# Loopback publish and subscribe

The Atlas MQTT quick start, and the project the [tutorial](https://youseful.software/delphi/atlas-mqtt.html#tutorial) builds step by step.
A console program that:

1. starts a `TAtlasMqttBroker` inside the program, on `127.0.0.1` and a port the system picks (`Port := 0`);
2. connects a `TAtlasMqttClient` to it with `ConnectAndWait`;
3. subscribes with the wildcard filter `sensors/+/temperature` at quality 2 and waits for `OnSubscribed`;
4. publishes three matching readings, at quality 0, 1 and 2, and one reading on `sensors/kitchen/humidity` that
   the filter does not match;
5. waits, with a timeout, for the three matching messages, for `WaitForIdle` and for an `OnPublished` event per
   acknowledged message, then checks every topic, payload and quality;
6. disconnects and stops the broker.

The client receives its own messages because its subscription matches them: MQTT 3.1.1 has no "no local"
option. Every check raises on a mismatch; the program exits with code 0 only when all of them pass. Nothing
leaves the machine and nothing needs to be installed.

## Build and run

Open `LoopbackPublishSubscribe.dpr` in Delphi 13; the IDE creates the project file. Add the Atlas MQTT `src` folder to the project's search path, then build and run for Win32 or Win64. No packages need to be installed. The program needs no input. It checks its own results and stops with an error if any of them differs from the output below.

Windows is required: the transport uses Winsock.

## Expected output

The port number changes on every run, and the three "acknowledged" lines can appear in a different order and
interleaved with the "received" lines, because acknowledgements and deliveries travel independently. Packet
identifier 1 went to the SUBSCRIBE request, so the three publications above quality 0 get 2, 3 and 4.

```
Broker listening on 127.0.0.1:62668
ok: the client connects
ok: the broker counts one client
  subscribed to sensors/+/temperature: granted=True quality=2
ok: the broker grants the subscription at quality 2
  received  sensors/kitchen/temperature=21.5 q0
  received  sensors/hall/temperature=19.0 q1
  broker acknowledged packet 2
  received  sensors/garage/temperature=12.5 q2
ok: three messages arrive
ok: nothing is left in flight
  broker acknowledged packet 4
  broker acknowledged packet 3
ok: OnPublished fired for each of the three acknowledged messages
ok: the humidity message was filtered out by the broker
ok: kitchen reading at quality 0
ok: hall reading at quality 1
ok: garage reading at quality 2
ok: in-flight count is zero
ok: the broker accepted four messages
ok: the client is disconnected
ok: the broker is stopped
Done.
```

`RoutedCount` is 4 because the broker counts every message it accepts from a publisher, including the humidity
reading that no subscription matched.

## Adapting it

- To talk to a broker that is already running, delete the `TAtlasMqttBroker` lines and set `Client.Host` and
  `Client.Port` to its address. Atlas MQTT speaks MQTT 3.1.1 over plain TCP only; there is no TLS.
- Try other filters: `sensors/#` also matches the humidity reading, `sensors/kitchen/+` matches both kitchen
  readings.
- In a VCL application, drop the `CheckSynchronize` loop: with `SynchronizeEvents` at its default the events
  already run on the main thread through the application's message loop.