Distributed counters pattern in Firebase - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize a distributed counter document reference in Firebase.

Firebase

const counterRef = firestore.collection('counters').doc([1]);

Drag options to blanks, or click blank then click option'

A'counterDoc'

B'counter'

C'distributedCounter'

D'countDoc'

Attempts:

3 left

💡 Hint

Common Mistakes

Using a collection name instead of a document ID.

Leaving the document ID empty or undefined.

The document ID 'distributedCounter' clearly identifies the counter document in the 'counters' collection.

2fill in blank

medium

Complete the code to increment a shard's count atomically using a transaction.

Firebase

await firestore.runTransaction(async (transaction) => {
  const shardRef = counterRef.collection('shards').doc([1]);
  const shardDoc = await transaction.get(shardRef);
  if (!shardDoc.exists) {
    transaction.set(shardRef, { count: 1 });
  } else {
    transaction.update(shardRef, { count: shardDoc.data().count + 1 });
  }
});

Drag options to blanks, or click blank then click option'

A'shard0'

B'shard1'

C'shardA'

D'shardX'

Attempts:

3 left

💡 Hint

Common Mistakes

Using shard IDs that do not match the naming convention.

Using an undefined or empty shard ID.

Using 'shard0' as the shard document ID is a common pattern to identify one shard among many.

3fill in blank

hard

Fix the error in the code to correctly sum counts from all shards.

Firebase

const shardsSnapshot = await counterRef.collection('shards').get();
let totalCount = 0;
shardsSnapshot.forEach(doc => {
  totalCount += doc.data().[1];
});

Drag options to blanks, or click blank then click option'

Avalue

Btotal

Cnumber

Dcount

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect field names like 'value' or 'total' which do not exist.

Assuming the field is named 'number' instead of 'count'.

The field storing the count in each shard document is named 'count'.

4fill in blank

hard

Fill both blanks to create a function that initializes shards with zero counts.

Firebase

async function initializeShards(counterRef, numShards) {
  const batch = firestore.batch();
  for (let i = 0; i < numShards; i++) {
    const shardRef = counterRef.collection('shards').doc([1]);
    batch.set(shardRef, { [2]: 0 });
  }
  await batch.commit();
}

Drag options to blanks, or click blank then click option'

A`shard${i}`

Bcount

Cvalue

D`shard_${i}`

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect shard naming without template literals.

Using wrong field names like 'value' instead of 'count'.

The shard document IDs use template literals like `shard${i}` to create unique shard names. The count field is named 'count' and initialized to 0.

5fill in blank

hard

Fill all three blanks to implement a function that reads and sums counts from all shards.

Firebase

async function getDistributedCount(counterRef) {
  const shardsSnapshot = await counterRef.collection([1]).get();
  let totalCount = 0;
  shardsSnapshot.forEach(doc => {
    totalCount += doc.data()[[2]];
  });
  return totalCount;
}

const count = await getDistributedCount([3]);

Drag options to blanks, or click blank then click option'

A'shards'

B'count'

CcounterRef

D'counters'

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong collection names like 'counters' instead of 'shards'.

Using incorrect field names or passing wrong arguments.

The function reads from the 'shards' collection, sums the 'count' field from each shard, and is called with the counterRef.