# NATS + JetStream 101

Learning-gate paper for subject 1 of the curriculum
(see [the curriculum](../README.md#learning-per-technology-resources)).
We build ON knowledge, not around it.

## 1. Source of truth

Read these, in this order, and quote them back rather than paraphrase:

- NATS core concepts: <https://docs.nats.io/nats-concepts/what-is-nats>
- Subjects and wildcards: <https://docs.nats.io/nats-concepts/subjects>
- JetStream overview: <https://docs.nats.io/nats-concepts/jetstream>
- Durable consumers and replay semantics: <https://docs.nats.io/nats-concepts/jetstream/js_walkthrough>
- Reference (for drills): <https://docs.nats.io/reference/reference-protocols/nats-protocol>

No blogs, no tutorials, no chat summaries. If something is missing from
the official sources, that is itself a finding worth recording here.

## 2. Estate mapping (our reality, host-verified)

| Item | Value | Truth of record |
|------|-------|-----------------|
| Host | `nats-dev.eachpath.local` (Saturn VM 122), `10.0.30.122` | [host record](../../eachpath.local/saturn.eachpath.local/vms/nats-dev.eachpath.local/nats-dev.eachpath.local.md) |
| NATS server | v2.10.24, `/usr/local/bin/nats-server` | host record |
| Ports | client `30000`, HTTP/monitoring `30001` | host record |
| JetStream | enabled, file store `/var/lib/nats/jetstream` (max 10 GB file / 1 GB memory) | host record |
| Streams | `REGISTER` -> `nimmerverse.register.>` (durable), `CYCLE` -> `nimmerverse.cycle.>` (100k msgs) | host record, 2026-11-06 |
| First round-trip | `VM123 ALIVE 2026-11-06` published to `nimmerverse.register.pulse`, replayed by durable consumer `nyx-probe` | host record, 2026-11-06 |
| Client library | `nats-py 2.16.0` in nimmer-lib venv (aynee) | verified 2026-11-06 |

## 3. Drills

Hands-on, on OUR estate, answers written into this paper:

- D1: Is the server alive right now? Read its identity from the HTTP
  monitoring port *without* a client library. (curl is a client too.)
- D2: List the streams and their message counts, from the monitoring
  port only.
- D3: Publish one message of your own words and read it back via a
  durable consumer. Prove the bus *remembers* after the publisher is
  gone.
- D4: Prove the bus *forgets on schedule*: find the retention setting
  on `CYCLE` and show where 100k lives in `nats stream info`.
- D5: Subjects are names, and names carry meaning: publish to
  `nimmerverse.register.pulse` and subscribe `nimmerverse.register.*`.
  Explain why the `*` matched and when you would use `>` instead.
- D6: Replay from sequence 1 for a *second* durable consumer and watch
  the first one's position remain untouched. Explain why two listeners
  on the same bus can disagree about *where they are* without ever
  corrupting each other.

## 4. Understand-check (ten words)

Said back, in our own words, before any primitive builds on this bus.

> draft (nyx, 2026-11-06): *the bus keeps every message until retention says stop; consumers read independently.*
> dafit's version: (to be said back)

## 5. Open questions

dafit's question log starts here. Every answer that teaches something
gets written into this paper, dated, host-verified if it touches our
estate.

- (waiting for the first question)