Firebase Firestore/Storage security rules minifier

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Firebase Firestore/Storage security rules minifier

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If you've ever reached Firestore's maximum security rule file size of 256KB, the next question is... "now what?".

At the moment, the limit is actually around 50KB. As you get close to this size the rules upload API will occasionally begin throwing...

HTTP Error: 400, Request contains an invalid argument.

This will continue to get worse as the rules file gets larger until it happens so frequently that the rules are undeployable.

See more information here...

Firebase Support's answer

  • The Firestore rules max file size limit cannot be lifted or raised
  • You can define custom functions, which can be reusable throughout the ruleset
    • This will definitely save you a lot of data space, and makes it look organized, as overused conditions can be called in one place
  • If possible, reconsider the database structure by making it efficient This means the less the number of collections and subcollections, the less rules are written, which makes the ruleset smaller in size
    • Refactor your database structure and security rules as much as possible by removing unnecessary or redundant parts
  • Minimize the use of data validation rules, and put it on application-level instead
    • Not only it can reduce lines of code, but it can reduce the number of expressions evaluated to avoid reaching the given limit of 1000 expressions per request
    • As much as possible, use your app logic to ensure your data is on the right character length, correct data type, meets the patter criteria, etc. You may also use your app's advanced UI elements like a password textbox, a textbox that you can limit the character length, among others

Thoughts on those options

  • The Firestore rules max file size limit cannot be lifted or raised

    • Well shit...
  • Use custom functions

    • This make sense and should definitely be used, but what if I've already done that?
  • Reconsider the database structure

    • Having to overhaul my application for the sake of reducing my rules file size is a huge ask for little gain.
  • Minimize the use of data validation rules

    • This to me was suprising. Firebase is designed in such a way that you can directly connect your client to your database which removes the middle application logic. The fact that Firebase is suggesting to do this seems to go directly against the architectural nature of Firebase. So, if you're building your application in this way and want to keep that architecture (and your security), this is a non-option.

Another approach

  • Technically, this problem is not too dissimilar from trying to keep javascript file size down on the web. We can use a lot of the same approaches taken by javascript minifiers to reduce the size of our firebase rules file.
  • A minifier can do the following before deployment
    • Remove comments
    • Remove unnecessary whitespace
    • Remove unused functions
    • Replace function names and function parameter names with shorter single character names
    • Collapse single use functions
    • and much more...

Enter Firemin...

  • Firemin is a minifier for Firebase Firestore security rules


  • Removes comments from firestore rules
  • Removes unnecessary whitespace
  • Removes unused functions from your rules file
  • Collapses single use functions (functions that are only invoked once)
  • Collapse small functions (functions that only perform few operation do not necessarily save us anything on code size and create more operation overhead)


  • Allow for option to preserve white space (for debugging)
  • Replace function names and function parameter names with shorter single character names
  • Collapse single use let declarations


npm install --save-dev firemin


Switch your firestore rules in your firebase.json to firestore.min.rules and add a predeploy hook to run firemin

  "firestore": {
    "predeploy": "firemin minimize -f './firestore.rules -o ./firestore.min.rules",
    "rules": "firestore.min.rules"

Now when firestore deploys, it will minify your firestore.rules file and output the minified file to firestore.min.rules before deploying the rules.

Using the Binary

You can using the binary directly by installing globally

npm install -g firemin

To minimize a rules file, use the minimize command

firemin minimize -f ./path/to/my-firestore.rules

By default the output is STDOUT. To output to a file path you can use the -o option.

firemin minimie -f ./path/to/my-firestore.rules -o ./output/file/my-firestore.min.rules

Using Programmatically

You can also use this project programmatically if need be. To do so, simply install firemin as a project dependency and then import the necessary methods from the firemin package

npm install --save firemin
import { minimize, setupContext } from 'firemin'

const context = setupContext()
const result = await minimize(context, {
  filePath: './path/to/firestore.rules'