This page describes how to use Cloud Functions for event-based DAG triggers.
While Airflow is designed to run DAGs on a regular schedule, you can trigger
DAGs in response to events, such as a change in a Cloud Storage bucket or a
message pushed to Pub/Sub. To accomplish this,
Cloud Composer DAGs can be triggered by
Cloud Functions.
The example in this guide runs a DAG every time a change occurs in a
Cloud Storage bucket. Object-change metadata are passed into the DAG's
config.
Enabling APIs for your project
Enable the
Cloud Composer, Cloud Functions, and Cloud Identity and Access Management (Cloud IAM) APIs.
Enable the
APIs
Granting blob signing permissions to the Cloud Functions Service Account
To authenticate to IAP, grant the Appspot Service Account (used
by Cloud Functions) the Service Account Token Creator role on itself.
To do this, execute the following command in the gcloud
command-line tool or Cloud Shell:
gcloud iam service-accounts add-iam-policy-binding \
your-project-id@appspot.gserviceaccount.com \
--member=serviceAccount:your-project-id@appspot.gserviceaccount.com \
--role=roles/iam.serviceAccountTokenCreator
Getting the client ID
To construct a token to authenticate to
IAP,
the function requires the client ID of the proxy that protects the Airflow webserver.
The Cloud Composer API does not provide this information directly. Instead,
make an unauthenticated request to the Airflow webserver and capture the client
ID from the redirect URL. The following Python code sample demonstrates how to
get the client ID:
Creating your function
Node.js
Using the Node.js 8 runtime or above, create a function with the index.js
and package.json files shown below, filling in the first four constants (see
Create a Function).
Enable Retry on failure.
When you have finished, your function should resemble the following graphic:
index.js
'use strict';
const fetch = require('node-fetch');
const FormData = require('form-data');
/**
* Triggered from a message on a Cloud Storage bucket.
*
* IAP authorization based on:
* https://stackoverflow.com/questions/45787676/how-to-authenticate-google-cloud-functions-for-access-to-secure-app-engine-endpo
* and
* https://cloud.google.com/iap/docs/authentication-howto
*
* @param {!Object} data The Cloud Functions event data.
* @returns {Promise}
*/
exports.triggerDag = async data => {
// Fill in your Composer environment information here.
// The project that holds your function
const PROJECT_ID = 'your-project-id';
// Navigate to your webserver's login page and get this from the URL
const CLIENT_ID = 'your-iap-client-id';
// This should be part of your webserver's URL:
// {tenant-project-id}.appspot.com
const WEBSERVER_ID = 'your-tenant-project-id';
// The name of the DAG you wish to trigger
const DAG_NAME = 'composer_sample_trigger_response_dag';
// Other constants
const WEBSERVER_URL = `https://${WEBSERVER_ID}.appspot.com/api/experimental/dags/${DAG_NAME}/dag_runs`;
const USER_AGENT = 'gcf-event-trigger';
const BODY = {conf: JSON.stringify(data)};
// Make the request
try {
const iap = await authorizeIap(CLIENT_ID, PROJECT_ID, USER_AGENT);
return makeIapPostRequest(WEBSERVER_URL, BODY, iap.idToken, USER_AGENT);
} catch (err) {
console.error('Error authorizing IAP:', err.message);
throw new Error(err);
}
};
/**
* @param {string} clientId The client id associated with the Composer webserver application.
* @param {string} projectId The id for the project containing the Cloud Function.
* @param {string} userAgent The user agent string which will be provided with the webserver request.
