Ordered messages, also known as first-in-first-out (FIFO) messages, are a type of message provided by Message Queue for Apache RocketMQ. Such messages are published and consumed in a strict order. This topic provides the sample code about how to send and subscribe to ordered messages. In this topic, HTTP client SDK for Python is used as an example.

Background information

Ordered messages are classified into the following types:
  • Globally ordered messages: All messages of a specified topic are published and consumed in first-in-first-out (FIFO) order.
  • Partitionally ordered messages: All messages of a specified topic are distributed to different partitions by using partition keys. The messages in each partition are published and consumed in FIFO order. A partition key is a key field that is used for ordered messages to distinguish among different partitions. It is different from the key of a normal message.

For more information, see Ordered 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 ordered messages

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

import sys

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

# 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 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 eight messages. 
msg_count = 8
print("%sPublish Message To %s\nTopicName:%s\nMessageCount:%s\n" % (10 * "=", 10 * "=", topic_name, msg_count))

try:
    for i in range(msg_count):
        msg = TopicMessage(
            # The content of the message. 
            "I am test message %s.hello" % i, 
            # The message tag. 
            "tag %s" % i
        )
        # The custom attributes of the message. 
        msg.put_property("a", str(i))
        # The partition key that is used to distribute ordered messages to different partitions in a topic. A partition key is different from a message key. 
        msg.set_sharding_key(str(i % 3))
        re_msg = producer.publish_message(msg)
        print("Publish Message Succeed. MessageID:%s, BodyMD5:%s" % (re_msg.message_id, re_msg.message_body_md5))
except MQExceptionBase as e:
    if e.type == "TopicNotExist":
        print("Topic not exist, please create it.")
        sys.exit(1)
    print("Publish Message Fail. Exception:%s" % e)
            

Subscribe to ordered messages

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

from mq_http_sdk.mq_exception import MQExceptionBase
from mq_http_sdk.mq_producer 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)
# Consume messages in long polling mode. The consumer may pull partitionally ordered messages from multiple partitions. The consumer consumes the messages in the same partition in the order that the messages are sent. 
# Assume that the consumer pulls partitionally ordered messages. If the broker does not receive an ACK for a message in a partition from the consumer, the broker delivers the same message in the partition to the consumer next time. 
# The consumer can consume the next batch of messages from a partition only after all the messages pulled from the partition in the previous batch are consumed. 
# 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. 
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:
        recv_msgs = consumer.consume_message_orderly(batch, wait_seconds)
        print("=======>Receive %d messages:" % len(recv_msgs))
        for msg in recv_msgs:
            print("\tMessageId: %s, MessageBodyMD5: %s,NextConsumeTime: %s,ConsumedTimes: %s,PublishTime: %s\n\tBody: %s \
                    \n\tReceiptHandle: %s \
                    \n\tProperties: %s,ShardingKey: %s\n" % \
                  (msg.message_id, msg.message_body_md5,
                   msg.next_consume_time, msg.consumed_times,
                   msg.publish_time, msg.message_body,
                   msg.receipt_handle, msg.properties, msg.get_sharding_key()))
    except MQExceptionBase as e:
        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"])))