add filter for logging principal of incoming requests

core/src/main/java/io/confluent/kafka/schemaregistry/rest/PrincipalLoggingFilter.java

@@ -0,0 +1,67 @@
+/*
+ * Copyright 2025 Confluent Inc.
+ *
+ * Licensed under the Confluent Community License (the "License"); you may not use
+ * this file except in compliance with the License.  You may obtain a copy of the
+ * License at
+ *
+ * http://www.confluent.io/confluent-community-license
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OF ANY KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations under the License.
+ */
+
+package io.confluent.kafka.schemaregistry.rest;
+
+import javax.servlet.Filter;
+import javax.servlet.FilterChain;
+import javax.servlet.FilterConfig;
+import javax.servlet.ServletException;
+import javax.servlet.ServletRequest;
+import javax.servlet.ServletResponse;
+import javax.servlet.http.HttpServletRequest;
+import java.io.IOException;
+import java.security.Principal;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import io.confluent.kafka.schemaregistry.utils.PrincipalContext;
+
+/**
+* This class is a servlet filter that logs the user principal for each incoming request to 
+* Schema Registry. It is a necessary step to allow for building resource associations
+*/
+public class PrincipalLoggingFilter implements Filter {
+
+  private static final Logger log = LoggerFactory.getLogger(PrincipalLoggingFilter.class.getName());
+
+  @Override
+  public void init(FilterConfig filterConfig) throws ServletException {
+  }
+
+  @Override
+  public void doFilter(ServletRequest request, ServletResponse servletResponse,
+                       FilterChain filterChain) throws IOException, ServletException {
+    HttpServletRequest req = (HttpServletRequest) request;
+    Principal principal = req.getUserPrincipal();
+
+    if (principal != null) {
+      log.info("User Principal: {}", principal.getName());
+      PrincipalContext.setPrincipal(principal.getName());
+    } else {
+      log.info("No User Principal found for the request.");
+      PrincipalContext.clear();
+    }
+
+    try {
+      filterChain.doFilter(request, servletResponse);
+    } finally {
+      PrincipalContext.clear(); // Clear the principal after the request is processed
+    }
+  }
+
+  @Override
+  public void destroy() {
+  }
+}

core/src/main/java/io/confluent/kafka/schemaregistry/rest/SchemaRegistryRestApplication.java

@@ -35,14 +35,17 @@
 import io.confluent.kafka.schemaregistry.storage.serialization.SchemaRegistrySerializer;
 import io.confluent.rest.Application;
 import io.confluent.rest.RestConfigException;
+import org.eclipse.jetty.servlet.FilterHolder;
 import org.eclipse.jetty.servlet.ServletContextHandler;
 import org.eclipse.jetty.util.resource.Resource;
 import org.eclipse.jetty.util.resource.ResourceCollection;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
+import javax.servlet.DispatcherType;
 import javax.ws.rs.core.Configurable;
 import java.io.IOException;
+import java.util.EnumSet;
 import java.util.List;
 import java.util.Map;
 import java.util.Properties;
@@ -59,6 +62,10 @@ public SchemaRegistryRestApplication(Properties props) throws RestConfigExceptio
   @Override
   protected void configurePreResourceHandling(ServletContextHandler context) {
     super.configurePreResourceHandling(context);
+    PrincipalLoggingFilter principalLoggingFilter = new PrincipalLoggingFilter();
+    FilterHolder filterHolder = new FilterHolder(principalLoggingFilter);
+    filterHolder.setName("PrincipalLoggingFilter");
+    context.addFilter(filterHolder, "/*", EnumSet.of(DispatcherType.REQUEST));
     context.setErrorHandler(new JsonErrorHandler());
     // This handler runs before first Session, Security or ServletHandler
     context.insertHandler(new RequestHeaderHandler());

core/src/main/java/io/confluent/kafka/schemaregistry/storage/KafkaSchemaRegistry.java

