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1. Create a custom ClientCallbackHandler and configure the client to use it
   1. This handler should handle the `SaslExtensionsCallback` in his `handle()` method and attach custom extensions

      
      

      Code Block

      @Override public void handle(Callback[] callbacks) { for (Callback callback : callbacks) { if (callback instanceof OAuthBearerTokenCallback) handleTokenCallback((OAuthBearerTokenCallback) callback); else if (callback instanceof SaslExtensionsCallback) { Map<String, String> extensions = new HashMap<>(); extensions.put("trace-id", "123"); ((SaslExtensionsCallback) callback).extensions(extensions); } else throw new UnsupportedCallbackException(callback); } }

2. The extensions can be fetched in the server-side through `OAuthBearerSaslServer#getNegotiatedProperty("trace-id")`

3. 1. 
      

4. A custom principal builder can then make use of the new extension

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      Code Block

      public class CustomPrincipalBuilder implements KafkaPrincipalBuilder { @Override public KafkaPrincipal build(AuthenticationContext context) { if (context instanceof SaslAuthenticationContext) { SaslServer saslServer = ((SaslAuthenticationContext) context).server(); String traceId = saslServer.getNegotiatedPropery("trace-id"); return new CustomPrincipal("", saslServer, traceId); } throw new Exception(); } }

      
      

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