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<?xml version="1.0" encoding="utf-8"?>
<!--
Licensed to the Apache Software Foundation (ASF) under one
or more contributor license agreements. See the NOTICE file
distributed with this work for additional information
regarding copyright ownership. The ASF licenses this file
to you under the Apache License, Version 2.0 (the
"License"); you may not use this file except in compliance
with the License. You may obtain a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing,
software distributed under the License is distributed on an
"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
KIND, either express or implied. See the License for the
specific language governing permissions and limitations
under the License.
-->
<section id="Message-Groups-Guide">
<title>Using Message Groups</title>
<para>
This section describes how messaging applications can use the Message Group feature
provided by the C++ Broker.
</para>
<note>
The content of this section assumes the reader is familiar with the Message Group
feature as described in the AMQP Messaging Broker (C++) user's guide. Please read the
section <emphasis>Using Message Groups</emphasis> in the user's guide before using the
examples given in this section.
</note>
<section role="h2" id="messagegroups-setup">
<title>Creating Message Group Queues</title>
<para>
The following examples show how to create a message group queue that enforces
ordered group consumption across multiple consumers.
</para>
<example>
<title>Message Group Queue Creation - Python</title>
<programlisting lang="python">
sender = connection.session().sender("msg-group-q;" +
" {create:always, delete:receiver," +
" node: {x-declare: {arguments:" +
" {'qpid.group_header_key':'THE-GROUP'," +
" 'qpid.shared_msg_group':1}}}}")
</programlisting>
</example>
<example>
<title>Message Group Queue Creation - C++</title>
<programlisting lang="c++">
std::string addr("msg-group-q; "
" {create:always, delete:receiver,"
" node: {x-declare: {arguments:"
" {qpid.group_header_key:'THE-GROUP',"
" qpid.shared_msg_group:1}}}}");
Sender sender = session.createSender(addr);
</programlisting>
</example>
<example>
<title>Message Group Queue Creation - Java</title>
<programlisting lang="java">
Session s = c.createSession(false, Session.CLIENT_ACKNOWLEDGE);
String addr = "msg-group-q; {create:always, delete:receiver," +
" node: {x-declare: {arguments:" +
" {'qpid.group_header_key':'THE-GROUP'," +
" 'qpid.shared_msg_group':1}}}}";
Destination d = (Destination) new AMQAnyDestination(addr);
MessageProducer sender = s.createProducer(d);
</programlisting>
</example>
<para>
The example code uses the x-declare map to specify the message group configuration
that should be used for the queue. See the AMQP Messaging Broker (C++) user's guide
for a detailed description of these arguments. Note that the
qpid.group_header_key's value MUST be a string type.
</para>
</section>
<section role="h2" id="messagegroups-sending">
<title>Sending Grouped Messages</title>
<para>
When sending grouped messages, the client must add a message property containing the
group identifier to the outgoing message. The group identifier must be a string
type. The key used for the property must exactly match the value passed in the
'qpid.group_header_key' configuration argument.
</para>
<example>
<title>Sending Grouped Messages - Python</title>
<programlisting lang="python">
group = "A"
m = Message(content="some data", properties={"THE-GROUP": group})
sender.send(m)
group = "B"
m = Message(content="some other group's data", properties={"THE-GROUP": group})
sender.send(m)
group = "A"
m = Message(content="more data for group 'A'", properties={"THE-GROUP": group})
sender.send(m)
</programlisting>
</example>
<example>
<title>Sending Grouped Messages - C++</title>
<programlisting lang="C++">
const std::string groupKey("THE-GROUP");
{
Message msg("some data");
msg.getProperties()[groupKey] = std::string("A");
sender.send(msg);
}
{
Message msg("some other group's data");
msg.getProperties()[groupKey] = std::string("B");
sender.send(msg);
}
{
Message msg("more data for group 'A'");
msg.getProperties()[groupKey] = std::string("A");
sender.send(msg);
}
</programlisting>
</example>
<example>
<title>Sending Grouped Messages - Java</title>
<programlisting lang="java">
String groupKey = "THE-GROUP";
TextMessage tmsg1 = s.createTextMessage("some data");
tmsg1.setStringProperty(groupKey, "A");
sender.send(tmsg1);
TextMessage tmsg2 = s.createTextMessage("some other group's data");
tmsg2.setStringProperty(groupKey, "B");
sender.send(tmsg2);
TextMessage tmsg3 = s.createTextMessage("more data for group 'A'");
tmsg3.setStringProperty(groupKey, "A");
sender.send(tmsg3);
</programlisting>
</example>
<para>
The examples above send two groups worth of messages to the queue created in the
previous example. Two messages belong to group "A", and one belongs to group
"B". Note that it is not necessary to complete sending one group's messages before
starting another. Also note that there is no need to indicate to the broker when a
new group is created or an existing group retired - the broker tracks group state
automatically.
</para>
</section>
<section role="h2" id="messagegroups-receiving">
<title>Receiving Grouped Messages</title>
<para>
Since the broker enforces group policy when delivering messages, no special actions
are necessary for receiving grouped messages from the broker. However, applications
must adhere to the rules for message group consumption as described in the AMQP
Messaging Broker (C++) user's guide. Refer to the section <emphasis>Well Behaved
Consumers</emphasis> for details.
</para>
</section>
</section>
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