Explain the concept and mechanics of DataLoader.

DataLoader is a utility pattern for data fetching that provides request-scoped batching and caching.

You provide it a batch loading function that accepts an array of keys and returns a Promise resolving to an array of values of the same length and order. When resolvers call .load(key), DataLoader returns a Promise. It waits until the next tick of the event loop, groups all accumulated keys, calls the batch function once, and resolves all promises.