Subscribe to Messages from a Topic
Subscribing to receive message from a topic with the Go CDK takes three steps:
- Open a subscription to a topic with the Pub/Sub service of your choice (once per subscription).
- Receive and acknowledge messages from the topic. After completing any work related to the message, use the Ack method to prevent it from being redelivered.
Opening a Subscription🔗
The first step in subscribing to receive messages from a topic is
to instantiate a portable *pubsub.Subscription
for your service.
The easiest way to do so is to use pubsub.OpenSubscription
and a service-specific URL pointing to the topic, making sure you
“blank import” the driver package to link it in.
import (
"context"
"gocloud.dev/pubsub"
_ "gocloud.dev/pubsub/<driver>"
)
...
ctx := context.Background()
subs, err := pubsub.OpenSubscription(ctx, "<driver-url>")
if err != nil {
return fmt.Errorf("could not open topic subscription: %v", err)
}
defer subs.Shutdown(ctx)
// subs is a *pubsub.Subscription; see usage below
...
See Concepts: URLs for general background and the guide below for URL usage for each supported service.
Alternatively, if you need fine-grained
control over the connection settings, you can call the constructor function in
the driver package directly (like gcppubsub.OpenSubscription
).
import "gocloud.dev/pubsub/<driver>"
...
subs, err := <driver>.OpenSubscription(...)
...
You may find the wire
package useful for managing your initialization code
when switching between different backing services.
See the guide below for constructor usage for each supported service.
Receiving and Acknowledging Messages🔗
A simple subscriber that operates on messages serially looks like this:
// Loop on received messages.
for {
msg, err := subscription.Receive(ctx)
if err != nil {
// Errors from Receive indicate that Receive will no longer succeed.
log.Printf("Receiving message: %v", err)
break
}
// Do work based on the message, for example:
fmt.Printf("Got message: %q\n", msg.Body)
// Messages must always be acknowledged with Ack.
msg.Ack()
}
If you want your subscriber to operate on incoming messages concurrently, you can start multiple goroutines:
// Loop on received messages. We can use a channel as a semaphore to limit how
// many goroutines we have active at a time as well as wait on the goroutines
// to finish before exiting.
const maxHandlers = 10
sem := make(chan struct{}, maxHandlers)
recvLoop:
for {
msg, err := subscription.Receive(ctx)
if err != nil {
// Errors from Receive indicate that Receive will no longer succeed.
log.Printf("Receiving message: %v", err)
break
}
// Wait if there are too many active handle goroutines and acquire the
// semaphore. If the context is canceled, stop waiting and start shutting
// down.
select {
case sem <- struct{}{}:
case <-ctx.Done():
break recvLoop
}
// Handle the message in a new goroutine.
go func() {
defer func() { <-sem }() // Release the semaphore.
defer msg.Ack() // Messages must always be acknowledged with Ack.
// Do work based on the message, for example:
fmt.Printf("Got message: %q\n", msg.Body)
}()
}
// We're no longer receiving messages. Wait to finish handling any
// unacknowledged messages by totally acquiring the semaphore.
for n := 0; n < maxHandlers; n++ {
sem <- struct{}{}
}
Note that the semantics of message delivery can vary by backing service.
Other Usage Samples🔗
Supported Pub/Sub Services🔗
Google Cloud Pub/Sub🔗
The Go CDK can receive messages from a Google Cloud Pub/Sub subscription. The URLs use the project ID and the subscription ID.
pubsub.OpenSubscription
will use Application Default Credentials; if you have
authenticated via gcloud auth application-default login
, it will use those credentials. See
Application Default Credentials to learn about authentication
alternatives, including using environment variables.
import (
"context"
"gocloud.dev/pubsub"
_ "gocloud.dev/pubsub/gcppubsub"
)
subscription, err := pubsub.OpenSubscription(ctx,
"gcppubsub://projects/my-project/subscriptions/my-subscription")
if err != nil {
return err
}
defer subscription.Shutdown(ctx)
Google Cloud Pub/Sub Constructor🔗
The gcppubsub.OpenSubscription
constructor opens a Cloud Pub/Sub
subscription. You must first obtain GCP credentials and then
create a gRPC connection to Cloud Pub/Sub. (This gRPC connection can be
reused among subscriptions.)
import (
"context"
"gocloud.dev/gcp"
"gocloud.dev/pubsub/gcppubsub"
)
// Your GCP credentials.
