MQTT backend documentation

2 files changed, 59 insertions, 2 deletions

diff --git a/backends/mqtt.c b/backends/mqtt.c
index 29f0436..41fa772 100644
--- a/backends/mqtt.c
+++ b/backends/mqtt.c
@@ -580,7 +580,7 @@ static int mqtt_maintenance(){
 
 static int mqtt_handle_publish(instance* inst, uint8_t type, uint8_t* variable_header, size_t length){
 	mqtt_instance_data* data = (mqtt_instance_data*) inst->impl;
-	char* topic = NULL, *payload = NULL;
+	char* topic = NULL, *payload = NULL, *conversion_end = NULL;
 	channel* changed = NULL;
 	channel_value val;
 	uint8_t qos = (type & 0x06) >> 1, content_utf8 = 0;
@@ -647,7 +647,11 @@ static int mqtt_handle_publish(instance* inst, uint8_t type, uint8_t* variable_h
 		//FIXME implement input mappings
 		changed = mm_channel(inst, u, 0);
 		if(changed){
-			val.normalised = strtod(payload, NULL);
+			val.normalised = clamp(strtod(payload, &conversion_end), 1.0, 0.0);
+			if(payload == conversion_end){
+				LOGPF("Failed to parse incoming data for %s.%s", inst->name, data->channel[u].topic);
+				return 0;
+			}
 			mm_channel_event(changed, val);
 		}
 	}
diff --git a/backends/mqtt.md b/backends/mqtt.md
new file mode 100644
index 0000000..895831c
--- /dev/null
+++ b/backends/mqtt.md
@@ -0,0 +1,53 @@
+### The `mqtt` backend
+
+This backend provides input from and output to an message queueing telemetry transport (MQTT)
+broker. The MQTT protocol is used in lightweight sensor/actor applications, a wide selection
+of smart home implementations and as a generic message bus in many other domains.
+
+The backend implements both the older protocol version MQTT v3.1.1 as well as the current specification
+for MQTT v5.0.
+
+#### Global configuration
+
+This backend does not take any global configuration.
+
+#### Instance configuration
+
+| Option	| Example value		| Default value 	| Description				|
+|---------------|-----------------------|-----------------------|---------------------------------------|
+| `host`	| `mqtt://10.23.23.1`	| none			| Host or URI of the MQTT broker	|
+| `user`	| `midimonster`		| none			| User name for broker authentication	|
+| `password`	| `mm`			| none			| Password for broker authentication	|
+| `clientid`	| `MM-main`		| random		| MQTT client identifier (generated randomly at start) |
+| `protocol`	| `3.1.1`		| `5`			| MQTT protocol version (`5` or `3.1.1`) to use for the connection |
+
+The `host` option can be specified as an URI of the form `mqtt[s]://[username][:password]@host.domain[:port]`.
+This allows specifying all necessary settings in one configuration option.
+
+#### Data exchange format
+
+The MQTT protocol places very few restrictions on the exchanged data. Thus, it is necessary to specify the input
+and output data formats accepted respectively output by the MIDIMonster.
+
+The basic format, without further channel-specific configuration, is an ASCII/UTF-8 string representing a floating
+point number between `0.0` and `1.0`. The MIDIMonster will read these and use the value as the normalised event value.
+
+Values above the maximum or below the minimum will be clamped. The MIDIMonster will not output values out of those
+bounds.
+
+#### Channel specification
+
+A channel specification may be any MQTT topic designator not containing the wildcard characters `+` and `#`.
+
+Example mapping: 
+```
+mq1./midimonster/in > mq2./midimonster/out
+```
+
+#### Known bugs / problems
+
+If the connection to a server is lost, the connection will be retried in approximately 10 seconds.
+If the server rejects the connection with reason code `0x01`, a protocol failure is assumed. If the initial
+connection was made with `MQTT v5.0`, it is retried with the older protocol version `MQTT v3.1.1`.
+
+Support for TLS-secured connections is planned, but not yet implemented.