Design an async batched key-value fetcher
Overview
This question evaluates understanding of asynchronous batching, request coalescing, and consistency guarantees in single-threaded event-loop environments, including competencies in concurrency control, error propagation, backpressure, and graceful shutdown.
Implement getKeyAsync with 100 ms batching and a single GET per batch
You are working in a single-threaded environment with concurrent I O (think event loop). Implement a function:
- getKeyAsync(key, callback) -> void
Within any 100 ms window, the implementation must batch multiple incoming key requests and issue exactly one HTTP request per batch:
- HTTP GET to path: /read?keys=k1,k2,...
You are given two helpers:
- httpGetAsync(urlPath, callback) -> void, which asynchronously returns either an error or a JSON payload
- parseKvs(json) -> Map<String, Integer>, which parses the JSON into a map from key to integer
After the batch response arrives, parse it and invoke each original per-key callback with its value.
Consistency requirement:
- A batch response may only serve callbacks for requests that arrived before the batch HTTP call was initiated. It must not serve later requests.
Also specify and design for the following:
- Handling duplicate keys within the same window
- Missing keys in the response
- Callback ordering
- Late arrivals that straddle batch boundaries
- Timeouts and retries
- Backpressure when requests outpace the network
- Error propagation to callbacks
- Graceful shutdown or drain behavior
Include a brief rationale about when you would choose callbacks versus multi-threading for this problem, and the trade-offs in complexity, latency, and throughput.
Solution
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