feat(rumqttd): Example for external authentication setup (#765)

rumqttd/examples/external_auth.rs

@@ -0,0 +1,41 @@
+use rumqttd::{Broker, Config};
+
+use std::sync::Arc;
+
+fn main() {
+    let builder = tracing_subscriber::fmt()
+        .pretty()
+        .with_line_number(false)
+        .with_file(false)
+        .with_thread_ids(false)
+        .with_thread_names(false);
+
+    builder
+        .try_init()
+        .expect("initialized subscriber succesfully");
+
+    let config = config::Config::builder()
+        .add_source(config::File::with_name("rumqttd.toml"))
+        .build()
+        .unwrap();
+    let mut config: Config = config.try_deserialize().unwrap();
+
+    // for e.g. if you want it for [v4.1] server, you can do something like
+    let server = config.v4.as_mut().and_then(|v4| v4.get_mut("1")).unwrap();
+
+    // set the external_auth field in ConnectionSettings
+    // external_auth function / closure signature must be:
+    // Fn(ClientId, AuthUser, AuthPass) -> bool
+    // type for ClientId, AuthUser and AuthPass is String
+    server.connections.external_auth = Some(Arc::new(auth));
+
+    let mut broker = Broker::new(config);
+
+    broker.start().unwrap();
+}
+
+fn auth(_client_id: String, _username: String, _password: String) -> bool {
+    // users can fetch data from DB or tokens and use them!
+    // do the verification and return true if verified, else false
+    true
+}