*/
const authorizeIap = async (clientId, projectId, userAgent) => {
const SERVICE_ACCOUNT = `${projectId}@appspot.gserviceaccount.com`;
const JWT_HEADER = Buffer.from(
JSON.stringify({alg: 'RS256', typ: 'JWT'})
).toString('base64');
let jwt = '';
let jwtClaimset = '';
// Obtain an Oauth2 access token for the appspot service account
const res = await fetch(
`http://metadata.google.internal/computeMetadata/v1/instance/service-accounts/${SERVICE_ACCOUNT}/token`,
{
headers: {'User-Agent': userAgent, 'Metadata-Flavor': 'Google'},
}
);
const tokenResponse = await res.json();
if (tokenResponse.error) {
console.error('Error in token reponse:', tokenResponse.error.message);
return Promise.reject(tokenResponse.error);
}
const accessToken = tokenResponse.access_token;
const iat = Math.floor(new Date().getTime() / 1000);
const claims = {
iss: SERVICE_ACCOUNT,
aud: 'https://www.googleapis.com/oauth2/v4/token',
iat: iat,
exp: iat + 60,
target_audience: clientId,
};
jwtClaimset = Buffer.from(JSON.stringify(claims)).toString('base64');
const toSign = [JWT_HEADER, jwtClaimset].join('.');
const blob = await fetch(
`https://iam.googleapis.com/v1/projects/${projectId}/serviceAccounts/${SERVICE_ACCOUNT}:signBlob`,
{
method: 'POST',
body: JSON.stringify({
bytesToSign: Buffer.from(toSign).toString('base64'),
}),
headers: {
'User-Agent': userAgent,
Authorization: `Bearer ${accessToken}`,
},
}
);
const blobJson = await blob.json();
if (blobJson.error) {
console.error('Error in blob signing:', blobJson.error.message);
return Promise.reject(blobJson.error);
}
// Request service account signature on header and claimset
const jwtSignature = blobJson.signature;
jwt = [JWT_HEADER, jwtClaimset, jwtSignature].join('.');
const form = new FormData();
form.append('grant_type', 'urn:ietf:params:oauth:grant-type:jwt-bearer');
form.append('assertion', jwt);
const token = await fetch('https://www.googleapis.com/oauth2/v4/token', {
method: 'POST',
body: form,
});
const tokenJson = await token.json();
if (tokenJson.error) {
console.error('Error fetching token:', tokenJson.error.message);
return Promise.reject(tokenJson.error);
}
return {
idToken: tokenJson.id_token,
};
};
/**
* @param {string} url The url that the post request targets.
* @param {string} body The body of the post request.
* @param {string} idToken Bearer token used to authorize the iap request.
* @param {string} userAgent The user agent to identify the requester.
*/
const makeIapPostRequest = async (url, body, idToken, userAgent) => {
const res = await fetch(url, {
method: 'POST',
headers: {
'User-Agent': userAgent,
Authorization: `Bearer ${idToken}`,
},
body: JSON.stringify(body),
});
if (!res.ok) {
const err = await res.text();
console.error('Error making IAP post request:', err.message);
throw new Error(err);
}
};
Update your package.json with the following dependencies.
package.json
Python
Create a function with the main.py and requirements.txt files shown below,
filling in the first four constants (see
Create a Function).
Enable Retry on failure.
When you have finished, your function should resemble the following graphic:
main.py
import google.auth
import google.auth.compute_engine.credentials
import google.auth.iam
from google.auth.transport.requests import Request
import google.oauth2.credentials
import google.oauth2.service_account
import requests
IAM_SCOPE = 'https://www.googleapis.com/auth/iam'
OAUTH_TOKEN_URI = 'https://www.googleapis.com/oauth2/v4/token'
def trigger_dag(data, context=None):
"""Makes a POST request to the Composer DAG Trigger API
When called via Google Cloud Functions (GCF),
data and context are Background function parameters.
For more info, refer to
https://cloud.google.com/functions/docs/writing/background#functions_background_parameters-python
To call this function from a Python script, omit the ``context`` argument
and pass in a non-null value for the ``data`` argument.
"""
# Fill in with your Composer info here
# Navigate to your webserver's login page and get this from the URL
# Or use the script found at
# https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/composer/rest/get_client_id.py
client_id = 'YOUR-CLIENT-ID'
# This should be part of your webserver's URL:
# {tenant-project-id}.appspot.com
webserver_id = 'YOUR-TENANT-PROJECT'
# The name of the DAG you wish to trigger
dag_name = 'composer_sample_trigger_response_dag'
webserver_url = (
'https://'
+ webserver_id
+ '.appspot.com/api/experimental/dags/'
+ dag_name
+ '/dag_runs'
)
# Make a POST request to IAP which then Triggers the DAG
make_iap_request(webserver_url, client_id, method='POST', json=data)
# This code is copied from
# https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/iap/make_iap_request.py
# START COPIED IAP CODE
def make_iap_request(url, client_id, method='GET', **kwargs):
"""Makes a request to an application protected by Identity-Aware Proxy.
Args:
url: The Identity-Aware Proxy-protected URL to fetch.
client_id: The client ID used by Identity-Aware Proxy.
method: The request method to use
('GET', 'OPTIONS', 'HEAD', 'POST', 'PUT', 'PATCH', 'DELETE')
**kwargs: Any of the parameters defined for the request function:
https://github.com/requests/requests/blob/master/requests/api.py
If no timeout is provided, it is set to 90 by default.
