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. In this topic, HTTP client SDK for Python is used as an example.

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 Python is installed. For more information, see Prepare the 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:

#!/usr/bin/env python
# coding=utf8
import sys

from mq_http_sdk.mq_exception import MQExceptionBase
from mq_http_sdk.mq_producer import *
from mq_http_sdk.mq_client import *
import time
import threading
# Initialize the producer client. 
mq_client = 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. 
     "${HTTP_ENDPOINT}",
    # The AccessKey ID that you created in the Resource Access Management (RAM) console for identity authentication. 
     "${ACCESS_KEY}",
    # The AccessKey secret that you created in the RAM console for identity authentication. 
    "${SECRET_KEY}"
    )
# The topic of the message. The topic must have been created in the Message Queue for Apache RocketMQ console. 
topic_name = "${TOPIC}"
# The group ID that you created in the Message Queue for Apache RocketMQ console. 
group_id = "${GROUP_ID}"
# The ID of the instance to which the topic belongs. The instance must have been created in the Message Queue for Apache RocketMQ console. 
# If the instance has a namespace, the instance ID must be passed in. If the instance does not have a namespace, pass in null or an empty string. Check whether your instance has a namespace on the Instance Details page in the Message Queue for Apache RocketMQ console.
instance_id = "${INSTANCE_ID}"

producer = mq_client.get_producer(instance_id, topic_name)

# Cyclically send four transactional messages. 
msg_count = 4
print("%sPublish Transaction Message To %s\nTopicName:%s\nMessageCount:%s\n" \
      % (10 * "=", 10 * "=", topic_name, msg_count))


def process_trans_error(exp):
    print("\nCommit/Roll Transaction Message Fail! Exception:%s" % exp)
    # If the transactional message is committed or rolled back after the time specified by the TransCheckImmunityTime parameter for the handle of the transactional message elapses
       or after the timeout period specified for the handle of consumeHalfMessage elapses, the commit or rollback fails. In this example, the timeout period is set to 10 seconds for the handle of consumeHalfMessage. 
    if exp.sub_errors:
        for sub_error in exp.sub_errors:
            print("\tErrorHandle:%s,ErrorCode:%s,ErrorMsg:%s" % \
                  (sub_error["ReceiptHandle"], sub_error["ErrorCode"], sub_error["ErrorMessage"]))


# The producer needs a thread or process to process unacknowledged transactional messages. 
# Start a thread to process unacknowledged transactional messages. 
class ConsumeHalfMessageThread(threading.Thread):
    def __init__(self):
        threading.Thread.__init__(self)
        self.count = 0
        # Create another client.
        self.mq_client = 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}"
                )
        self.trans_producer = self.mq_client.get_trans_producer(instance_id, topic_name, group_id)

    def run(self):
        while 1:
            if self.count == 3:
                break;
            try:
                half_msgs = self.trans_producer.consume_half_message(1, 3)
                for half_msg in half_msgs:
                    print("Receive Half Message, MessageId: %s\nMessageBodyMD5: %s \
                                                  \nMessageTag: %s\nConsumedTimes: %s \
                                                  \nPublishTime: %s\nBody: %s \
                                                  \nNextConsumeTime: %s \
                                                  \nReceiptHandle: %s \
                                                  \nProperties: %s" % \
                          (half_msg.message_id, half_msg.message_body_md5,
                           half_msg.message_tag, half_msg.consumed_times,
                           half_msg.publish_time, half_msg.message_body,
                           half_msg.next_consume_time, half_msg.receipt_handle, half_msg.properties))

                a = int(half_msg.get_property("a"))
                try:
                    if a == 1:
                        # Confirm to commit the transactional message. 
                        self.trans_producer.commit(half_msg.receipt_handle)
                        self.count += 1
                        print("------>commit")
                    elif a == 2 and half_msg.consumed_times > 1:
                        # Confirm to commit the transactional message. 
                        self.trans_producer.commit(half_msg.receipt_handle)
                        self.count += 1
                        print("------>commit")
                    elif a == 3:
                        # Confirm to roll back the transactional message. 
                        self.trans_producer.rollback(half_msg.receipt_handle)
                        self.count += 1
                        print("------>rollback")
                    else:
                        # Check the status next time. 
                        print("------>unknown")
                except MQExceptionBase as rec_commit_roll_e:
                    process_trans_error(rec_commit_roll_e)
            except MQExceptionBase as half_e:
                if half_e.type == "MessageNotExist":
                    print("No half message! RequestId: %s" % half_e.req_id)
                    continue

                print("Consume half message Fail! Exception:%s\n" % half_e)
                break


consume_half_thread = ConsumeHalfMessageThread()
consume_half_thread.setDaemon(True)
consume_half_thread.start()

