Message Queue for Apache RocketMQ provides a distributed transaction processing feature similar to X/Open XA to ensure transaction consistency in Message Queue for Apache RocketMQ. This topic provides the sample code about how to send and subscribe to transactional messages by using HTTP client SDK for C++.

Background information

The following figure shows the interaction process of transactional messages. 事务消息流程图For more information, see Transactional messages.

Prerequisites

Before you begin, make sure that the following operations are complete:

  • SDK for C++ is installed. For more information, see Prepare an environment.
  • Resources are created. The resources that you want to specify in the code are created in advance. For example, the instance, topic, and group ID that you want to specify in the code are created in the Message Queue for Apache RocketMQ console in advance. For more information, see Create resources.

Send transactional messages

The following sample code provides an example on how to send transactional messages:

//#include <iostream>
#include <fstream>
#ifdef _WIN32
#include <windows.h>
#include <process.h>
#else
#include "pthread.h"
#endif
#include "mq_http_sdk/mq_client.h"

using namespace std;
using namespace mq::http::sdk;


const int32_t pubMsgCount = 4;
const int32_t halfCheckCount = 3;

void processCommitRollError(AckMessageResponse& bdmResp, const std::string& messageId) {
    if (bdmResp.isSuccess()) {
        cout << "Commit/Roll Transaction Suc: " << messageId << endl;
        return;
    }
    const std::vector<AckMessageFailedItem>& failedItems =
        bdmResp.getAckMessageFailedItem();
    for (std::vector<AckMessageFailedItem>::const_iterator iter = failedItems.begin();
            iter != failedItems.end(); ++iter)
    {
        cout << "Commit/Roll Transaction ERROR: " << iter->errorCode
            << "  " << iter->receiptHandle << endl;
    }
}

#ifdef WIN32
unsigned __stdcall consumeHalfMessageThread(void *arg)
#else
void* consumeHalfMessageThread(void *arg)
#endif
{
    MQTransProducerPtr transProducer = *(MQTransProducerPtr*)(arg);
    int count = 0;
    do {
        std::vector<Message> halfMsgs;
        try {
            // Consume messages in long polling mode. 
            // In long polling mode, if no message in the topic is available for consumption, the request is suspended on the broker for a specified period of time. If a message is available for consumption within the duration, a response is immediately sent to the consumer. In this example, the duration is 3 seconds. 
            transProducer->consumeHalfMessage(
                    1, // The maximum number of messages that can be consumed at a time. In this example, the value is set to 1. The largest value you can set is 16. 
                    3, // The duration of a long polling cycle. Unit: seconds. In this example, the value is set to 3. The largest value you can set is 30. 
                    halfMsgs
                    );
        } catch (MQServerException& me) {
            if (me.GetErrorCode() == "MessageNotExist") {
                cout << "No half message to consume! RequestId: " + me.GetRequestId() << endl;
                continue;
            }
            cout << "Request Failed: " + me.GetErrorCode() + ".RequestId: " + me.GetRequestId() << endl;
        }
        if (halfMsgs.size() == 0) {
            continue;
        }

        cout << "Consume Half: " << halfMsgs.size() << " Messages!" << endl;
        // Process the half message. 
        std::vector<std::string> receiptHandles;
        for (std::vector<Message>::iterator iter = halfMsgs.begin();
                iter != halfMsgs.end(); ++iter)
        {
            cout << "MessageId: " << iter->getMessageId()
                << " PublishTime: " << iter->getPublishTime()
                << " Tag: " << iter->getMessageTag()
                << " Body: " << iter->getMessageBody()
                << " FirstConsumeTime: " << iter->getFirstConsumeTime()
                << " NextConsumeTime: " << iter->getNextConsumeTime()
                << " ConsumedTimes: " << iter->getConsumedTimes()
                << " Properties: " << iter->getPropertiesAsString()
                << " Key: " << iter->getMessageKey() << endl;

            int32_t consumedTimes = iter->getConsumedTimes();
            const std::string propA = iter->getProperty("a");
            const std::string handle = iter->getReceiptHandle();
            AckMessageResponse bdmResp;
            if (propA == "1") {
                cout << "Commit msg.." << endl;
                transProducer->commit(handle, bdmResp);
                count++;
            } else if(propA == "2") {
                if (consumedTimes > 1) {
                    cout << "Commit msg.." << endl;
                    transProducer->commit(handle, bdmResp);
                    count++;
                } else {
                    cout << "Commit Later!!!" << endl;
                }
            } else if(propA == "3") {
                cout << "Rollback msg.." << endl;
                transProducer->rollback(handle, bdmResp);
                count++;
            } else {
                transProducer->commit(handle, bdmResp);
                cout << "Unkown msg.." << endl;
            }
            // If the transactional message is committed or rolled back after the time specified by the NextConsumeTime parameter elapses, the commit or rollback fails. 
            processCommitRollError(bdmResp, iter->getMessageId());
        }

