Using push subscriptions

Pub/Sub supports both push and pull message delivery. For an overview and comparison of pull and push subscriptions, see the Subscriber Overview. This document describes push delivery. For a discussion of pull delivery, see the Pull Subscriber Guide.

If a subscription uses push delivery, the Pub/Sub service delivers messages to a push endpoint. The push endpoint must be a publicly accessible HTTPS address. The server for the push endpoint must have a valid SSL certificate signed by a certificate authority.

The Pub/Sub service delivers messages to push endpoints from the same Google Cloud region that the Pub/Sub service stores the messages. The Pub/Sub service delivers messages from the same Google Cloud region on a best-effort basis.

In addition, push subscriptions can be configured to provide an authorization header to allow the endpoints to authenticate the requests. Automatic authentication and authorization mechanisms are available for App Engine Standard and Cloud Functions endpoints hosted in the same project as the subscription.

Receiving messages

When Pub/Sub delivers a message to a push endpoint, Pub/Sub sends the message in the body of a POST request. The body of the request is a JSON object and the message data is in the message.data field. The message data is base64-encoded.

The following example is the body of a POST request to a push endpoint:

{
    "message": {
        "attributes": {
            "key": "value"
        },
        "data": "SGVsbG8gQ2xvdWQgUHViL1N1YiEgSGVyZSBpcyBteSBtZXNzYWdlIQ==",
        "messageId": "2070443601311540",
        "message_id": "2070443601311540",
        "publishTime": "2021-02-26T19:13:55.749Z",
        "publish_time": "2021-02-26T19:13:55.749Z",
    },
   "subscription": "projects/myproject/subscriptions/mysubscription"
}

To receive messages from push subscriptions, use a webhook and process the POST requests that Pub/Sub sends to the push endpoint. For more information about processing these POST requests in App Engine, see Writing and responding to Pub/Sub messages.

After you receive a push request, return an HTTP status code. To acknowledge the message, return one of the following status codes:

  • 102
  • 200
  • 201
  • 202
  • 204

To send a negative acknowledgement for the message, return any other status code. If you send a negative acknowledgement or the acknowledgement deadline expires, Pub/Sub resends the message. You can't modify the acknowledgement deadline of individual messages that you receive from push subscriptions.

Authentication and authorization

If a push subscription uses authentication, the Pub/Sub service signs a JSON Web Token (JWT) and sends the JWT in the authorization header of the push request. The JWT includes claims and a signature.

Subscribers can decode the JWT and verify the following:

  • The claims are accurate.
  • The Pub/Sub service signed the claims.

If subscribers use a firewall, they can't receive push requests. To receive push requests, you must turn off the firewall and verify the JWT.

JWT format

The JWT is an OpenIDConnect JWT that consists of a header, claim set, and signature. The Pub/Sub service encodes the JWT as a base64 string with period delimiters.

For example, the following authorization header includes an encoded JWT:

"Authorization" : "Bearer
eyJhbGciOiJSUzI1NiIsImtpZCI6IjdkNjgwZDhjNzBkNDRlOTQ3MTMzY2JkNDk5ZWJjMWE2MWMzZDVh
YmMiLCJ0eXAiOiJKV1QifQ.eyJhdWQiOiJodHRwczovL2V4YW1wbGUuY29tIiwiYXpwIjoiMTEzNzc0M
jY0NDYzMDM4MzIxOTY0IiwiZW1haWwiOiJnYWUtZ2NwQGFwcHNwb3QuZ3NlcnZpY2VhY2NvdW50LmNvb
SIsImVtYWlsX3ZlcmlmaWVkIjp0cnVlLCJleHAiOjE1NTAxODU5MzUsImlhdCI6MTU1MDE4MjMzNSwia
XNzIjoiaHR0cHM6Ly9hY2NvdW50cy5nb29nbGUuY29tIiwic3ViIjoiMTEzNzc0MjY0NDYzMDM4MzIxO
TY0In0.QVjyqpmadTyDZmlX2u3jWd1kJ68YkdwsRZDo-QxSPbxjug4ucLBwAs2QePrcgZ6hhkvdc4UHY
4YF3fz9g7XHULNVIzX5xh02qXEH8dK6PgGndIWcZQzjSYfgO-q-R2oo2hNM5HBBsQN4ARtGK_acG-NGG
WM3CQfahbEjZPAJe_B8M7HfIu_G5jOLZCw2EUcGo8BvEwGcLWB2WqEgRM0-xt5-UPzoa3-FpSPG7DHk7
z9zRUeq6eB__ldb-2o4RciJmjVwHgnYqn3VvlX9oVKEgXpNFhKuYA-mWh5o7BCwhujSMmFoBOh6mbIXF
cyf5UiVqKjpqEbqPGo_AvKvIQ9VTQ"

