## Publish/Subscribe and subscriptions to topics

SAM allows messages to be sent either to queues or, for WebSphere MQ and WPM, to publish/subscribe topics. A topic destination is specified to SAM in the usual way, i.e. in the form 'topic://fred', rather than the form 'queue://AQUEUE' used for point to point operation. To use publish/subscribe it is simply necessary to specify the correct broker name on the SAMConnect "connect" call and the desired topic in the destination argument to the SAMConnect "send" and "receive" calls. The PHP interface is otherwise identical to the point to point model.

By default, SAM creates non-durable subscriptions when using publish/subscribe. This means that if a client application is inactive when messages are published to a topic, then it will not receive them when it subsequently restarted. SAM does also allow durable subscriptions to be made to topics when using WPM or WebSphere MQ publish/subscribe. The purpose of these subscriptions is to allow data to be received by a client application even if that client was not active at the time the data was published.

Durable subscriptions are specified by using the SAMConnect "subscribe" call. This method takes the destination topic as an input parameter and returns a subscription identifier that may be used on subsequent "receive" calls. When the subscription is no longer required the SAMConnection "unsubscribe" method should be used to delete the subscription.

Example #1 Creating a durable subscription to a topic

`<?php
$subName = $conn->subscribe('topic://A');
if (!$subName) {
echo "Subscribe failed";
} else {
# Subscribe was OK
// ...
}
?>`

Example #2 Subscribing to a topic using a WebSphere Platform Messaging (WPM) server

`<?php
$conn = new SAMConnection();
// Note: For pub/sub on WPM, when connecting the name of a messaging engine
// to hold the durable subscription (SAM_WPM_DUR_SUB_HOME) must be specified.
$conn->connect(SAM_WMQ, array(SAM_ENDPOINTS => 'localhost:7278:BootstrapBasicMessaging',
SAM_BUS => 'Bus1',
SAM_TARGETCHAIN => 'InboundBasicMessaging',
SAM_WPM_DUR_SUB_HOME => 'MyMachineNode01.server1-Bus1'));
$subName = $conn->subscribe('topic://A');
if (!$subName) {
echo "Subscribe failed";
} else {
# Subscribe was OK
// ...
}
?>`

Example #3 Receiving published data using a durable subscription

`<?php
$msg = $conn->receive($subName);
if ($msg) {
echo "Received a message OK";
} else {
echo "The receive failed";
}
?>`

Example #4 Deleting a durable subscription to a topic

`<?php
if (!$conn->unsubscribe($subName)) {
echo "Unsubscribe failed";
}
?>`