    } while(count < halfCheckCount);

#ifdef WIN32
    return 0;
#else
    return NULL;
#endif
}

int main() {

    MQClient mqClient(
            // The HTTP endpoint. To obtain the HTTP endpoint, log on to the Message Queue for Apache RocketMQ console. In the left-side navigation pane, click Instances. On the Instances page, click the name of your instance. On the Instance Details page, scroll to the Basic Information section and view the endpoint on the Endpoints tab. 
            "${HTTP_ENDPOINT}",
            // The AccessKey ID that you created in the RAM console for identity authentication. 
            "${ACCESS_KEY}",
            // The AccessKey secret that you created in the RAM console for identity authentication. 
            "${SECRET_KEY}"
            );

    // The topic of the message. The topic must have been created in the Message Queue for Apache RocketMQ console. 
    string topic = "${TOPIC}";
    // The ID of the instance to which the topic belongs. The instance must have been created in the Message Queue for Apache RocketMQ console. 
    // If the instance has a namespace, the instance ID must be passed in. If the instance does not have a namespace, pass in null or an empty string. Check whether your instance has a namespace on the Instance Details page in the Message Queue for Apache RocketMQ console.
    string instanceId = "${INSTANCE_ID}";
    // The group ID that you created in the Message Queue for Apache RocketMQ console. 
    string groupId = "${GROUP_ID}";

    MQTransProducerPtr transProducer;
    if (instanceId == "") {
        transProducer = mqClient.getTransProducerRef(topic, groupId);
    } else {
        transProducer = mqClient.getTransProducerRef(instanceId, topic, groupId);
    }

    // The producer needs a thread or process to process unacknowledged transactional messages. 
    // Start a thread to process unacknowledged transactional messages. 
#ifdef WIN32
    HANDLE thread;
    unsigned int threadId;
    thread = (HANDLE)_beginthreadex(NULL, 0, consumeHalfMessageThread, &transProducer, 0, &threadId);
#else
    pthread_t thread;
    pthread_create(&thread, NULL, consumeHalfMessageThread, static_cast<void *>(&transProducer));
#endif

    try {
        for (int i = 0; i < pubMsgCount; i++)
        {
            PublishMessageResponse pmResp;
            TopicMessage pubMsg("Hello, mq, trans_msg!");
            pubMsg.putProperty("a",std::to_string(i));
            pubMsg.setMessageKey("ImKey");
            pubMsg.setTransCheckImmunityTime(10);
            transProducer->publishMessage(pubMsg, pmResp);
            cout << "Publish mq message success. Topic:" << topic
                << ", msgId:" << pmResp.getMessageId()
                << ", bodyMD5:" << pmResp.getMessageBodyMD5()
                << ", Handle:" << pmResp.getReceiptHandle() << endl;

            if (i == 0) {
                // After the producer sends the transactional message, the broker obtains the handle of the half message that corresponds to the transactional message, and can directly commit or roll back the half message. 
                // If the transactional message is committed or rolled back after the time specified by the TransCheckImmunityTime parameter elapses, the commit or rollback fails. 
                AckMessageResponse bdmResp;
                transProducer->commit(pmResp.getReceiptHandle(), bdmResp);
                processCommitRollError(bdmResp, pmResp.getMessageId());
            }
        }
    } catch (MQServerException& me) {
        cout << "Request Failed: " + me.GetErrorCode() << ", requestId is:" << me.GetRequestId() << endl;
    } catch (MQExceptionBase& mb) {
        cout << "Request Failed: " + mb.ToString() << endl;
    }

#ifdef WIN32
    WaitForSingleObject(thread, INFINITE);
    CloseHandle(thread);
#else
    pthread_join(thread, NULL);
#endif

    return 0;
}

Subscribe to transactional messages

The following sample code provides an example on how to subscribe to transactional messages:

#include <vector>
#include <fstream>
#include "mq_http_sdk/mq_client.h"

