File metadata in Firebase - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ service_behavior

intermediate

Understanding Firebase Storage Metadata Update Behavior

You upload a file to Firebase Storage and then update its metadata to add a custom field. What happens to the file content after the metadata update?

Firebase

const storageRef = firebase.storage().ref('files/myfile.txt');
await storageRef.put(file);
await storageRef.updateMetadata({ customMetadata: { owner: 'user123' } });

A The file content remains unchanged; only the metadata is updated.

B The file content is overwritten with empty content during metadata update.

C The file content is deleted and replaced with metadata only.

D The file content is duplicated and stored twice after metadata update.

💡 Hint

Think about whether metadata updates affect the actual file data.

Updating metadata in Firebase Storage only changes the metadata fields. The file content stays the same and is not modified or duplicated.

❓ Configuration

intermediate

Setting Cache Control Metadata for Firebase Storage Files

You want to set a cache control header on a file in Firebase Storage so browsers cache it for 1 hour. Which metadata configuration achieves this?

Firebase

const storageRef = firebase.storage().ref('images/photo.jpg');
await storageRef.updateMetadata({ cacheControl: 'public, max-age=3600' });

A { cacheControl: 'no-cache' }

B { cacheControl: 'public, max-age=3600' }

C { cacheControl: 'private, max-age=3600' }

D { cacheControl: 'max-age=86400' }

💡 Hint

Cache control headers define how long browsers keep files before checking for updates.

The value 'public, max-age=3600' tells browsers to cache the file publicly for 3600 seconds (1 hour). Other options either disable caching or set different durations.

❓ security

advanced

Restricting Metadata Updates with Firebase Storage Security Rules

You want to allow users to update only the customMetadata.owner field of their files in Firebase Storage, but not other metadata fields. Which security rule snippet enforces this?

Firebase

service firebase.storage {
  match /b/{bucket}/o {
    match /files/{fileId} {
      allow update: if request.resource.metadata.customMetadata.owner == resource.metadata.customMetadata.owner &&
                    request.resource.metadata.keys().hasOnly(['customMetadata']) &&
                    request.auth.uid == request.resource.metadata.customMetadata.owner;
    }
  }
}

A allow update: if request.resource.metadata.customMetadata.owner == resource.metadata.customMetadata.owner && request.auth.uid == request.resource.metadata.customMetadata.owner;

B allow update: if request.auth.uid == resource.metadata.customMetadata.owner && request.resource.metadata.keys().hasOnly(['customMetadata']);

C allow update: if request.resource.metadata.customMetadata.owner == resource.metadata.customMetadata.owner && request.resource.metadata.keys().hasOnly(['customMetadata']) && request.auth.uid == request.resource.metadata.customMetadata.owner;

D allow update: if request.resource.metadata.customMetadata.owner == request.auth.uid && request.resource.metadata.keys().hasOnly(['customMetadata']);

💡 Hint

Check that only the customMetadata field is changed and the owner matches the authenticated user.

This rule ensures the update only changes customMetadata, the owner field remains consistent, and the user is authenticated as the owner.

❓ Architecture

advanced

Designing Metadata for Versioning Files in Firebase Storage

You want to keep track of file versions in Firebase Storage using metadata. Which approach best supports versioning without duplicating files?

A Use metadata <code>cacheControl</code> to indicate version numbers.

B Rename files with version numbers in their names and keep metadata unchanged.

C Store version info only in Firestore, not in Storage metadata.

D Add a <code>version</code> field in <code>customMetadata</code> and update it on each file overwrite.

💡 Hint

Think about how to track versions without creating multiple files.

Adding a version field in customMetadata allows tracking versions on the same file without duplicating storage. Renaming files duplicates storage, and cacheControl is unrelated.

🧠 Conceptual

expert

Effect of Metadata on Firebase Storage Download URLs

You generate a download URL for a file in Firebase Storage. Later, you update the file's metadata (e.g., add customMetadata). What happens to the previously generated download URL?

A The download URL remains valid and unchanged after metadata updates.

B The download URL expires immediately when metadata is updated.

C The download URL changes automatically to reflect new metadata.

D The download URL becomes invalid and must be regenerated after metadata changes.

💡 Hint

Consider whether metadata updates affect file access URLs.

Download URLs in Firebase Storage are tied to the file content and security token, not metadata. Updating metadata does not invalidate or change existing URLs.

Practice

1. What is the main purpose of file metadata in Firebase Storage?

easy

A. To store extra information about files like type and custom tags

B. To store the actual file content

C. To manage user authentication

D. To create database records

Solution

1. Step 1: Understand what metadata means

   Metadata is data about data, so it holds extra info about files, not the files themselves.