The header and claim set are JSON strings. Once decoded, they take the following form:

{"alg":"RS256","kid":"7d680d8c70d44e947133cbd499ebc1a61c3d5abc","typ":"JWT"}

{
   "aud":"https://example.com",
   "azp":"113774264463038321964",
   "email":"gae-gcp@appspot.gserviceaccount.com",
   "sub":"113774264463038321964",
   "email_verified":true,
   "exp":1550185935,
   "iat":1550182335,
   "iss":"https://accounts.google.com"
  }

The tokens attached to requests sent to push endpoints may be up to an hour old.

Setting up Pub/Sub for push authentication

Authentication configuration for a subscription consists of two parameters:

  • Service account: The GCP service account associated with the push subscription. Push requests carry the identity of this service account. As an example, a push subscription configured with a service account that has the role roles/run.invoker and is bound to a particular Cloud Run service can invoke that Cloud Run service.
  • Token audience (optional): A single, case-insensitive string that can be used by the webhook to validate the intended audience of this particular token.

In addition to configuring these fields, you must also grant Pub/Sub the permissions needed to create tokens for your service account. Pub/Sub creates and maintains a special service account for your project: service-{PROJECT_NUMBER}@gcp-sa-pubsub.iam.gserviceaccount.com where {PROJECT_NUMBER} is the GCP project that contains the subscription. This service account needs the Service Account Token Creator role. If you use the Cloud Console to set up the subscription for push authentication or you use a project created after April 8, 2021, the role is granted automatically as part of the Pub/Sub service agent role. Otherwise, you must explicitly grant the role to the account.

COMMAND-LINE

# Only for projects created on or before April 8, 2021:
# grant Cloud Pub/Sub the permission to create tokens
PUBSUB_SERVICE_ACCOUNT="service-${PROJECT_NUMBER}@gcp-sa-pubsub.iam.gserviceaccount.com"
gcloud projects add-iam-policy-binding ${PROJECT_ID} \
 --member="serviceAccount:${PUBSUB_SERVICE_ACCOUNT}"\
 --role='roles/iam.serviceAccountTokenCreator'

# For all projects:
# configure the subscription push identity
gcloud pubsub subscriptions (create|update|modify-push-config) ${SUBSCRIPTION} \
 --topic=${TOPIC} \
 --push-endpoint=${PUSH_ENDPOINT_URI} \
 --push-auth-service-account=${SERVICE_ACCOUNT_EMAIL} \
 --push-auth-token-audience=${OPTIONAL_AUDIENCE_OVERRIDE}

console

  1. Go to the Pub/Sub Topics page.

    Go to the Topics page

  2. Click a topic name.

  3. Create or update a subscription.

  4. Enter an identity and (optionally) an audience.

Authentication and authorization by the push endpoint

Claims

The JWT can be used to validate that the claims -- including email and aud claims -- are signed by Google. For more information about how Google's OAuth 2.0 APIs can be used for both authentication and authorization, see OpenID Connect.

There are two mechanisms that make these claims meaningful. First, Pub/Sub requires that the user or service account used to associate a service account identity with a push subscription have the Service Account User role (roles/iam.serviceAccountUser) for the project or the service account.

Second, access to the certificates used to sign the tokens is tightly controlled. To create the token, Pub/Sub must call an internal Google service using a separate signing service account identity. The signing service account must be authorized to create tokens for the claimed service account or the project containing the account. This is done using the iam.serviceAccounts.getOpenIdToken permission or a Service Account Token Creator role (roles/iam.serviceAccountTokenCreator).

Validating tokens

Validating tokens sent by Pub/Sub to the push endpoint involves:

  • Checking the token integrity via signature validation.
  • Ensuring that the email and audience claims in the token match the values set in the push subscription configuration.