// See https://cloud.google.com/docs/authentication/production
// for more info on alternatives.
creds, err := gcp.DefaultCredentials(ctx)
if err != nil {
return err
}
// Open a gRPC connection to the GCP Pub/Sub API.
conn, cleanup, err := gcppubsub.Dial(ctx, creds.TokenSource)
if err != nil {
return err
}
defer cleanup()
// Construct a SubscriberClient using the connection.
subClient, err := gcppubsub.SubscriberClient(ctx, conn)
if err != nil {
return err
}
defer subClient.Close()
// Construct a *pubsub.Subscription.
subscription, err := gcppubsub.OpenSubscriptionByPath(
subClient, "projects/myprojectID/subscriptions/example-subscription", nil)
if err != nil {
return err
}
defer subscription.Shutdown(ctx)
Amazon Simple Queueing Service🔗
The Go CDK can subscribe to an Amazon Simple Queueing Service (SQS)
topic. SQS URLs closely resemble the the queue URL, except the leading
https://
is replaced with awssqs://
. You should specify the region
query parameter to ensure your application connects to the correct region.
pubsub.OpenSubscription
will create a default AWS Session with the
SharedConfigEnable
option enabled; if you have authenticated with the AWS CLI,
it will use those credentials. See AWS Session to learn about authentication
alternatives, including using environment variables.
import (
"context"
"gocloud.dev/pubsub"
_ "gocloud.dev/pubsub/awssnssqs"
)
// pubsub.OpenSubscription creates a *pubsub.Subscription from a URL.
// This URL will open the subscription with the URL
// "https://sqs.us-east-2.amazonaws.com/123456789012/myqueue".
subscription, err := pubsub.OpenSubscription(ctx,
"awssqs://sqs.us-east-2.amazonaws.com/123456789012/"+
"myqueue?region=us-east-2")
if err != nil {
return err
}
defer subscription.Shutdown(ctx)
If your messages are being sent to SQS directly, or if they are being delivered
via an SNS topic with RawMessageDelivery
enabled, set a raw=true
query
parameter in your URL, or set SubscriberOptions.Raw
to true
if you’re using
the constructors. By default, the subscription will use heuristics to identify
whether the message bodies are raw or SNS JSON.
Messages with a base64encoded
message attribute will be automatically
Base64 decoded before being returned. See the SNS publishing guide
or the SQS publishing guide for more details.
Amazon SQS Constructor🔗
The awssnssqs.OpenSubscription
constructor opens an SQS queue. You must
first create an AWS session with the same region as your topic:
import (
"context"
"github.com/aws/aws-sdk-go/aws"
"github.com/aws/aws-sdk-go/aws/session"
"gocloud.dev/pubsub/awssnssqs"
)
// Establish an AWS session.
// See https://docs.aws.amazon.com/sdk-for-go/api/aws/session/ for more info.
// The region must match the region for "MyQueue".
sess, err := session.NewSession(&aws.Config{
Region: aws.String("us-east-2"),
})
if err != nil {
return err
}
// Construct a *pubsub.Subscription.
// https://docs.aws.amazon.com/sdk-for-net/v2/developer-guide/QueueURL.html
const queueURL = "https://sqs.us-east-2.amazonaws.com/123456789012/MyQueue"
subscription := awssnssqs.OpenSubscription(ctx, sess, queueURL, nil)
defer subscription.Shutdown(ctx)
Azure Service Bus🔗
The Go CDK can recieve messages from an Azure Service Bus subscription.
The URL for subscribing is the topic name with the
subscription name in the subscription
query parameter.
pubsub.OpenSubscription
will use the environment variable
SERVICEBUS_CONNECTION_STRING
to obtain the Service Bus Connection String
you need to copy from the Azure portal.
import (
"context"
"gocloud.dev/pubsub"
_ "gocloud.dev/pubsub/azuresb"
)
// pubsub.OpenSubscription creates a *pubsub.Subscription from a URL.