2. Step 2: Identify the role of metadata in Firebase Storage

   Firebase Storage uses metadata to store info like file type and custom tags to help manage files better.

3. Final Answer:

   To store extra information about files like type and custom tags -> Option A

4. Quick Check:

   File metadata = extra info about files [OK]

Hint: Metadata holds info about files, not the files themselves [OK]

Common Mistakes:

• Confusing metadata with file content
• Thinking metadata manages users
• Assuming metadata creates database records

2. Which of the following is the correct way to set custom metadata for a file in Firebase Storage using JavaScript?

easy

A. storageRef.updateMetadata({ customMetadata: { owner: 'user1' } })

B. storageRef.setMetadata({ owner: 'user1' })

C. storageRef.metadata = { owner: 'user1' }

D. storageRef.uploadMetadata({ customMetadata: { owner: 'user1' } })

Solution

1. Step 1: Recall Firebase Storage method for updating metadata

   The correct method to update metadata is updateMetadata(), not setMetadata or uploadMetadata.

2. Step 2: Check the structure for custom metadata

   Custom metadata must be inside a customMetadata object within the metadata parameter.

3. Final Answer:

   storageRef.updateMetadata({ customMetadata: { owner: 'user1' } }) -> Option A

4. Quick Check:

   Use updateMetadata() with customMetadata object [OK]

Hint: Use updateMetadata() with customMetadata key [OK]

Common Mistakes:

• Using setMetadata() which does not exist
• Assigning metadata directly to storageRef
• Using uploadMetadata() which is invalid

3. Given the following code snippet, what will be logged to the console?

const metadata = await storageRef.getMetadata();
console.log(metadata.contentType);

medium

A. The file's download URL

B. The file size in bytes

C. The MIME type of the file, e.g., 'image/png'

D. An error because getMetadata() returns undefined

Solution

1. Step 1: Understand what getMetadata() returns

   getMetadata() returns an object with file metadata including contentType, size, and more.

2. Step 2: Identify what metadata.contentType holds

   contentType holds the MIME type of the file, such as 'image/png' or 'application/pdf'.

3. Final Answer:

   The MIME type of the file, e.g., 'image/png' -> Option C

4. Quick Check:

   metadata.contentType = MIME type [OK]

Hint: contentType in metadata shows file type [OK]

Common Mistakes:

• Thinking contentType is file size
• Expecting download URL from metadata
• Assuming getMetadata() returns undefined

4. You try to update file metadata with this code but get an error:

await storageRef.updateMetadata({ owner: 'user1' });

What is the likely cause?

medium

A. updateMetadata() cannot update metadata

B. storageRef is not defined

C. owner is not a valid metadata field

D. Missing the customMetadata wrapper around owner

Solution

1. Step 1: Check the structure required for custom metadata

   Custom metadata must be inside a customMetadata object, not directly as a field.

2. Step 2: Identify the error cause

   Passing { owner: 'user1' } directly causes an error because owner is not a top-level metadata field.

3. Final Answer:

   Missing the customMetadata wrapper around owner -> Option D

4. Quick Check:

   Custom metadata needs customMetadata key [OK]

Hint: Wrap custom fields inside customMetadata object [OK]

Common Mistakes:

• Passing custom fields directly without customMetadata
• Thinking updateMetadata() can't update metadata
• Ignoring storageRef definition errors

5. You want to tag files uploaded by users with their user ID and file category. Which is the best way to store this info in Firebase Storage metadata?

hard

A. Add user ID and category as top-level metadata fields

B. Use customMetadata with keys 'userId' and 'category' inside updateMetadata()

C. Save user ID and category in Firestore only, not in metadata

D. Store user ID and category in the file name

Solution

1. Step 1: Understand how to store extra info in Firebase Storage

   Firebase Storage supports customMetadata to store extra info like tags.

2. Step 2: Evaluate options for tagging files

   Using customMetadata keys 'userId' and 'category' inside updateMetadata() is the best practice for tagging files.

3. Step 3: Why other options are less ideal

   File names can get messy, Firestore is separate, and top-level metadata fields do not support custom keys.

4. Final Answer:

   Use customMetadata with keys 'userId' and 'category' inside updateMetadata() -> Option B

5. Quick Check:

   Tag files using customMetadata keys [OK]

Hint: Use customMetadata keys for tagging files [OK]

Common Mistakes:

• Putting tags in file names instead of metadata
• Relying only on Firestore for file tags
• Trying to add custom fields as top-level metadata