Example of triggering an appengine endpoint from a Firebase function.

1. Create a firebase project/cloud app (https://console.firebase.google.com/).
2. Enable firebase functions from the project's console.
3. Activate the app engine API (https://console.cloud.google.com -> getting started -> select API's, should be enabled by default)
4. Run npm i in the main directory and the /functions folder.
5. Install the Google cloud SDK if you haven't already (https://cloud.google.com/sdk/)
6. Set the firebase project (firebase use YOUR_PROJECT_ID) and gcloud project (gcloud config set project YOUR_PROJECT_ID) in the main directory.
7. Create a topic and a subscription (see About App engine)
8. Update variables in app.standard.yaml, app.js, /functions/index.js
9. To test locally see (Testing locally)
10. To test live: gcloud app deploy app.standard.yaml (please make sure you've select the correct Cloud project). Run the Firebase function locally (see About Firebase functions) and see the log on https://console.cloud.google.com/logs/viewer. See https://console.cloud.google.com/errors to see if the app engine server has encountered an error.

Firebase functions have a maximum timeout and memory. Some functions, such as video encoding, are too demanding for Firebase functions. Google Cloude app engine can handle these loads.

A Firebase function communicates to the Cloud App engine via Google PubSub.

We'll use NodeJs in this example.

You create a topic on a PubSub server. Topics can have multiple subscribers, meaning that if you publish a message to the PubSub topic, multiple 'endpoints' can receive the message.

Subscriptions can be based on a push or pull method (https://cloud.google.com/pubsub/docs/subscriber). With the push method the PubSub server will make calls to a predetermined endpoint until the endpoint responds with a ~200 status code or after 7 days (https://cloud.google.com/pubsub/docs/push). The server will make a call about every 7 seconds. For the pull method see https://cloud.google.com/pubsub/docs/pull#pubsub-pull-messages-async-nodejs.

The Firebase function uses the NodeJs pubsub module () to publish a message to a topic. The message needs to be given as a Buffer (https://nodejs.org/api/buffer.html).

You can manage your topics & subscriptions at https://console.cloud.google.com/cloudpubsub/.

To select a project: gcloud config set project YOUR_PROJECT_ID

To create a topic: gcloud pubsub topics create YOUR_TOPIC_NAME

To create a PubSub subscription:

gcloud pubsub subscriptions create YOUR_SUBSCRIPTION_NAME
    --topic YOUR_TOPIC_NAME
    --push-endpoint https://YOUR_PROJECT_ID.appspot.com/_ah/push-handlers/YOUR_HANDLER_NAME?token=YOUR_TOKEN
    --ack-deadline 10

The POST endpoint in your App engine should listen to _ah/push-handlers/YOUR_HANDLER_NAME.

Information on Firebase functions can be found on: https://firebase.google.com/docs/functions/

To select a project run firebase use YOUR_PROJECT_ID in the main directory.

To run the functions locally run firebase functions:shell in the main directory (not the /functions folder).

To run an onCall function: myCallableFunction({})

A Google cloud project can have multiple App engine instances. Each instance can be triggered by different endpoints, regulated in the dispatch.yaml file. Read more about it in this Medium article: https://medium.com/this-dot-labs/node-js-microservices-on-google-app-engine-b1193497fb4b, or the Cloud documentation: https://cloud.google.com/appengine/docs/standard/nodejs/how-instances-are-managed.

Each instance can be coupled with a PubSub topic by editing the .yaml file of the instance:

env_variables:
  PUBSUB_TOPIC: YOUR_TOPIC_ID
  PUBSUB_VERIFICATION_TOKEN: YOUR_SECRET_TOKEN

Instances can have three types of scaling: auto, manual and basic. The auto scaling options has a short deadline so we'll need to use one of the other types.

You need to set the environmental settings locally (these are normally picked up by the server from the .yaml file you deployed).

export PUBSUB_VERIFICATION_TOKEN=YOUR_TOKEN
export PUBSUB_TOPIC=YOUR_TOPIC_NAME

In order to run the App engine instance locally, you can use something like: curl -H "Content-Type: application/json" -i --data @sample_message.json "https://localhost:8080/_ah/push-handlers/YOUR_HANDLER_NAME?token=YOUR_TOKEN".

The sample_message.json should have the following structure:

{
  "messages": {
    "data" : "base64String"
  }
}