DataLoader batching and caching in GraphQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

How does DataLoader batch requests?

Imagine you have multiple requests for user data by ID coming in quickly. How does DataLoader handle these requests?

A It collects all requests within a tick and sends one batch request to the database.

B It sends each request immediately to the database without waiting.

C It caches the first request and ignores the rest.

D It queues requests and sends them one by one with delays.

💡 Hint

Think about how batching reduces the number of database calls.

DataLoader collects all requests made during the same event loop tick and sends them as a single batch to the database, improving efficiency.

❓ query_result

intermediate

What is the output of this DataLoader cache test?

Given this DataLoader usage, what will be the value of count after all calls?

GraphQL

let count = 0;
const loader = new DataLoader(async keys => {
  count += 1;
  return keys.map(key => ({ id: key, name: `User${key}` }));
});

await Promise.all([
  loader.load(1),
  loader.load(1),
  loader.load(2)
]);

// What is count?

A 1

B 2

C 0

D 3

💡 Hint

Consider how caching affects repeated keys.

DataLoader caches results for keys. The batch function runs once for keys [1,2]. The second load(1) uses the cache, so count increments only once.

📝 Syntax

advanced

Identify the syntax error in this DataLoader batch function

Which option contains a syntax error in the batch loading function?

GraphQL

const loader = new DataLoader(async keys => {
  return keys.map(key => {
    id: key,
    name: `User${key}`
  });
});

A Incorrect async keyword usage in batch function.

B Missing return statement inside map callback, causing undefined results.

C Keys parameter should be an object, not an array.

D Using parentheses instead of braces for object return in map callback.

💡 Hint

Check how arrow functions return objects.

The map callback uses braces without a return statement, so it returns undefined. Wrapping the object in parentheses or adding a return fixes this.

❓ optimization

advanced

How to optimize DataLoader cache for large datasets?

You have a DataLoader caching millions of keys. What is the best way to prevent memory issues?

A Store all keys permanently in the default cache.

B Disable caching and rely only on batching.

C Increase Node.js memory limit to handle large cache.

D Use a custom cache with limited size and eviction policy.

💡 Hint

Think about controlling cache size to avoid memory overflow.

Using a custom cache with size limits and eviction prevents memory bloat while keeping caching benefits.

🔧 Debug

expert

Why does this DataLoader batch function return results in wrong order?

Given this batch function, why might the results not match the order of keys?

GraphQL

const loader = new DataLoader(async keys => {
  const users = await database.getUsersByIds(keys);
  return users;
});

A Batch function must return a Map instead of an array.

B DataLoader requires keys to be sorted before batch function.

C The database returns users in arbitrary order, not matching keys order.

D Keys array is mutated inside batch function causing mismatch.

💡 Hint

Consider how the database returns results compared to requested keys.

DataLoader expects the batch function to return results in the same order as keys. If the database returns unordered results, the batch function must reorder them accordingly.