bmqa_messageeventbuilder.h File Reference

Provide a builder for bmqa::MessageEvent objects. More...

#include <bmqa_message.h>
#include <bmqa_messageevent.h>
#include <bmqt_resultcode.h>
#include <bsl_memory.h>

Go to the source code of this file.

Classes
class	BloombergLP::bmqa::MessageEventBuilder
	A builder for MessageEvent objects. More...

Namespaces
	BloombergLP
	BloombergLP::bmqa

Detailed Description

This component implements a mechanism, bmqa::MessageEventBuilder, that can be used for creating message events containing one or multiple messages. The resulting MessageEvent can be sent to the BlazingMQ broker using the bmqa::Session (refer to bmqa_session.h for details).

The builder holds a bmqa::MessageEvent under construction, and provides methods to return a reference to the current message (in order to set its various members), as well as to append a new message. Once the application is done, the builder provides a method to get the message event that has been constructed. See Usage section for more details.

Note that publishing events containing multiple messages may be more efficient than events limited to a single message for applications that send large volume of small messages.

Usage

• An instance of bmqa::MessageEventBuilder (the builder) can be used to create multiple bmqa::MessageEvent's, as long as the instance is reset in between. This reset is preferably to do right after sending the event to guarantee that all user resources bound to the bmqa::MessageEvent (e.g. CorrelationIds with user's shared_ptr) are kept no longer than expected (e.g. until related ACK event is received). The recommended approach is to create one instance of the builder and use that throughout the lifetime of the task (if the task is multi-threaded, an instance per thread must be created and maintained). See usage example 1 for an illustration.
• The lifetime of an instance of the builder is bound by the bmqa::Session from which it was created. In other words, bmqa::Session instance must outlive the builder instance.
• If it is desired to post the same bmqa::Message to different queues, bmqa::MessageEventBuilder::packMessage can be called multiple times in a row with different queue IDs. The builder will append the previously packed message with the new queue ID to the underlying message event. Note that after calling bmqa::MessageEventBuilder::packMessage, the message keeps all the attributes (payload, properties, etc) that were previously set (except for the correlationId which must be set explicitly for each individual message). If desired, any attribute can be tweaked before being packing the message again. Refer to usage example 2 for an illustration.

Example 1 - Basic Usage

// Note that error handling is omitted below for the sake of brevity
 
bmqa::Session session;
    // Session start up logic omitted for brevity.
 
// Obtain a valid instance of message properties.
bmqa::MessageProperties properties;
session.loadMessageProperties(&properties);
 
// Set common properties that will be applicable to all messages sent by
// this application.
int rc = properties.setPropertyAsChar(
                                "encoding",
                                static_cast<char>(MyEncodingEnum::e_BER));
 
rc = properties.setPropertyAsString("producerId", "MyUniqueId");
 
// Obtain a valid instance of message event builder.
bmqa::MessageEventBuilder builder;
session.loadMessageEventBuilder(&builder);
 
// Create and post a message event containing 1 message.  Associate
// properties with this message.
bmqa::Message& msg = builder.startMessage();
 
msg.setCorrelationId(myCorrelationId);
 
// Set payload (where 'myPayload' is of type 'bdlbb::Blob')
msg.setDataRef(&myPayload);
 
// Set current timestamp as one of the properties.
rc = properties.setPropertyAsInt64(
             "timestamp",
             bdlt::EpochUtil::convertToTimeT64(bdlt::CurrentTime::now()));
 
// Set properties.
msg.setPropertiesRef(&properties);
 
// Pack the message.
rc = builder.packMessage(myQueueId);
 
// Post message event
rc = session.post(builder.messageEvent());
 
 
// Create and post another message event containing 1 message.
 
// bmqa::MessageEventBuilder must be reset before reuse.
builder.reset();
 
// Start a new message.
bmqa::Message& msg = builder.startMessage();
 
msg.setCorrelationId(myAnotherCorrelationId);
msg.setDataRef(&myAnotherPayload);
 
// It's okay (and recommended) to use same properties instance.
rc = properties.setPropertyAsInt64(
             "timestamp",
             bdlt::EpochUtil::convertToTimeT64(bdlt::CurrentTime::now()));
 
msg.setPropertiesRef(&properties);
rc = builder.packMessage(myAnotherQueueId);
 
// Post second message event
rc = session.post(builder.messageEvent());
 
// Reset the builder to free resources earlier.
builder.reset();

Example 2 - Packing multiple messages in a message event

// Note that error handling is omitted below for the sake of brevity
 
bmqa::Session session;
// Session start up logic omitted for brevity.
 
// Obtain a valid instance of message properties.
bmqa::MessageProperties properties;
session.loadMessageProperties(&properties);
 
// Set common properties that will be applicable to all messages sent by
// this application.
int rc = properties.setPropertyAsChar(
                                "encoding",
                                static_cast<char>(MyEncodingEnum::e_BER));
 
rc = properties.setPropertyAsString("producerId", "MyUniqueId");
 
// Obtain a valid instance of message event builder.
bmqa::MessageEventBuilder builder;
session.loadMessageEventBuilder(&builder);
 
// Create and post a message event containing 4 messages.
bmqa::Message& msg = builder.startMessage();
 
// Pack message #1
msg.setCorrelationId(correlationId1);
msg.setDataRef(&payload1);  // where 'payload1' is of type 'bdlbb::Blob'
 
// Set current timestamp as one of the properties.
int rc = properties.setPropertyAsInt64(
             "timestamp",
             bdlt::EpochUtil::convertToTimeT64(bdlt::CurrentTime::now()));
 
// Pack the message.
rc = builder.packMessage(queueId1);
 
// Pack message #2
// We want to send message #1 to another queue.  In order to do so, we
// just update the correlation ID of message #1.  There is no need to set
// the payload or properties again.  Because 'payload1' and 'properties'
// objects are being reused for the second message, they should not be
// destroyed before packing the second message.
 
msg.setCorrelationId(correlationId2);
 
// Also note that the "timestamp" property for the second message will be
// updated for this message.  There is no need to invoke
// 'setPropertiesRef' on the message though.
rc = properties.setPropertyAsInt64(
             "timestamp",
             bdlt::EpochUtil::convertToTimeT64(bdlt::CurrentTime::now()));
 
rc = builder.packMessage(queueId2);
 
// 'payload1' can be safely destroyed at this point if it will not be
// reused again for another message.
 
// Pack message #3
// Message #3 has a different payload, no properties and destined to
// 'queueId1'.
msg.setCorrelationId(correlationId3);
msg.setDataRef(&payload2);  // where 'payload2' is of type 'bdlbb::Blob'
 
// We need to explicitly clear out the associated properties.
msg.clearProperties();
 
rc = builder.packMessage(queueId1);
 
// Pack message #4
// Message #4 has different payload and destined to 'queueId3'.
msg.setCorrelationId(correlationId4);
msg.setDataRef(&payload3);  // where 'payload3' is of type 'bdlbb::Blob'
 
// Update "timestamp" property.
rc = properties.setPropertyAsInt64(
             "timestamp",
             bdlt::EpochUtil::convertToTimeT64(bdlt::CurrentTime::now()));
 
// Need to associate properties with the message, since they were cleared
// out while packing message #3 above.
msg.setPropertiesRef(&properties);
 
rc = builder.packMessage(queueId3);
 
// Post second message event
rc = session.post(builder.messageEvent());
 
// Reset the builder to free resources earlier.
builder.reset();

Thread Safety

This component is NOT thread safe.