Sending and Receiving Transactional Messages

DMS for RocketMQ ensures transaction consistency between the service logic and message transmission, and implements transaction support in two phases. Figure 1 illustrates the interaction of transactional messages.

Figure 1 Transactional message interaction

The producer sends a half message and then executes the local transaction. If the execution is successful, the transaction is committed. If the execution fails, the transaction is rolled back. If the server does not receive any commit or rollback request after a period of time, it initiates a check. After receiving the check request, the producer resends a transaction commit or rollback request. The message is delivered to the consumer only after being committed. The consumer is unaware of the rollback.

Before sending and receiving transactional messages, collect RocketMQ connection information by referring to Collecting Connection Information.

Preparing the Environment

Run the following command to check whether Go has been installed:
```
go version
```
If the following information is displayed, Go has been installed:
```
go version go1.16.5 linux/amd64
```
If Go is not installed, download and install it.

Add the following code to go.mod to add the dependency:

module rocketmq-example-go

go 1.13

require (
	github.com/apache/rocketmq-client-go/v2 v2.1.1
)

Sending Transactional Messages

The following code is an example. Replace the information in bold with the actual values.

package main

import (
	"context"
	"fmt"
	"os"
	"strconv"
	"sync"
	"sync/atomic"
	"time"

	"github.com/apache/rocketmq-client-go/v2"
	"github.com/apache/rocketmq-client-go/v2/primitive"
	"github.com/apache/rocketmq-client-go/v2/producer"
)

type DemoListener struct {
	localTrans       *sync.Map
	transactionIndex int32
}

func NewDemoListener() *DemoListener {
	return &DemoListener{
		localTrans: new(sync.Map),
	}
}

func (dl *DemoListener) ExecuteLocalTransaction(msg *primitive.Message) primitive.LocalTransactionState {
	nextIndex := atomic.AddInt32(&dl.transactionIndex, 1)
	fmt.Printf("nextIndex: %v for transactionID: %v\n", nextIndex, msg.TransactionId)
	status := nextIndex % 3
	dl.localTrans.Store(msg.TransactionId, primitive.LocalTransactionState(status+1))

	fmt.Printf("dl")
	return primitive.UnknowState
}

func (dl *DemoListener) CheckLocalTransaction(msg *primitive.MessageExt) primitive.LocalTransactionState {
	fmt.Printf("%v msg transactionID : %v\n", time.Now(), msg.TransactionId)
	v, existed := dl.localTrans.Load(msg.TransactionId)
	if !existed {
		fmt.Printf("unknow msg: %v, return Commit", msg)
		return primitive.CommitMessageState
	}
	state := v.(primitive.LocalTransactionState)
	switch state {
	case 1:
		fmt.Printf("checkLocalTransaction COMMIT_MESSAGE: %v\n", msg)
		return primitive.CommitMessageState
	case 2:
		fmt.Printf("checkLocalTransaction ROLLBACK_MESSAGE: %v\n", msg)
		return primitive.RollbackMessageState
	case 3:
		fmt.Printf("checkLocalTransaction unknow: %v\n", msg)
		return primitive.UnknowState
	default:
		fmt.Printf("checkLocalTransaction default COMMIT_MESSAGE: %v\n", msg)
		return primitive.CommitMessageState
	}
}

func main() {
	p, _ := rocketmq.NewTransactionProducer(
		NewDemoListener(),
		producer.WithNsResolver(primitive.NewPassthroughResolver([]string{"192.168.0.1:8100"})),
		producer.WithRetry(1),
	)
	err := p.Start()
	if err != nil {
		fmt.Printf("start producer error: %s\n", err.Error())
		os.Exit(1)
	}

	for i := 0; i < 10; i++ {
		res, err := p.SendMessageInTransaction(context.Background(),
			primitive.NewMessage("topic1", []byte("Hello RocketMQ again "+strconv.Itoa(i))))

		if err != nil {
			fmt.Printf("send message error: %s\n", err)
		} else {
			fmt.Printf("send message success: result=%s\n", res.String())
		}
	}
	time.Sleep(5 * time.Minute)
	err = p.Shutdown()
	if err != nil {
		fmt.Printf("shutdown producer error: %s", err.Error())
	}
}

The parameters in the example code are described as follows. For details about how to obtain the parameter values, see Collecting Connection Information.

192.168.0.1:8100: instance address and port.
topic1: topic name.

The producer needs to implement two callback functions. The ExecuteLocalTransaction callback function is called after the half message is sent (see step 3 in the diagram). The CheckLocalTransaction callback function is called after the check request is received (see step 6 in the diagram). The two callback functions can return three transaction states:

primitive.CommitMessageState: Transaction committed. The consumer can retrieve the message.
primitive.RollbackMessageState: Transaction rolled back. The message will be discarded and cannot be retrieved.
primitive.UnknowState: The status cannot be determined and the server is expected to check the message status from the producer again.