// This URL will open the subscription "mysubscription" for the topic
// "mytopic" using a connection string from the environment variable
// SERVICEBUS_CONNECTION_STRING.
subscription, err := pubsub.OpenSubscription(ctx,
"azuresb://mytopic?subscription=mysubscription")
if err != nil {
return err
}
defer subscription.Shutdown(ctx)
Azure Service Bus Constructor🔗
The azuresb.OpenSubscription
constructor opens an Azure Service Bus
subscription. You must first connect to the topic and subscription using the
Azure Service Bus library and then pass the subscription to
azuresb.OpenSubscription
. There are also helper functions in the azuresb
package to make this easier.
import (
"context"
"os"
"gocloud.dev/pubsub/azuresb"
)
// Change these as needed for your application.
serviceBusConnString := os.Getenv("SERVICEBUS_CONNECTION_STRING")
const topicName = "test-topic"
const subscriptionName = "test-subscription"
// Connect to Azure Service Bus for the given subscription.
sbClient, err := azuresb.NewClientFromConnectionString(serviceBusConnString, nil)
if err != nil {
return err
}
sbReceiver, err := azuresb.NewReceiver(sbClient, topicName, subscriptionName, nil)
if err != nil {
return err
}
defer sbReceiver.Close(ctx)
// Construct a *pubsub.Subscription.
subscription, err := azuresb.OpenSubscription(ctx, sbClient, sbReceiver, nil)
if err != nil {
return err
}
defer subscription.Shutdown(ctx)
RabbitMQ🔗
The Go CDK can receive messages from an AMQP 0.9.1 queue, the dialect of
AMQP spoken by RabbitMQ. A RabbitMQ URL only includes the queue name.
The RabbitMQ’s server is discovered from the RABBIT_SERVER_URL
environment
variable (which is something like amqp://guest:guest@localhost:5672/
).
import (
"context"
"gocloud.dev/pubsub"
_ "gocloud.dev/pubsub/rabbitpubsub"
)
// pubsub.OpenSubscription creates a *pubsub.Subscription from a URL.
// This URL will Dial the RabbitMQ server at the URL in the environment
// variable RABBIT_SERVER_URL and open the queue "myqueue".
subscription, err := pubsub.OpenSubscription(ctx, "rabbit://myqueue")
if err != nil {
return err
}
defer subscription.Shutdown(ctx)
RabbitMQ Constructor🔗
The rabbitpubsub.OpenSubscription
constructor opens a RabbitMQ queue.
You must first create an *amqp.Connection
to your RabbitMQ instance.
import (
"context"
amqp "github.com/rabbitmq/amqp091-go"
"gocloud.dev/pubsub/rabbitpubsub"
)
rabbitConn, err := amqp.Dial("amqp://guest:guest@localhost:5672/")
if err != nil {
return err
}
defer rabbitConn.Close()
subscription := rabbitpubsub.OpenSubscription(rabbitConn, "myqueue", nil)
defer subscription.Shutdown(ctx)
NATS🔗
The Go CDK can publish to a NATS subject. A NATS URL only includes the
subject name. The NATS server is discovered from the NATS_SERVER_URL
environment variable (which is something like nats://nats.example.com
).
import (
"context"
"gocloud.dev/pubsub"
_ "gocloud.dev/pubsub/natspubsub"
)
// pubsub.OpenSubscription creates a *pubsub.Subscription from a URL.
// This URL will Dial the NATS server at the URL in the environment variable
// NATS_SERVER_URL and receive messages with subject "example.mysubject".
subscription, err := pubsub.OpenSubscription(ctx, "nats://example.mysubject")
if err != nil {
return err
}
defer subscription.Shutdown(ctx)
NATS guarantees at-most-once delivery; it will never redeliver a message.
Therefore, Message.Ack
is a no-op.
