:branch: master

=== Google Cloud Background Functions Sample Application

Google Cloud Functions supports deploying https://cloud.google.com/functions/docs/writing/background[Background Functions] which are invoked indirectly in response to an event, such as a message on a https://cloud.google.com/pubsub[Pub/Sub] topic, a change in a https://cloud.google.com/storage[Cloud Storage] bucket, or a https://firebase.google.com/[Firebase] event.

This sample demonstrates a simple background function which triggers from a Pub/Sub event.

===== Test locally

In a terminal window, run:

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mvn function:run
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In a separate window, invoke the background function by issuing the `curl` command below.

NOTE: In this sample, we demonstrate a background function which is triggered when a message is https://cloud.google.com/functions/docs/calling/pubsub[published to a specified Cloud Pub/Sub topic].
The `curl` request simulates sending the message that would be received by your function when a Pub/Sub event occurs.

In order to simulate how Pub/Sub will send you this message, we use a base64-encoded string as the input because Pub/Sub encodes the message data in base64 in the Pub/Sub event.
See the notes section below for a more information on the Pub/Sub event structure.

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curl localhost:8080 -H "Content-Type: application/json" -d '{"data":"aGVsbG8="}'
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In the original terminal window where the `mvn function:run` was invoked, you should see a message printed.

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Received Pub/Sub message with data: hello
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===== Deploy to GCP

The steps below will demonstrate how to deploy your background function to GCP and have it be invoked by a Pub/Sub event.

To complete the next steps, make sure that you have the https://cloud.google.com/sdk/install[Cloud SDK CLI] installed.

1. Create a new Pub/Sub topic. This topic will be used as the trigger for the background function.
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gcloud pubsub topics create my-functions-topic
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2. In this sample directory, package the application by running:
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mvn package
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You should see the JAR to deploy in the `target/deploy` directory.

3. Deploy the JAR that you created by running
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gcloud functions deploy function-sample-gcp-background \
--entry-point org.springframework.cloud.function.adapter.gcp.GcfJarLauncher \
--runtime java17 \
--trigger-topic my-functions-topic \
--source target/deploy \
--memory 512MB
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Notice the parameter `--trigger-topic` which topic will trigger the function invocation when new messages are published to it.

4. Invoke the background function by publishing a message to your topic.
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gcloud pubsub topics publish my-functions-topic --message="Hello world"
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5. To verify that this sample background function was invoked, check the logs of the background function on GCF by running:
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gcloud functions logs read function-sample-gcp-background --filter=Received
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You should see a log message that looks like below.
It might take a minute or two for the log to appear.
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function-sample-gcp-background-2  h8q1jt46069r  2020-05-19 19:48:27.960  Received Pub/Sub message with data: Hello world
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This log output is produced by the sample background function, so this entry in the logs means that the background function was successfully invoked.

===== Notes

One important note about the Pub/Sub background function provided in `BackgroundFunctionMain` is that the `data` field must be decoded from base64 encoding because Pub/Sub will encode the published message in base64 according to the https://cloud.google.com/functions/docs/calling/pubsub#event_structure[PubSubMessage event structure].

[source, java]
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@Bean
public Consumer<PubSubMessage> pubSubFunction() {
	return message -> {
		// The PubSubMessage data field arrives as a base-64 encoded string and must be decoded.
		// See: https://cloud.google.com/functions/docs/calling/pubsub#event_structure
		String decodedMessage = new String(Base64.decode(message.getData()), StandardCharsets.UTF_8);
		System.out.println("Received Pub/Sub message with data: " + decodedMessage);
	};
}
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