#ifdef _WIN32
#include <windows.h>
#else
#include <unistd.h>
#endif

using namespace std;
using namespace mq::http::sdk;


int main() {

    MQClient mqClient(
            // The HTTP endpoint. To obtain the HTTP endpoint, log on to the Message Queue for Apache RocketMQ console. In the left-side navigation pane, click Instances. On the Instances page, click the name of your instance. On the Instance Details page, scroll to the Basic Information section and view the endpoint on the Endpoints tab. 
            "${HTTP_ENDPOINT}",
            // The AccessKey ID that you created in the RAM console for identity authentication. 
            "${ACCESS_KEY}",
            // The AccessKey secret that you created in the RAM console for identity authentication. 
            "${SECRET_KEY}"
            );

    // The topic of the message. The topic must have been created in the Message Queue for Apache RocketMQ console. 
    string topic = "${TOPIC}";
    // The group ID that you created in the Message Queue for Apache RocketMQ console. 
    string groupId = "${GROUP_ID}";
    // The ID of the instance to which the topic belongs. The instance must have been created in the Message Queue for Apache RocketMQ console. 
    // If the instance has a namespace, the instance ID must be passed in. If the instance does not have a namespace, pass in null or an empty string. Check whether your instance has a namespace on the Instance Details page in the Message Queue for Apache RocketMQ console.
    string instanceId = "${INSTANCE_ID}";

    MQConsumerPtr consumer;
    if (instanceId == "") {
        consumer = mqClient.getConsumerRef(topic, groupId);
    } else {
        consumer = mqClient.getConsumerRef(instanceId, topic, groupId, "");
    }

    do {
        try {
            std::vector<Message> messages;
            // Consume messages in long polling mode. 
            // In long polling mode, if no message in the topic is available for consumption, the request is suspended on the broker for a specified period of time. If a message is available for consumption within the duration, a response is immediately sent to the consumer. In this example, the duration is 3 seconds. 
            consumer->consumeMessage(
                    3, // The maximum number of messages that can be consumed at a time. In this example, the value is set to 3. The largest value you can set is 16. 
                    3, // The duration of a long polling cycle. Unit: seconds. In this example, the value is set to 3. The largest value you can set is 30. 
                    messages
            );
            cout << "Consume: " << messages.size() << " Messages!" << endl;

            // Specify the business processing logic. 
            std::vector<std::string> receiptHandles;
            for (std::vector<Message>::iterator iter = messages.begin();
                    iter != messages.end(); ++iter)
            {
                cout << "MessageId: " << iter->getMessageId()
                    << " PublishTime: " << iter->getPublishTime()
                    << " Tag: " << iter->getMessageTag()
                    << " Body: " << iter->getMessageBody()
                    << " FirstConsumeTime: " << iter->getFirstConsumeTime()
                    << " NextConsumeTime: " << iter->getNextConsumeTime()
                    << " ConsumedTimes: " << iter->getConsumedTimes()
                    << " Properties: " << iter->getPropertiesAsString()
                    << " Key: " << iter->getMessageKey() << endl;
                receiptHandles.push_back(iter->getReceiptHandle());
            }

            // Obtain an acknowledgment (ACK) from the consumer. 
            // If the broker does not receive an ACK from the consumer before the period of time specified by the Message.NextConsumeTime parameter elapses, the message will be consumed again. 
            // A unique timestamp is specified for the handle of a message each time the message is consumed. 
            AckMessageResponse bdmResp;
            consumer->ackMessage(receiptHandles, bdmResp);
            if (!bdmResp.isSuccess()) {
                // The broker may fail to receive an ACK for a message from the consumer if the handle of the message times out. 
                const std::vector<AckMessageFailedItem>& failedItems =
                    bdmResp.getAckMessageFailedItem();
                for (std::vector<AckMessageFailedItem>::const_iterator iter = failedItems.begin();
                        iter != failedItems.end(); ++iter)
                {
                    cout << "AckFailedItem: " << iter->errorCode
                        << "  " << iter->receiptHandle << endl;
                }
            } else {
                cout << "Ack: " << messages.size() << " messages suc!" << endl;
            }
        } catch (MQServerException& me) {
            if (me.GetErrorCode() == "MessageNotExist") {
                cout << "No message to consume! RequestId: " + me.GetRequestId() << endl;
                continue;
            }
            cout << "Request Failed: " + me.GetErrorCode() + ".RequestId: " + me.GetRequestId() << endl;
#ifdef _WIN32
            Sleep(2000);
#else
            usleep(2000 * 1000);
#endif
        } catch (MQExceptionBase& mb) {
            cout << "Request Failed: " + mb.ToString() << endl;
#ifdef _WIN32
            Sleep(2000);
#else
            usleep(2000 * 1000);
#endif
        }

    } while(true);
}