To parse messages published via the Go CDK, the NATS driver
will first attempt to decode the payload using gob. Failing that, it will
return the message payload as the Data
with no metadata to accomodate
subscribing to messages coming from a source not using the Go CDK.
NATS Constructor🔗
The natspubsub.OpenSubscription
constructor opens a NATS subject as a
topic. You must first create an *nats.Conn
to your NATS instance.
import (
"context"
"github.com/nats-io/nats.go"
"gocloud.dev/pubsub/natspubsub"
)
natsConn, err := nats.Connect("nats://nats.example.com")
if err != nil {
return err
}
defer natsConn.Close()
subscription, err := natspubsub.OpenSubscription(
natsConn,
"example.mysubject",
nil)
if err != nil {
return err
}
defer subscription.Shutdown(ctx)
Kafka🔗
The Go CDK can receive messages from a Kafka cluster.
A Kafka URL includes the consumer group name, plus at least one instance
of a query parameter specifying the topic to subscribe to.
The brokers in the Kafka cluster are discovered from the
KAFKA_BROKERS
environment variable (which is a comma-delimited list of
hosts, something like 1.2.3.4:9092,5.6.7.8:9092
).
import (
"context"
"gocloud.dev/pubsub"
_ "gocloud.dev/pubsub/kafkapubsub"
)
// pubsub.OpenSubscription creates a *pubsub.Subscription from a URL.
// The host + path are used as the consumer group name.
// The "topic" query parameter sets one or more topics to subscribe to.
// The set of brokers must be in an environment variable KAFKA_BROKERS.
subscription, err := pubsub.OpenSubscription(ctx,
"kafka://my-group?topic=my-topic")
if err != nil {
return err
}
defer subscription.Shutdown(ctx)
Kafka Constructor🔗
The kafkapubsub.OpenSubscription
constructor creates a consumer in a
consumer group, subscribed to one or more topics.
In addition to the list of brokers, you’ll need a *sarama.Config
, which
exposes many knobs that can affect performance and semantics; review and set
them carefully. kafkapubsub.MinimalConfig
provides a minimal config to
get you started.
import (
"context"
"gocloud.dev/pubsub/kafkapubsub"
)
// The set of brokers in the Kafka cluster.
addrs := []string{"1.2.3.4:9092"}
// The Kafka client configuration to use.
config := kafkapubsub.MinimalConfig()
// Construct a *pubsub.Subscription, joining the consumer group "my-group"
// and receiving messages from "my-topic".
subscription, err := kafkapubsub.OpenSubscription(
addrs, config, "my-group", []string{"my-topic"}, nil)
if err != nil {
return err
}
defer subscription.Shutdown(ctx)
In-Memory🔗
The Go CDK includes an in-memory Pub/Sub provider useful for local testing.
The names in mem://
URLs are a process-wide namespace, so subscriptions to
the same name will receive messages posted to that topic. For instance, if
you open a topic mem://topicA
and open two subscriptions with
mem://topicA
, you will have two subscriptions to the same topic.
import (
"context"
"gocloud.dev/pubsub"
_ "gocloud.dev/pubsub/mempubsub"
)
// Create a topic.
topic, err := pubsub.OpenTopic(ctx, "mem://topicA")
if err != nil {
return err
}
defer topic.Shutdown(ctx)
// Create a subscription connected to that topic.
subscription, err := pubsub.OpenSubscription(ctx, "mem://topicA")
if err != nil {
return err
}
defer subscription.Shutdown(ctx)
In-Memory Constructor🔗
To create a subscription to an in-memory Pub/Sub topic, pass the topic you
created into the mempubsub.NewSubscription
function.
You will also need to pass an acknowledgement deadline: once a message is
received, if it is not acknowledged after the deadline elapses, then it will be
redelivered.
import (
"context"
"time"
"gocloud.dev/pubsub/mempubsub"
)
// Construct a *pubsub.Topic.
topic := mempubsub.NewTopic()
defer topic.Shutdown(ctx)
// Construct a *pubsub.Subscription for the topic.
subscription := mempubsub.NewSubscription(topic, 1*time.Minute /* ack deadline */)
defer subscription.Shutdown(ctx)