The following example illustrates how to authenticate a push request to a App Engine application.

protocol

Request:

GET https://oauth2.googleapis.com/tokeninfo?id_token={BEARER_TOKEN}

Response:

200 OK
{
    "alg": "RS256",
    "aud": "example.com",
    "azp": "104176025330667568672",
    "email": "{SERVICE_ACCOUNT_NAME}@{YOUR_PROJECT_NAME}.iam.gserviceaccount.com",
    "email_verified": "true",
    "exp": "1555463097",
    "iat": "1555459497",
    "iss": "https://accounts.google.com",
    "kid": "3782d3f0bc89008d9d2c01730f765cfb19d3b70e",
    "sub": "104176025330667568672",
    "typ": "JWT"
}

Go

// receiveMessagesHandler validates authentication token and caches the Pub/Sub
// message received.
func (a *app) receiveMessagesHandler(w http.ResponseWriter, r *http.Request) {
	if r.Method != "POST" {
		http.Error(w, http.StatusText(http.StatusMethodNotAllowed), http.StatusMethodNotAllowed)
		return
	}

	// Verify that the request originates from the application.
	// a.pubsubVerificationToken = os.Getenv("PUBSUB_VERIFICATION_TOKEN")
	if token, ok := r.URL.Query()["token"]; !ok || len(token) != 1 || token[0] != a.pubsubVerificationToken {
		http.Error(w, "Bad token", http.StatusBadRequest)
		return
	}

	// Get the Cloud Pub/Sub-generated JWT in the "Authorization" header.
	authHeader := r.Header.Get("Authorization")
	if authHeader == "" || len(strings.Split(authHeader, " ")) != 2 {
		http.Error(w, "Missing Authorization header", http.StatusBadRequest)
		return
	}
	token := strings.Split(authHeader, " ")[1]
	// Verify and decode the JWT.
	// If you don't need to control the HTTP client used you can use the
	// convenience method idtoken.Validate instead of creating a Validator.
	v, err := idtoken.NewValidator(r.Context(), option.WithHTTPClient(a.defaultHTTPClient))
	if err != nil {
		http.Error(w, "Unable to create Validator", http.StatusBadRequest)
		return
	}
	// Please change http://example.com to match with the value you are
	// providing while creating the subscription.
	payload, err := v.Validate(r.Context(), token, "http://example.com")
	if err != nil {
		http.Error(w, fmt.Sprintf("Invalid Token: %v", err), http.StatusBadRequest)
		return
	}
	if payload.Issuer != "accounts.google.com" && payload.Issuer != "https://accounts.google.com" {
		http.Error(w, "Wrong Issuer", http.StatusBadRequest)
		return
	}

	// IMPORTANT: you should validate claim details not covered by signature
	// and audience verification above, including:
	//   - Ensure that `payload.Claims["email"]` is equal to the expected service
	//     account set up in the push subscription settings.
	//   - Ensure that `payload.Claims["email_verified"]` is set to true.
	if payload.Claims["email"] != "test-service-account-email@example.com" || payload.Claims["email_verified"] != true {
		http.Error(w, "Unexpected email identity", http.StatusBadRequest)
		return
	}

	var pr pushRequest
	if err := json.NewDecoder(r.Body).Decode(&pr); err != nil {
		http.Error(w, fmt.Sprintf("Could not decode body: %v", err), http.StatusBadRequest)
		return
	}

	a.messagesMu.Lock()
	defer a.messagesMu.Unlock()
	// Limit to ten.
	a.messages = append(a.messages, pr.Message.Data)
	if len(a.messages) > maxMessages {
		a.messages = a.messages[len(a.messages)-maxMessages:]
	}

	fmt.Fprint(w, "OK")
}

Java

@WebServlet(value = "/pubsub/authenticated-push")
public class PubSubAuthenticatedPush extends HttpServlet {
  private final String pubsubVerificationToken = System.getenv("PUBSUB_VERIFICATION_TOKEN");
  private final MessageRepository messageRepository;
  private final GoogleIdTokenVerifier verifier =
      new GoogleIdTokenVerifier.Builder(new NetHttpTransport(), new JacksonFactory())
          /**
           * Please change example.com to match with value you are providing while creating
           * subscription as provided in @see <a
           * href="https://github.com/GoogleCloudPlatform/java-docs-samples/tree/main/appengine-java8/pubsub">README</a>.
           */
          .setAudience(Collections.singletonList("example.com"))
          .build();
  private final Gson gson = new Gson();
  private final JsonParser jsonParser = new JsonParser();

  @Override
  public void doPost(HttpServletRequest req, HttpServletResponse resp)
      throws IOException, ServletException {

