Common rule patterns in Firebase - Step-by-Step Execution

Process Flow - Common rule patterns

Start Request

↓

Match Request Path

↓

Check Rule Conditions

↓

Allow

↓

End Request

A request comes in, matches a path, checks conditions, then allows or denies access.

Execution Sample

Firebase

match /documents/{docId} {
  allow read: if request.auth != null;
  allow write: if request.auth.uid == resource.data.owner;
}

This rule allows reading if the user is logged in, and writing only if the user owns the document.

Process Table

Step	Request Path	Condition Checked	Condition Result	Access Decision
1	/documents/abc123	request.auth != null	true	Allow read
2	/documents/abc123	request.auth.uid == resource.data.owner	false	Deny write
3	/documents/abc123	request.auth != null	false	Deny read
4	/documents/abc123	request.auth.uid == resource.data.owner	true	Allow write

💡 Access decision made based on condition results for read or write.

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4
request.auth	null or object	object (logged in)	object (logged in)	null	object (logged in)
request.auth.uid	undefined	user123	user123	undefined	user123
resource.data.owner	user123	user123	user456	user123	user123

Key Moments - 3 Insights

Why does a read request get denied even if the user is logged in?

If request.auth is null (user not logged in), the condition 'request.auth != null' fails, so read access is denied as shown in step 3 of the execution table.

Why can a user read but not write the same document?

Read requires only that the user is logged in (request.auth != null), but write requires the user ID to match the document owner. If they don't match, write is denied (step 2).

What happens if the request path does not match any rule?

No matching rule means no access is granted, so the request is denied by default.

Visual Quiz - 3 Questions

Look at the execution table, what is the access decision at step 2?

A Allow write

B Deny write

C Allow read

D Deny read

💡 Hint

Check the 'Access Decision' column in row 2 of the execution table.

At which step does the condition 'request.auth != null' evaluate to false?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Condition Result' column for 'request.auth != null' in the execution table.

If the resource.data.owner changes to 'user123' at step 2, what would happen to the write access decision?

A Write would be allowed

B Write would be denied

C Read would be denied

D No change

💡 Hint

Compare 'request.auth.uid' and 'resource.data.owner' values in variable_tracker after step 2.

Concept Snapshot

Firebase rules check requests by matching paths and conditions.
Use 'allow' with conditions like 'request.auth != null' to control access.
Read and write can have different conditions.
If no rule matches or condition fails, access is denied.
Rules protect data by verifying user identity and ownership.

Full Transcript

Firebase security rules work by matching the incoming request path to defined rules. Then, each rule checks conditions such as whether the user is logged in or owns the data. If the condition is true, access is allowed; otherwise, it is denied. For example, a read request is allowed if the user is authenticated, while a write request requires the user to be the owner of the document. If no rules match or conditions fail, access is denied by default. This ensures data is protected and only accessible to authorized users.