis also allows the rate a producer can emit message to be limited,
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in units of messages per second. By specifying such a rate, Apache ActiveMQ Artemis
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will ensure that producer never produces messages at a rate higher than
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that specified. This is controlled by the `producerMaxRate` URL parameter.
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The `producerMaxRate` must be a positive integer to enable this functionality and is
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the maximum desired message production rate specified in units of
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messages per second. Setting this to `-1` disables rate limited flow
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control. The default value is `-1`.
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Please see [the examples chapter](examples.md) for a working example of limiting
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producer rate.
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