Returns:
The page body, or raises an exception if the page couldn't be retrieved.
"""
# Set the default timeout, if missing
if 'timeout' not in kwargs:
kwargs['timeout'] = 90
# Figure out what environment we're running in and get some preliminary
# information about the service account.
bootstrap_credentials, _ = google.auth.default(
scopes=[IAM_SCOPE])
# For service account's using the Compute Engine metadata service,
# service_account_email isn't available until refresh is called.
bootstrap_credentials.refresh(Request())
signer_email = bootstrap_credentials.service_account_email
if isinstance(bootstrap_credentials,
google.auth.compute_engine.credentials.Credentials):
# Since the Compute Engine metadata service doesn't expose the service
# account key, we use the IAM signBlob API to sign instead.
# In order for this to work:
# 1. Your VM needs the https://www.googleapis.com/auth/iam scope.
# You can specify this specific scope when creating a VM
# through the API or gcloud. When using Cloud Console,
# you'll need to specify the "full access to all Cloud APIs"
# scope. A VM's scopes can only be specified at creation time.
# 2. The VM's default service account needs the "Service Account Actor"
# role. This can be found under the "Project" category in Cloud
# Console, or roles/iam.serviceAccountActor in gcloud.
signer = google.auth.iam.Signer(
Request(), bootstrap_credentials, signer_email)
else:
# A Signer object can sign a JWT using the service account's key.
signer = bootstrap_credentials.signer
# Construct OAuth 2.0 service account credentials using the signer
# and email acquired from the bootstrap credentials.
service_account_credentials = google.oauth2.service_account.Credentials(
signer, signer_email, token_uri=OAUTH_TOKEN_URI, additional_claims={
'target_audience': client_id
})
# service_account_credentials gives us a JWT signed by the service
# account. Next, we use that to obtain an OpenID Connect token,
# which is a JWT signed by Google.
google_open_id_connect_token = get_google_open_id_connect_token(
service_account_credentials)
# Fetch the Identity-Aware Proxy-protected URL, including an
# Authorization header containing "Bearer " followed by a
# Google-issued OpenID Connect token for the service account.
resp = requests.request(
method, url,
headers={'Authorization': 'Bearer {}'.format(
google_open_id_connect_token)}, **kwargs)
if resp.status_code == 403:
raise Exception('Service account {} does not have permission to '
'access the IAP-protected application.'.format(
signer_email))
elif resp.status_code != 200:
raise Exception(
'Bad response from application: {!r} / {!r} / {!r}'.format(
resp.status_code, resp.headers, resp.text))
else:
return resp.text
def get_google_open_id_connect_token(service_account_credentials):
"""Get an OpenID Connect token issued by Google for the service account.
This function:
1. Generates a JWT signed with the service account's private key
containing a special "target_audience" claim.
2. Sends it to the OAUTH_TOKEN_URI endpoint. Because the JWT in #1
has a target_audience claim, that endpoint will respond with
an OpenID Connect token for the service account -- in other words,
a JWT signed by *Google*. The aud claim in this JWT will be
set to the value from the target_audience claim in #1.
For more information, see
https://developers.google.com/identity/protocols/OAuth2ServiceAccount .
The HTTP/REST example on that page describes the JWT structure and
demonstrates how to call the token endpoint. (The example on that page
shows how to get an OAuth2 access token; this code is using a
modified version of it to get an OpenID Connect token.)
"""
service_account_jwt = (
service_account_credentials._make_authorization_grant_assertion())
request = google.auth.transport.requests.Request()
body = {
'assertion': service_account_jwt,
'grant_type': google.oauth2._client._JWT_GRANT_TYPE,
}
token_response = google.oauth2._client._token_endpoint_request(
request, OAUTH_TOKEN_URI, body)
return token_response['id_token']
# END COPIED IAP CODE
Update your requirements.txt with the following dependencies.
requirements.txt
Setting up your DAG
The following DAG contains a BashOperator which prints object-change metadata.
In order for it to run in response to Cloud Functions events, copy it
into your environment's DAGs folder (see
Manage DAGs).
trigger_response_dag.py
Testing your function
Upload a file to your Cloud Storage bucket. You should see that the DAG has
been triggered and information about the Cloud Storage change has been
logged by the BashOperator.
What's next
