Firebase Cloud Firestore Overview

Antoine Marandon
Dec 2, 2019 · 4 min read

I spent over a year working on a Cloud Firestore application. I wrote this post to help people understand the power and limitations of the Firestore service.

What is firebase ?

Firebase Firestore is a cloud NOSQL database system. Clients can fetch part of the cloud database to consult on locally, and publish change that will be reflected almost instantly to other clients through the cloud master.

You could for exemple implement an app with a live drawing board or a chat where every new user input is a new row in a database, with changes reflected in real time to all users.

You can also use it 100% locally without cloud sync.

Here is a quick demo of two iOS apps sharing the same textfield through firebase:

See the github project here:

Disclaimer: the codes exemples are in Swift but Firestore is available on many other languages/platforms through a similar API.

Quick look into the API

Data structures

Firestore requires you to represent data into collections and documents.

In effect the database can be represented into a single JSON.

If you come from a relational world, this will force you to rethink the way you establish relationships.

Keeping the data integrity is entirely up to the developer (altho there are tools to help you with this), and lot of the logic that used to be guaranteed by the database/serverside is going to be entirely in your hands.

One good exemple is that DocumentReference can be stored, but unlike a SQL ForeignKey, it is not going to be nullified if the document pointed to is removed. DocumentReference are in fact simple URLS

Code exemples:

Do a one time fetch:

If collection “aCollection” and document “aDocument” don’t exist, they will be automatically created.

Subscribe to live objects update:

As long as the ListenerRegistration returned by addSnapshotListener is retained, updates will be published to the callback.

You can observe any fetch request, (queries, collections changes, documents changes, …)

Set a value in a document:

Together with a single setup function, we can say that boilerplate is kept to a minimum here. (Yes I’m thinking of you Core Data! )

Triggers and Web functions

Firebase have a Function tool. It works similarly to Amazon lambda, and comes preloaded with Firstore support.

Functions are written in NodeJS, and can be called either directly from a REST API, using the function SDK, or from a database trigger.

Database triggers are: onWrite, onCreate, onUpdate, onDelete

Some common use cases could be:

  • Generating a thumbnail
  • Sending a notification/email
  • Removing reference to deleted objects
  • Updating reciprocal values (it’s sometime easier/faster to duplicate data across the DB due to the inherent limitations of NOSQL)

Limitations and pitfalls

No OR Query

One of the most striking limitation is the absence of an OR or similar search keyword. It’s probably a performance tradeoff, so it’s unlikely this will be added in the future. The database is just not designed to be run this way.

You are left with two solutions:

  • Run multiple queries
  • Use an external database, such as elastic search.

Errors happen !

While it can be tempting to ignore the error parameter provided by firebase (and might even be fine for a prototype), be warned that error WILL happen. They will also happen where you might not have expected them, such as when accessing the database on a server side triggers. You can use batch operation and commits to guarantee to guard against partial failures, but it will require you to have some thoughtful design.

The firebase SDK updates for iOS can be buggy

I am not sure about the other platforms but I encountered a few crash (including one that required the affected users to reinstall the app) by updating the SDK to a defective version. I suggest to pin a version and upgrade carefully.


Firebase is an awesome tool. It allows you to implement in minutes things that would have required months of years of man hours before. The free tiers should also be enough for small/hobbist applications. There are however a few limitations to it, and I hope you now understand it better!

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Antoine Marandon

Written by

French software engineer working in Tokyo

Eureka Engineering

Learn about Eureka’s engineering efforts, product developments and more.

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