Rule syntax and structure in Firebase - Step-by-Step Execution

Process Flow - Rule syntax and structure

Start: Define rules block

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Specify service (e.g., firestore)

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Define match path (document or collection)

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Set allow conditions (read, write)

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Use expressions to check auth, data

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Return true or false

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End of rule block

This flow shows how Firebase security rules are structured: start with a rules block, specify the service, match data paths, set allow conditions, and return true or false to permit or deny access.

Execution Sample

Firebase

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {
    match /users/{userId} {
      allow read, write: if request.auth.uid == userId;
    }
  }
}

This rule allows users to read and write only their own user document based on matching userId with their authentication ID.

Process Table

Step	Rule Part	Action	Evaluation	Result
1	rules_version = '2'	Set rules version	Version set to 2	Success
2	service cloud.firestore	Specify Firestore service	Service identified	Success
3	match /databases/{database}/documents	Match all documents in database	Wildcard {database} captures database name	Success
4	match /users/{userId}	Match user documents	Wildcard {userId} captures user ID	Success
5	allow read, write: if request.auth.uid == userId	Check if authenticated user ID equals userId in path	If equal, allow access	Access granted if true, denied if false
6	End of rules	No more rules to evaluate	Evaluation complete	Rules applied

💡 Rules evaluation ends after checking all defined matches and conditions.

Status Tracker

Variable	Start	After Step 4	After Step 5	Final
rules_version	undefined	undefined	'2'	'2'
service	undefined	'cloud.firestore'	'cloud.firestore'	'cloud.firestore'
database	undefined	captured from path	captured from path	captured from path
userId	undefined	captured from path	captured from path	captured from path
request.auth.uid	undefined	undefined	provided by user auth	provided by user auth
allow	undefined	undefined	true if request.auth.uid == userId	true or false

Key Moments - 3 Insights

Why do we use wildcards like {userId} in match paths?

Wildcards capture parts of the path dynamically, allowing rules to apply to many documents without writing separate rules for each. See execution_table step 4 where {userId} captures the user ID.

What happens if the condition in 'allow' is false?

Access is denied if the condition evaluates to false. In execution_table step 5, if request.auth.uid does not equal userId, the rule returns false and denies access.

Can rules apply to multiple operations at once?

Yes, you can specify multiple operations like 'read, write' together in one allow statement, as shown in execution_table step 5.

Visual Quiz - 3 Questions

Look at the execution_table step 5, what condition must be true to allow access?

A request.auth.uid equals userId

B request.auth.uid is undefined

C userId is empty

D request.auth.uid is different from userId

💡 Hint

Check the 'Evaluation' column in step 5 of execution_table.

At which step does the rule capture the userId from the document path?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look for 'Wildcard {userId} captures user ID' in execution_table.

If we remove 'write' from the allow statement, what changes in the rule behavior?

A Users can only write their documents

B Users can only read their documents

C Users can read and write their documents

D Users cannot read or write their documents

💡 Hint

Refer to the 'allow read, write' part in execution_sample and think about removing 'write'.

Concept Snapshot

Firebase rules start with a version and service declaration.
Use match blocks with wildcards to specify document paths.
Allow statements define read/write permissions with conditions.
Conditions use request.auth and path variables.
Rules return true to allow or false to deny access.

Full Transcript

Firebase security rules control access to your database. You start by setting the rules version and specifying the service, like Firestore. Then you define match blocks that point to document paths, using wildcards to capture dynamic parts like user IDs. Inside these matches, you write allow statements that say when read or write is allowed. These statements use conditions comparing the authenticated user's ID to the document's userId. If the condition is true, access is granted; otherwise, it is denied. This structure helps keep your data safe by ensuring users can only access their own documents.