try:
    trans_producer = mq_client.get_trans_producer(instance_id, topic_name, group_id)
    for i in range(msg_count):
        msg = TopicMessage(
            # The content of the message. 
            "I am test message %s." % i,
            # The message tag. 
            "tagA"
        )
        # The custom attributes of the message. 
        msg.put_property("xy", i)
        # The key of the message. 
        msg.set_message_key("MessageKey")
        # The time interval between the sending time of the transactional message and the start time of the first back-check for the status of local transaction. The time interval is a relative time interval. Unit: seconds. Valid values: 10 to 300. 
        # If the message is not committed or rolled back after the first back-check for the status of local transaction, the broker initiates a back-check request for the status of local transaction every 10 seconds within the 24 hours. 
        msg.set_trans_check_immunity_time(10)
        re_msg = trans_producer.publish_message(msg)
        print("Publish Transaction Message Succeed. MessageID:%s, BodyMD5:%s, Handle:%s" \
              % (re_msg.message_id, re_msg.message_body_md5, re_msg.receipt_handle))
        time.sleep(1)
        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. 
            try:
                trans_producer.commit(re_msg.receipt_handle)
            except MQExceptionBase as pub_commit_roll_e:
                process_trans_error(pub_commit_roll_e)

except MQExceptionBase as pub_e:
    if pub_e.type == "TopicNotExist":
        print("Topic not exist, please create it.")
        sys.exit(1)
    print("Publish Message Fail. Exception:%s" % pub_e)

while 1:
    if not consume_half_thread.isAlive():
        break
    time.sleep(1)

Subscribe to transactional messages

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

from mq_http_sdk.mq_exception import MQExceptionBase
from mq_http_sdk.mq_consumer import *
from mq_http_sdk.mq_client import *

# Initialize the consumer client. 
mq_client = 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. 
    "${SECRET_KEY}"
     )
# The topic of the message. The topic must have been created in the Message Queue for Apache RocketMQ console. 
topic_name = "${TOPIC}"
# The group ID that you created in the Message Queue for Apache RocketMQ console. 
group_id = "${GROUP_ID}"
# The ID of the instance to which the topic belongs. The instance must have been created in the Message Queue for Apache RocketMQ console. 
# If the instance has a namespace, the instance ID must be passed in. If the instance does not have a namespace, pass in null or an empty string. Check whether your instance has a namespace on the Instance Details page in the Message Queue for Apache RocketMQ console.
instance_id = "${INSTANCE_ID}"

consumer = mq_client.get_consumer(instance_id, topic_name, group_id)

# 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. 
# 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. 
wait_seconds = 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. 
batch = 3
print(("%sConsume And Ak Message From Topic%s\nTopicName:%s\nMQConsumer:%s\nWaitSeconds:%s\n" \
        % (10 * "=", 10 * "=", topic_name, group_id, wait_seconds)))
while True:
    try:
        # Consume messages in long polling mode. 
        recv_msgs = consumer.consume_message(batch, wait_seconds)
        for msg in recv_msgs:
            print(("Receive, MessageId: %s\nMessageBodyMD5: %s \
                              \nMessageTag: %s\nConsumedTimes: %s \
                              \nPublishTime: %s\nBody: %s \
                              \nNextConsumeTime: %s \
                              \nReceiptHandle: %s \
                              \nProperties: %s\n" % \
                  (msg.message_id, msg.message_body_md5,
                   msg.message_tag, msg.consumed_times,
                   msg.publish_time, msg.message_body,
                   msg.next_consume_time, msg.receipt_handle, msg.properties)))
            print(msg.get_property(""))
    except MQExceptionBase as e:
        # No messages in the topic are available for consumption. 
        if e.type == "MessageNotExist":
            print(("No new message! RequestId: %s" % e.req_id))
            continue

        print(("Consume Message Fail! Exception:%s\n" % e))
        time.sleep(2)
        continue

    # If the broker does not receive an acknowledgment (ACK) from the consumer before the period of time specified by the msg.next_consume_time 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. 
    try:
        receipt_handle_list = [msg.receipt_handle for msg in recv_msgs]
        consumer.ack_message(receipt_handle_list)
        print(("Ak %s Message Succeed.\n\n" % len(receipt_handle_list)))
    except MQExceptionBase as e:
        print(("\nAk Message Fail! Exception:%s" % e))
        # The broker may fail to receive an ACK for a message from the consumer if the handle of the message times out. 
        if e.sub_errors:
            for sub_error in e.sub_errors:
                print(("\tErrorHandle:%s,ErrorCode:%s,ErrorMsg:%s" % \
                      (sub_error["ReceiptHandle"], sub_error["ErrorCode"], sub_error["ErrorMessage"])))