KAFKA-19644: Enhance the documentation for producer headers and integration tests

clients/clients-integration-tests/src/test/java/org/apache/kafka/clients/consumer/PlaintextConsumerTest.java

@@ -203,7 +203,10 @@ private void testHeaders(Map<String, Object> consumerConfig) throws Exception {
         ) {
             var record = new ProducerRecord<>(TP.topic(), TP.partition(), null, "key".getBytes(), "value".getBytes());
             record.headers().add("headerKey", "headerValue".getBytes());
+            record.headers().add("headerKey2", "headerValue2".getBytes());
+            record.headers().add("headerKey3", "headerValue3".getBytes());
             producer.send(record);
+            producer.flush();
 
             assertEquals(0, consumer.assignment().size());
             consumer.assign(List.of(TP));
@@ -212,8 +215,15 @@ var record = new ProducerRecord<>(TP.topic(), TP.partition(), null, "key".getByt
             consumer.seek(TP, 0);
             var records = consumeRecords(consumer, numRecords);
             assertEquals(numRecords, records.size());
+
             var header = records.get(0).headers().lastHeader("headerKey");
             assertEquals("headerValue", header == null ? null : new String(header.value()));
+
+            // Test the order of headers in a record is preserved when producing and consuming
+            Header[] headers = records.get(0).headers().toArray();
+            assertEquals("headerKey", headers[0].key());
+            assertEquals("headerKey2", headers[1].key());
+            assertEquals("headerKey3", headers[2].key());
         }
     }
 

clients/clients-integration-tests/src/test/java/org/apache/kafka/clients/consumer/ShareConsumerTest.java

@@ -467,6 +467,8 @@ public void testHeaders() {
             int numRecords = 1;
             ProducerRecord<byte[], byte[]> record = new ProducerRecord<>(tp.topic(), tp.partition(), null, "key".getBytes(), "value".getBytes());
             record.headers().add("headerKey", "headerValue".getBytes());
+            record.headers().add("headerKey2", "headerValue2".getBytes());
+            record.headers().add("headerKey3", "headerValue3".getBytes());
             producer.send(record);
             producer.flush();
 
@@ -475,11 +477,15 @@ public void testHeaders() {
             List<ConsumerRecord<byte[], byte[]>> records = consumeRecords(shareConsumer, numRecords);
             assertEquals(numRecords, records.size());
 
-            for (ConsumerRecord<byte[], byte[]> consumerRecord : records) {
-                Header header = consumerRecord.headers().lastHeader("headerKey");
-                if (header != null)
-                    assertEquals("headerValue", new String(header.value()));
-            }
+            Header header = records.get(0).headers().lastHeader("headerKey");
+            assertEquals("headerValue", new String(header.value()));
+
+            // Test the order of headers in a record is preserved when producing and consuming
+            Header[] headers = records.get(0).headers().toArray();
+            assertEquals("headerKey", headers[0].key());
+            assertEquals("headerKey2", headers[1].key());
+            assertEquals("headerKey3", headers[2].key());
+
             verifyShareGroupStateTopicRecordsProduced();
         }
     }

clients/src/main/java/org/apache/kafka/common/header/Header.java

@@ -16,10 +16,23 @@
  */
 package org.apache.kafka.common.header;
 
+/**
+ * A header is a key-value pair.
+ */
 public interface Header {
-
+
+    /**
+     * Returns the key of the header.
+     *
+     * @return the header's key; must not be null.
+     */
     String key();
 
+    /**
+     * Returns the value of the header.
+     *
+     * @return the header's value; may be null.
+     */
     byte[] value();
-   
+
 }

clients/src/main/java/org/apache/kafka/common/header/Headers.java

@@ -16,12 +16,18 @@
  */
 package org.apache.kafka.common.header;
 
+
+/**
+ * A mutable ordered collection of {@link Header} objects. Note that multiple headers may have the same {@link Header#key() key}.
+ * <p>
+ * The order of headers is preserved in the order they were added.
+ */
 public interface Headers extends Iterable<Header> {
 
     /**
      * Adds a header (key inside), to the end, returning if the operation succeeded.
      * 
-     * @param header the Header to be added
+     * @param header the Header to be added.
      * @return this instance of the Headers, once the header is added.
      * @throws IllegalStateException is thrown if headers are in a read-only state.
      */
@@ -30,8 +36,8 @@ public interface Headers extends Iterable<Header> {
     /**
      * Creates and adds a header, to the end, returning if the operation succeeded.
      *
-     * @param key of the header to be added.
-     * @param value of the header to be added.
+     * @param key of the header to be added; must not be null.
+     * @param value of the header to be added; may be null.
      * @return this instance of the Headers, once the header is added.
      * @throws IllegalStateException is thrown if headers are in a read-only state.
      */
@@ -40,7 +46,7 @@ public interface Headers extends Iterable<Header> {
     /**
      * Removes all headers for the given key returning if the operation succeeded.
      * 
-     * @param key to remove all headers for.
+     * @param key to remove all headers for; must not be null.
      * @return this instance of the Headers, once the header is removed.
      * @throws IllegalStateException is thrown if headers are in a read-only state.
      */
@@ -49,15 +55,15 @@ public interface Headers extends Iterable<Header> {
     /**
      * Returns just one (the very last) header for the given key, if present.
      * 
-     * @param key to get the last header for.
+     * @param key to get the last header for; must not be null.
      * @return this last header matching the given key, returns null if not present.
      */
     Header lastHeader(String key);
 
     /**
      * Returns all headers for the given key, in the order they were added in, if present.
      *
-     * @param key to return the headers for.
+     * @param key to return the headers for; must not be null.
      * @return all headers for the given key, in the order they were added in, if NO headers are present an empty iterable is returned. 
      */
     Iterable<Header> headers(String key);

clients/src/test/java/org/apache/kafka/common/header/internals/RecordHeadersTest.java

@@ -47,6 +47,21 @@ public void testAdd() {
         assertEquals(2, getCount(headers));
     }
 
+    @Test
+    public void testAddHeadersPreserveOder() {
+        Headers headers = new RecordHeaders();
+        headers.add(new RecordHeader("key", "value".getBytes()));
+        headers.add(new RecordHeader("key2", "value2".getBytes()));
+        headers.add(new RecordHeader("key3", "value3".getBytes()));
+
+        Header[] headersArr = headers.toArray();
+        assertHeader("key", "value", headersArr[0]);
+        assertHeader("key2", "value2", headersArr[1]);
+        assertHeader("key3", "value3", headersArr[2]);
+
+        assertEquals(3, getCount(headers));
+    }
+
     @Test
     public void testRemove() {
         Headers headers = new RecordHeaders();

docs/implementation.html

@@ -101,6 +101,7 @@ <h5 class="anchor-heading"><a id="recordheader" class="anchor-link"></a><a href=
 headerKey: String
 headerValueLength: varint
 Value: byte[]</code></pre>
+    <p>The key of a record header is guaranteed to be non-null, while the value of a record header may be null. The order of headers in a record is preserved when producing and consuming.</p>
     <p>We use the same varint encoding as Protobuf. More information on the latter can be found <a href="https://developers.google.com/protocol-buffers/docs/encoding#varints">here</a>. The count of headers in a record
     is also encoded as a varint.</p>