Guarantees of Sends and Commits
When committing or rolling back a transaction with Apache ActiveMQ Artemis, the request to commit or rollback is sent to the server, and the call will block on the client side until a response has been received from the server that the commit or rollback was executed.
When the commit or rollback is received on the server, it will be
committed to the journal, and depending on the value of the parameter
journal-sync-transactional the server will ensure that the commit or
rollback is durably persisted to storage before sending the response
back to the client. If this parameter has the value
false then commit
or rollback may not actually get persisted to storage until some time
after the response has been sent to the client. In event of server
failure this may mean the commit or rollback never gets persisted to
storage. The default value of this parameter is
true so the client can
be sure all transaction commits or rollbacks have been persisted to
storage by the time the call to commit or rollback returns.
Setting this parameter to
false can improve performance at the expense
of some loss of transaction durability.
This parameter is set in
Non Transactional Message Sends
If you are sending messages to a server using a non transacted session, Apache ActiveMQ Artemis can be configured to block the call to send until the message has definitely reached the server, and a response has been sent back to the client. This can be configured individually for durable and non-durable messages, and is determined by the following two URL parameters:
blockOnDurableSend. If this is set to
truethen all calls to send for durable messages on non transacted sessions will block until the message has reached the server, and a response has been sent back. The default value is
blockOnNonDurableSend. If this is set to
truethen all calls to send for non-durable messages on non transacted sessions will block until the message has reached the server, and a response has been sent back. The default value is
Setting block on sends to
true can reduce performance since each send
requires a network round trip before the next send can be performed.
This means the performance of sending messages will be limited by the
network round trip time (RTT) of your network, rather than the bandwidth
of your network. For better performance we recommend either batching
many messages sends together in a transaction since with a transactional
session, only the commit / rollback blocks not every send, or, using
Apache ActiveMQ Artemis's advanced asynchronous send acknowledgements feature
described in Asynchronous Send Acknowledgements.
When the server receives a message sent from a non transactional
session, and that message is durable and the message is routed to at
least one durable queue, then the server will persist the message in
permanent storage. If the journal parameter
journal-sync-non-transactional is set to
true the server will not
send a response back to the client until the message has been persisted
and the server has a guarantee that the data has been persisted to disk.
The default value for this parameter is
Non Transactional Acknowledgements
If you are acknowledging the delivery of a message at the client side
using a non transacted session, Apache ActiveMQ Artemis can be configured to block the
call to acknowledge until the acknowledge has definitely reached the
server, and a response has been sent back to the client. This is
configured with the parameter
BlockOnAcknowledge. If this is set to
true then all calls to acknowledge on non transacted sessions will
block until the acknowledge has reached the server, and a response has
been sent back. You might want to set this to
true if you want to
implement a strict at most once delivery policy. The default value is
Asynchronous Send Acknowledgements
If you are using a non transacted session but want a guarantee that every message sent to the server has reached it, then, as discussed in Guarantees of Non Transactional Message Sends, you can configure Apache ActiveMQ Artemis to block the call to send until the server has received the message, persisted it and sent back a response. This works well but has a severe performance penalty - each call to send needs to block for at least the time of a network round trip (RTT) - the performance of sending is thus limited by the latency of the network, not limited by the network bandwidth.
Let's do a little bit of maths to see how severe that is. We'll consider a standard 1Gib ethernet network with a network round trip between the server and the client of 0.25 ms.
With a RTT of 0.25 ms, the client can send at most 1000/ 0.25 = 4000 messages per second if it blocks on each message send.
If each message is < 1500 bytes and a standard 1500 bytes MTU (Maximum Transmission Unit) size is used on the network, then a 1GiB network has a theoretical upper limit of (1024 * 1024 * 1024 / 8) / 1500 = 89478 messages per second if messages are sent without blocking! These figures aren't an exact science but you can clearly see that being limited by network RTT can have serious effect on performance.
To remedy this, Apache ActiveMQ Artemis provides an advanced new feature called asynchronous send acknowledgements. With this feature, Apache ActiveMQ Artemis can be configured to send messages without blocking in one direction and asynchronously getting acknowledgement from the server that the messages were received in a separate stream. By de-coupling the send from the acknowledgement of the send, the system is not limited by the network RTT, but is limited by the network bandwidth. Consequently better throughput can be achieved than is possible using a blocking approach, while at the same time having absolute guarantees that messages have successfully reached the server.
The window size for send acknowledgements is determined by the confirmation-window-size parameter on the connection factory or client session factory. Please see Client Reconnection and Session Reattachment for more info on this.
To use the feature using the core API, you implement the interface
org.apache.activemq.artemis.api.core.client.SendAcknowledgementHandler and set
a handler instance on your
Then, you just send messages as normal using your
as messages reach the server, the server will send back an
acknowledgement of the send asynchronously, and some time later you are
informed at the client side by Apache ActiveMQ Artemis calling your handler's
sendAcknowledged(ClientMessage message) method, passing in a reference
to the message that was sent.
To enable asynchronous send acknowledgements you must make sure
confirmationWindowSize is set to a positive integer value, e.g.
Please see the examples chapter for a full working example.