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 by using the HTTP-based SDK for Go.

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 Go 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:


package main

import (
    "fmt"
    "time"
    "strconv"

    "github.com/aliyunmq/mq-http-go-sdk"
)

func main() {
    // The HTTP endpoint. To obtain the HTTP endpoint, perform the following steps: 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. 
    endpoint := "${HTTP_ENDPOINT}"
    // The AccessKey ID that you create in the RAM console for identity authentication. 
    accessKey := "${ACCESS_KEY}"
    // The AccessKey secret that you create in the RAM console for identity authentication. 
    secretKey := "${SECRET_KEY}"
    // The topic of the message. The topic is created in the Message Queue for Apache RocketMQ console. 
    topic := "${TOPIC}"
    // The ID of the instance to which the topic belongs. The instance is 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. 
    instanceId := "${INSTANCE_ID}"

    client := mq_http_sdk.NewAliyunMQClient(endpoint, accessKey, secretKey, "")

    mqProducer := client.GetProducer(instanceId, topic)
    // Cyclically send eight messages. 
    for i := 0; i < 8; i++ {
        msg := mq_http_sdk.PublishMessageRequest{
            MessageBody: "hello mq!",         // The content of the message. 
            MessageTag:  "",                  // The message tag. 
            Properties:  map[string]string {}, // The message attributes. 
        }
        // The key of the message. 
        msg.MessageKey = "MessageKey"
        // The custom attributes of the message. 
        msg.Properties["a"] = strconv.Itoa(i)
        // The partition key that is used to distribute ordered messages to a specific partition. The partition key distinguishes a partition from other partitions. A partition key is different from a message key. 
        msg.ShardingKey = strconv.Itoa(i % 2)
        ret, err := mqProducer.PublishMessage(msg)

        if err != nil {
            fmt.Println(err)
            return
        } else {
            fmt.Printf("Publish ---->\n\tMessageId:%s, BodyMD5:%s, \n", ret.MessageId, ret.MessageBodyMD5)
        }
        time.Sleep(time.Duration(100) * time.Millisecond)
    }
}

Subscribe to ordered messages

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

package main

import (
    "fmt"
    "github.com/gogap/errors"
    "strings"
    "time"

    "github.com/aliyunmq/mq-http-go-sdk"
)

func main() {
    // The HTTP endpoint. To obtain the HTTP endpoint, perform the following steps: 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. 
    endpoint := "${HTTP_ENDPOINT}"
    // The AccessKey ID that you create in the RAM console for identity authentication. 
    accessKey := "${ACCESS_KEY}"
    // The AccessKey secret that you create in the RAM console for identity authentication. 
    secretKey := "${SECRET_KEY}"
    // The topic of the message. The topic is created in the Message Queue for Apache RocketMQ console. 
    topic := "${TOPIC}"
    // The ID of the instance to which the topic belongs. The instance is 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. 
    instanceId := "${INSTANCE_ID}"
    // The ID of the group that you create in the Message Queue for Apache RocketMQ console. 
    groupId := "${GROUP_ID}"

    client := mq_http_sdk.NewAliyunMQClient(endpoint, accessKey, secretKey, "")

    mqConsumer := client.GetConsumer(instanceId, topic, groupId, "")

    for {
        endChan := make(chan int)
        respChan := make(chan mq_http_sdk.ConsumeMessageResponse)
        errChan := make(chan error)
        go func() {
            select {
            case resp := <-respChan:
                {
                    // Specify the business processing logic. 
                    var handles []string
                    fmt.Printf("Consume %d messages---->\n", len(resp.Messages))
                    for _, v := range resp.Messages {
                        handles = append(handles, v.ReceiptHandle)
                        fmt.Printf("\tMessageID: %s, PublishTime: %d, MessageTag: %s\n"+
                            "\tConsumedTimes: %d, FirstConsumeTime: %d, NextConsumeTime: %d\n"+
                            "\tBody: %s\n"+
                            "\tProps: %s\n"+
                            "\tShardingKey: %s\n",
                            v.MessageId, v.PublishTime, v.MessageTag, v.ConsumedTimes,
                            v.FirstConsumeTime, v.NextConsumeTime, v.MessageBody, v.Properties, v.ShardingKey)
                    }

                    // If the broker does not receive an acknowledgment for a message from the consumer before the period of time specified by the 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. 
                    ackerr := mqConsumer.AckMessage(handles)
                    if ackerr != nil {
                        // The broker may fail to receive an acknowledgment for a message from the consumer if the handle of the message times out. 
                        fmt.Println(ackerr)
                        if errAckItems, ok := ackerr.(errors.ErrCode).Context()["Detail"].([]mq_http_sdk.ErrAckItem); ok {
                           for _, errAckItem := range errAckItems {
                              fmt.Printf("\tErrorHandle:%s, ErrorCode:%s, ErrorMsg:%s\n",
                                 errAckItem.ErrorHandle, errAckItem.ErrorCode, errAckItem.ErrorMsg)
                           }
                        } else {
                           fmt.Println("ack err =", ackerr)
                        }
                        time.Sleep(time.Duration(3) * time.Second)
                    } else {
                        fmt.Printf("Ack ---->\n\t%s\n", handles)
                    }

                    endChan <- 1
                }
            case err := <-errChan:
                {
                    // No messages in the topic are available for consumption. 
                    if strings.Contains(err.(errors.ErrCode).Error(), "MessageNotExist") {
                        fmt.Println("\nNo new message, continue!")
                    } else {
                        fmt.Println(err)
                        time.Sleep(time.Duration(3) * time.Second)
                    }
                    endChan <- 1
                }
            case <-time.After(35 * time.Second):
                {
                    fmt.Println("Timeout of consumer message ??")
                    endChan <- 1
                }
            }
        }()

                // The consumer may pull partitionally ordered messages from multiple partitions. The consumer consumes the messages in each partition in the order that the messages are sent. 
                // If the consumer pulls partitionally ordered messages and a message in one partition is not consumed by the consumer, the consumer consumes the message 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, the network timeout period is 35 seconds. 
                // 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. 
        mqConsumer.ConsumeMessageOrderly(respChan, errChan,
            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. 
        )
        <-endChan
    }
}