google cloud firestore - Firebase centered architecture

Question

This is an architectural question.

I am currently in the process of designing a web application and I am used to a basic: frontend, api, database, microservices setup.

For the sake of saving money and making my architecture a little bit more modern than what I am used to I decided to look into serverless.

The two main parts I am interested in are google cloud functions and firebase. My understanding is that google cloud functions can be fired when a database entry in firebase has been manipulated.

The way I used to communicate between services was through message queues such as RabbitMQ but it seems to me that by using firebase and cloud functions you can build communication through the database without the need for message queues. What I mean by communication in this case, would be that one service would be able to react to the execution of another service by seeing that an entry in the database was changed.

My question therefore is, what are the upsides and downsides of letting all your "communication" between microservices run through firebase instead of message queues, and is this an architecture that is generally used?

Answer 1

AFAIK, cloud function triggers is a beta feature in Firebase, and according to the doc, there are some limitations for firestore trigger events:

• It can take up to 10 seconds for a function to respond to changes in Cloud Firestore.
• Ordering is not guaranteed. Rapid changes can trigger function invocations in an unexpected order.
• Events are delivered at least once, but a single event may result in multiple function invocations. Avoid depending on exactly-once mechanics, and write idempotent functions.
• Cloud Firestore triggers for Cloud Functions is available only for Cloud Firestore in Native mode. It is not available for Cloud Firestore in Datastore mode.

The most concerning limitation here is the first one. 10 seconds for an update is a long time if you need that update to be visible to the user.

Another disadvantage I see is that it may run out of control (in terms of system design) as the complexity increases. You may be tempted to add events for everything, and it may be hard to partition them by category, for example (in message queues, you can use topics for that).

Also, according to the doc, cloud functions are rate-limited to 16 invocations per 100 seconds, which may quickly be reached if you got some traffic on your app.

I would use trigger-events for isolated scenarios and use a message queue for the backbone communication between microservices.