01 · Problem
What is it for?
Discovery can produce several delayed answers; it is not a one-request/one-response RPC.
02 · Mental model
How should you think about it?
Announcements create discoverable records owned by a publisher. Queries collect matching records until a deadline.
Provides a deterministic local discovery model and an explicitly started multicast adapter. MDNS verifies TTL and cleanup locally; NetworkMDNS collects delayed network answers through multicast-dns. CI uses an injected transport; interface selection and cross-machine reliability remain integration work.
03 · Control flow
What happens in which order?
- Publish a typed service record.
- Match queries by name and record type.
- Collect all matching answers.
- Expire records by TTL.
- Remove owned records on shutdown.
04 · Building blocks
The important contracts
MDNS.query / announce / destroyDiscover matching local records, publish them and remove owned records.DNSRecord / DNSTypeDescribe supported record data and query types.05 · Build it yourself
Implement a small core
Start with this small independent implementation, then use the steps below to make its contract explicit.
- Publish a typed service record.
- Match queries by name and record type.
- Collect all matching answers.
- Expire records by TTL.
- Remove owned records on shutdown.
async function query(name: string, type: string, signal: AbortSignal) {
const answers: unknown[] = [];
const onResponse = (packet: any) => {
for (const record of [...packet.answers, ...packet.additionals]) {
if (record.name === name && (type === "ANY" || record.type === type)) {
answers.push(record);
}
}
};
socket.on("response", onResponse);
try { socket.query({ questions: [{ name, type }] }); await delay(500, signal); }
finally { socket.off("response", onResponse); }
return deduplicate(answers);
}06 · Verify
What should you test?
- Discover multiple publishers without duplicates.
- Test TTL expiry and publisher shutdown.
- Test transport timeouts using an injected clock or adapter.
Try it yourself
Turn the first verification claim into a failing test, implement the smallest fix, then add a boundary case.
Discover multiple publishers without duplicates.
Compare your approach
Your check should fail when the stated invariant is deliberately broken, and pass again after the fix. Keep setup deterministic and assert an observable result.
07 · Limits
Trade-offs and failure modes
- The in-process core cannot discover another machine.
- Real multicast needs interface selection and socket cleanup.
- TTL is part of the discovery contract.
MDNS verifies TTL and cleanup locally; NetworkMDNS collects delayed network answers through multicast-dns. CI uses an injected transport; interface selection and cross-machine reliability remain integration work.
08 · Keep exploring
Source code and related ideas
yarn tsx packages/mdns/examples/basic.ts