Health-Aware Resolver Pool

Problem

Design HealthAwareResolverPool, an in-memory DNS resolver that stores one current address answer per hostname, tracks the latest health observation for each address, and applies a fixed time to live.

Requirements

Replace an answer. publishAnswer replaces the hostname's cached answer with the supplied distinct addresses and observedAt.
Track latest health. recordHealth replaces the address's previous health observation with the supplied value.
Expire by TTL. An answer is valid through observedAt + ttl and expires when now is greater; expired addresses are not eligible.
Return eligible addresses. resolve returns exactly the healthy addresses in the hostname's current valid answer. The returned array's order is not judged.
Keep reads hostname-local. resolve runs in $O(k)$ time, where $k$ is the size of the requested hostname's current answer, independent of addresses stored for other hostnames.

API

Signature	Returns	Behavior
`HealthAwareResolverPool(ttl: integer)`	Not applicable	Creates an empty pool with one fixed positive TTL.
`publishAnswer(hostname: string, addresses: string[], observedAt: integer)`	`void`	Replaces one hostname's cached answer.
`recordHealth(address: string, healthy: boolean)`	`void`	Stores the address's latest health observation.
`resolve(hostname: string, now: integer)`	`string[]`	Returns the requested hostname's eligible address set as an array in $O(k)$ time.

Examples

Given HealthAwareResolverPool(10):

Step	Operation	Result
1	`publishAnswer("api.test", ["10.0.0.2", "10.0.0.1"], 5)`	`void`
2	`recordHealth("10.0.0.2", true)`	`void`
3	`recordHealth("10.0.0.1", false)`	`void`
4	`resolve("api.test", 15)`	`["10.0.0.2"]`
5	`resolve("api.test", 16)`	`[]`

Constraints

ttl is between 1 and 1,000,000,000.
Every string is non-empty ASCII text with at most 253 characters.
Each published answer contains 1 to 1,000 distinct addresses.
Time values are between 0 and 1,000,000,000.
observedAt + ttl fits in a signed 64-bit integer.
Every hostname supplied to resolve has a current published answer.
Before resolving an unexpired answer, every address in that current answer has a recorded health observation.
At most 50,000 public method calls occur per testcase.
Inputs satisfy these constraints.

Notes

Calls are sequential. Result-array order has no meaning.