@@ -84,6 +84,7 @@
 import io.confluent.kafka.schemaregistry.storage.exceptions.StoreInitializationException;
 import io.confluent.kafka.schemaregistry.storage.exceptions.StoreTimeoutException;
 import io.confluent.kafka.schemaregistry.storage.serialization.Serializer;
+import io.confluent.kafka.schemaregistry.utils.PrincipalContext;
 import io.confluent.kafka.schemaregistry.utils.QualifiedSubject;
 import io.confluent.rest.NamedURI;
 import io.confluent.rest.RestConfig;
@@ -658,6 +659,10 @@
         schema = new Schema(subject, version, schema.getId(), newSchema);
       }
 
+      String schemaHash = MD5.ofSchema(schema).toHexString();
+      String principal = PrincipalContext.getPrincipal();
+      logResourceAssociation(schemaHash, principal);
+
       return register(subject, schema, normalize, request.doPropagateSchemaTags());
     } catch (IllegalArgumentException e) {
       throw new InvalidSchemaException(e);
@@ -677,7 +682,7 @@
    *     it is set in the return object.
    */
   @Override
   public Schema register(String subject,
                          Schema schema,
                          boolean normalize,
                          boolean propagateSchemaTags)
@@ -1145,6 +1150,9 @@
       } else {
         kafkaStore.put(key, null);
       }
+      String schemaHash = MD5.ofSchema(schema).toHexString();
+      String principal = PrincipalContext.getPrincipal();
+      logResourceAssociation(schemaHash, principal);
     } catch (StoreTimeoutException te) {
       throw new SchemaRegistryTimeoutException("Write to the Kafka store timed out while", te);
     } catch (StoreException e) {
@@ -1715,6 +1723,11 @@
       if (schemaValue != null && (!schemaValue.isDeleted() || returnDeletedSchema)) {
         schema = toSchemaEntity(schemaValue);
       }
+      if (schema != null) {
+        String schemaHash = MD5.ofSchema(schema).toHexString();
+        String principal = PrincipalContext.getPrincipal();
+        logResourceAssociation(schemaHash, principal);
+      }
       return schema;
     }
   }
@@ -1887,6 +1900,9 @@
       if (schema == null) {
         return null;
       }
+      String schemaHash = MD5.ofSchema(schema).toHexString();
+      String principal = PrincipalContext.getPrincipal();
+      logResourceAssociation(schemaHash, principal);
 
       SchemaIdAndSubjects schemaIdAndSubjects =
           this.lookupCache.schemaIdAndSubjects(toSchemaEntity(schema));
@@ -2626,6 +2642,10 @@
     }
   }
 
+  private void logResourceAssociation(String schemaHash, String principal) {
+    log.info("Resource association log - (Principal, schemaHash): ({}, {})", principal, schemaHash);
+  }
+
   @Override
   public SchemaRegistryConfig config() {
     return config;

core/src/main/java/io/confluent/kafka/schemaregistry/storage/MD5.java

@@ -81,4 +81,16 @@ public boolean equals(Object o) {
     MD5 otherMd5 = (MD5) o;
     return Arrays.equals(this.md5, otherMd5.md5);
   }
+
+  public String toHexString() {
+    StringBuilder hexString = new StringBuilder();
+    for (byte b : md5) {
+      String hex = Integer.toHexString(0xFF & b);
+      if (hex.length() == 1) {
+        hexString.append('0');
+      }
+      hexString.append(hex);
+    }
+    return hexString.toString();
+  }
 }

core/src/main/java/io/confluent/kafka/schemaregistry/utils/PrincipalContext.java

@@ -0,0 +1,32 @@
+/*
+ * Copyright 2025 Confluent Inc.
+ *
+ * Licensed under the Confluent Community License (the "License"); you may not use
+ * this file except in compliance with the License.  You may obtain a copy of the
+ * License at
+ *
+ * http://www.confluent.io/confluent-community-license
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OF ANY KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations under the License.
+ */
+
+package io.confluent.kafka.schemaregistry.utils;
+
+public class PrincipalContext {
+  private static final ThreadLocal<String> principalHolder = new ThreadLocal<>();
+
+  public static void setPrincipal(String principal) {
+    principalHolder.set(principal);
+  }
+
+  public static String getPrincipal() {
+    return principalHolder.get();
+  }
+
+  public static void clear() {
+    principalHolder.remove();
+  }
+}