    // Verify that the request originates from the application.
    if (req.getParameter("token").compareTo(pubsubVerificationToken) != 0) {
      resp.setStatus(HttpServletResponse.SC_BAD_REQUEST);
      return;
    }
    // Get the Cloud Pub/Sub-generated JWT in the "Authorization" header.
    String authorizationHeader = req.getHeader("Authorization");
    if (authorizationHeader == null
        || authorizationHeader.isEmpty()
        || authorizationHeader.split(" ").length != 2) {
      resp.setStatus(HttpServletResponse.SC_BAD_REQUEST);
      return;
    }
    String authorization = authorizationHeader.split(" ")[1];

    try {
      // Verify and decode the JWT.
      // Note: For high volume push requests, it would save some network overhead
      // if you verify the tokens offline by decoding them using Google's Public
      // Cert; caching already seen tokens works best when a large volume of
      // messsages have prompted a singple push server to handle them, in which
      // case they would all share the same token for a limited time window.
      GoogleIdToken idToken = verifier.verify(authorization);

      GoogleIdToken.Payload payload = idToken.getPayload();
      // IMPORTANT: you should validate claim details not covered by signature
      // and audience verification above, including:
      //   - Ensure that `payload.getEmail()` is equal to the expected service
      //     account set up in the push subscription settings.
      //   - Ensure that `payload.getEmailVerified()` is set to true.

      messageRepository.saveToken(authorization);
      messageRepository.saveClaim(payload.toPrettyString());
      // parse message object from "message" field in the request body json
      // decode message data from base64
      Message message = getMessage(req);
      messageRepository.save(message);
      // 200, 201, 204, 102 status codes are interpreted as success by the Pub/Sub system
      resp.setStatus(102);
      super.doPost(req, resp);
    } catch (Exception e) {
      resp.setStatus(HttpServletResponse.SC_BAD_REQUEST);
    }
  }

  private Message getMessage(HttpServletRequest request) throws IOException {
    String requestBody = request.getReader().lines().collect(Collectors.joining("\n"));
    JsonElement jsonRoot = jsonParser.parse(requestBody);
    String messageStr = jsonRoot.getAsJsonObject().get("message").toString();
    Message message = gson.fromJson(messageStr, Message.class);
    // decode from base64
    String decoded = decode(message.getData());
    message.setData(decoded);
    return message;
  }

  private String decode(String data) {
    return new String(Base64.getDecoder().decode(data));
  }

  PubSubAuthenticatedPush(MessageRepository messageRepository) {
    this.messageRepository = messageRepository;
  }

  public PubSubAuthenticatedPush() {
    this(MessageRepositoryImpl.getInstance());
  }
}

Node.js

app.post('/pubsub/authenticated-push', jsonBodyParser, async (req, res) => {
  // Verify that the request originates from the application.
  if (req.query.token !== PUBSUB_VERIFICATION_TOKEN) {
    res.status(400).send('Invalid request');
    return;
  }

  // Verify that the push request originates from Cloud Pub/Sub.
  try {
    // Get the Cloud Pub/Sub-generated JWT in the "Authorization" header.
    const bearer = req.header('Authorization');
    const [, token] = bearer.match(/Bearer (.*)/);
    tokens.push(token);

    // Verify and decode the JWT.
    // Note: For high volume push requests, it would save some network
    // overhead if you verify the tokens offline by decoding them using
    // Google's Public Cert; caching already seen tokens works best when
    // a large volume of messages have prompted a single push server to
    // handle them, in which case they would all share the same token for
    // a limited time window.
    const ticket = await authClient.verifyIdToken({
      idToken: token,
      audience: 'example.com',
    });

    const claim = ticket.getPayload();

    // IMPORTANT: you should validate claim details not covered
    // by signature and audience verification above, including:
    //   - Ensure that `claim.email` is equal to the expected service
    //     account set up in the push subscription settings.
    //   - Ensure that `claim.email_verified` is set to true.

    claims.push(claim);
  } catch (e) {
    res.status(400).send('Invalid token');
    return;
  }

  // The message is a unicode string encoded in base64.
  const message = Buffer.from(req.body.message.data, 'base64').toString(
    'utf-8'
  );

  messages.push(message);

  res.status(200).send();
});

Python

@app.route('/push-handlers/receive_messages', methods=['POST'])
def receive_messages_handler():
    # Verify that the request originates from the application.
    if (request.args.get('token', '') !=
            current_app.config['PUBSUB_VERIFICATION_TOKEN']):
        return 'Invalid request', 400

    # Verify that the push request originates from Cloud Pub/Sub.
    try:
        # Get the Cloud Pub/Sub-generated JWT in the "Authorization" header.
        bearer_token = request.headers.get('Authorization')
        token = bearer_token.split(' ')[1]
        TOKENS.append(token)

