I am fairly new to Go, I want to make a pipeline that translate every requests I receive by send it to first queue (TEST), and get the final result from the last queue (RESULT) and send it back as a response.
The problem I am facing is, the response never wait til all result back from the queue. Here is the code:
func main() {
requests := []int{3, 4, 5, 6, 7}
var wg sync.WaitGroup
wg.Add(1)
resArr := []string{}
go func() {
for _, r := range requests {
rabbitSend("TEST", r)
resArr = append(resArr, <-rabbitReceive("RESULT"))
}
defer wg.Done()
}()
wg.Wait()
log.Println("Result", resArr)
}
rabbitSend method:
func rabbitSend(queueName string, msg int) {
conn, err := amqp.Dial("amqp://guest:guest@localhost:5672/")
failOnError(err, "Failed to connect to RabbitMQ")
defer conn.Close()
ch, err := conn.Channel()
failOnError(err, "Failed to open a channel")
defer ch.Close()
q, err := ch.QueueDeclare(
queueName, // name
true, // durable
false, // delete when unused
false, // exclusive
false, // no-wait
nil, // arguments
)
failOnError(err, "Failed to declare a queue")
body, _ := json.Marshal(msg)
err = ch.Publish(
"", // exchange
q.Name, // routing key
false, // mandatory
false, // immediate
amqp.Publishing{
ContentType: "application/json",
Body: []byte(body),
})
log.Printf("[x] Sent %s to %s", body, q.Name)
failOnError(err, "Failed to publish a message")
}
rabbitReceive method:
func rabbitReceive(queueName string) <-chan string {
conn, err := amqp.Dial("amqp://guest:guest@localhost:5672/")
failOnError(err, "Failed to connect to RabbitMQ")
defer conn.Close()
ch, err := conn.Channel()
failOnError(err, "Failed to open a channel")
defer ch.Close()
q, err := ch.QueueDeclare(
queueName, // name
true, // durable
false, // delete when usused
false, // exclusive
false, // no-waits
nil, // arguments
)
failOnError(err, "Failed to declare a queue")
msgs, err := ch.Consume(
q.Name, // queue
"", // consumer
true, // auto-ack
false, // exclusive
false, // no-local
false, // no-wait
nil, // args
)
failOnError(err, "Failed to register a consumer")
resCh := make(chan string)
go func() {
for d := range msgs {
log.Printf("Received a message: %v from %v", string(d.Body), q.Name)
resCh <- string(d.Body)
}
close(resCh)
}()
return resCh
}
Here is what I get when I run the program:
2018/11/12 05:11:54 [x] Sent 3 to TEST
2018/11/12 05:11:54 [x] Sent 4 to TEST
2018/11/12 05:11:54 Received a message: 9 from RESULT
2018/11/12 05:11:54 [x] Sent 5 to TEST
2018/11/12 05:11:54 [x] Sent 6 to TEST
2018/11/12 05:11:54 Received a message: 15 from RESULT
2018/11/12 05:11:54 [x] Sent 7 to TEST
2018/11/12 05:11:54 Received a message: 18 from RESULT
2018/11/12 05:11:54 Result [ 9 15 18]
What I want is that, I receive the result exactly after I send the request, so the request will not get the wrong result as a response. Something like:
2018/11/12 05:11:54 [x] Sent 3 to TEST
2018/11/12 05:11:54 Received a message: 9 from RESULT
2018/11/12 05:11:54 [x] Sent 4 to TEST
2018/11/12 05:11:54 Received a message: 12 from RESULT
2018/11/12 05:11:54 [x] Sent 5 to TEST
2018/11/12 05:11:54 Received a message: 15 from RESULT
2018/11/12 05:11:54 [x] Sent 6 to TEST
2018/11/12 05:11:54 Received a message: 18 from RESULT
2018/11/12 05:11:54 [x] Sent 7 to TEST
2018/11/12 05:11:54 Received a message: 21 from RESULT
2018/11/12 05:11:54 Result [ 9 12 15 18 21]
I believe I did not use goroutine or sync.WaitGroup correctly here. Thanks in advance :)
Modify your func rabbitReceive(queueName string) <-chan string as below:
func rabbitReceive(queueName string) <-chan string {
conn, err := amqp.Dial("amqp://guest:guest@localhost:5672/")
failOnError(err, "Failed to connect to RabbitMQ")
ch, err := conn.Channel()
failOnError(err, "Failed to open a channel")
q, err := ch.QueueDeclare(
queueName, // name
true, // durable
false, // delete when usused
false, // exclusive
false, // no-waits
nil, // arguments
)
failOnError(err, "Failed to declare a queue")
msgs, err := ch.Consume(
q.Name, // queue
"", // consumer
true, // auto-ack
false, // exclusive
false, // no-local
false, // no-wait
nil, // args
)
failOnError(err, "Failed to register a consumer")
resCh := make(chan string)
go func() {
d := <-msgs
log.Printf("Received a message: %v from %v", string(d.Body), q.Name)
resCh <- string(d.Body)
conn.Close()
ch.Close()
close(resCh)
}()
return resCh
}
The reason previous code cause you problem was defer ch.Close(). ch closes before response was written to resCh.