        # Verify and decode the JWT. `verify_oauth2_token` verifies
        # the JWT signature, the `aud` claim, and the `exp` claim.
        # Note: For high volume push requests, it would save some network
        # overhead if you verify the tokens offline by downloading Google's
        # Public Cert and decode them using the `google.auth.jwt` module;
        # caching already seen tokens works best when a large volume of
        # messages have prompted a single push server to handle them, in which
        # case they would all share the same token for a limited time window.
        claim = id_token.verify_oauth2_token(token, requests.Request(),
                                             audience='example.com')

        # IMPORTANT: you should validate claim details not covered by signature
        # and audience verification above, including:
        #   - Ensure that `claim["email"]` is equal to the expected service
        #     account set up in the push subscription settings.
        #   - Ensure that `claim["email_verified"]` is set to true.

        CLAIMS.append(claim)
    except Exception as e:
        return 'Invalid token: {}\n'.format(e), 400

    envelope = json.loads(request.data.decode('utf-8'))
    payload = base64.b64decode(envelope['message']['data'])
    MESSAGES.append(payload)
    # Returning any 2xx status indicates successful receipt of the message.
    return 'OK', 200

Ruby

post "/pubsub/authenticated-push" do
  halt 400 if params[:token] != PUBSUB_VERIFICATION_TOKEN

  begin
    bearer = request.env["HTTP_AUTHORIZATION"]
    token = /Bearer (.*)/.match(bearer)[1]
    claim = Google::Auth::IDTokens.verify_oidc token, aud: "example.com"
    claims.push claim
  rescue Google::Auth::IDTokens::VerificationError => e
    puts "VerificationError: #{e.message}"
    halt 400, "Invalid token"
  end

  message = JSON.parse request.body.read
  payload = Base64.decode64 message["message"]["data"]

  messages.push payload
end

For information on environment variables such as PUBSUB_VERIFICATION_TOKEN, see Writing and responding to Pub/Sub messages.

You will find additional examples of how to validate the bearer JWT in this Guide for Google Sign-in for Websites. A broader overview of OpenID tokens is available in the OpenID Connect Guide, including a list of client libraries that help validate JWTs.

Cloud Run and App Engine

Cloud Run and App Engine automatically authenticate HTTP calls by verifying Pub/Sub-generated tokens. The only configuration required of the user is that the necessary IAM roles be granted to the caller account. For example, you can authorize or revoke permission to call a particular Cloud Run endpoint for an account. For details, see the following tutorials:

Stopping and resuming delivery

To temporarily stop Pub/Sub from sending requests to the push endpoint, change the subscription to pull. Note that it can take several minutes for this changeover to take effect.

To resume push delivery, set the URL to a valid endpoint again. To permanently stop delivery, delete the subscription.

Quotas, limits and delivery rate

Note that push subscriptions are subject to a set of quotas and resource limits.

Push backoff

If a push subscriber sends negative acknowledgements, Pub/Sub might deliver messages using a push backoff. When Pub/Sub uses a push backoff, it stops delivering messages for 100 milliseconds to 60 seconds and then starts delivering messages again.

The push backoff is an exponential backoff that prevents a push subscriber from receiving messages that it can't process. The amount of time that Pub/Sub stops delivering messages depends on the number of negative acknowledgments that push subscribers send.

For example, if a push subscriber receives five messages per second and sends one negative acknowledgment per second, Pub/Sub delivers messages approximately every 500 milliseconds. If the push subscriber sends five negative acknowledgments per second, Pub/Sub delivers messages every 30-60 seconds.

Delivery rate

Pub/Sub adjusts the number of concurrent push requests using a slow-start algorithm. The maximum allowed number of concurrent push requests is the push window. The push window increases on any successful delivery and decreases on any failure. The system starts with a small window: 3 times N, where N is the number of publish regions.

When a subscriber acknowledges messages, the window increases exponentially up to 3,000 times N outstanding messages. For subscriptions where subscribers acknowledge greater than 99% of messages and average less than one second of push request latency, the push window increases up to 30,000 times N outstanding messages.

The push request latency includes the following:

After 3,000 outstanding messages, the window increases linearly to prevent the push endpoint from receiving too many messages. If the average latency exceeds one second or the subscriber acknowledges less than 99% of requests, the window decreases to the lower limit of 3,000 outstanding messages.

For more information about the metrics you can use to monitor push delivery, see Monitoring push subscriptions.