Validate initial credits is at least n x 2

For flow strategy. The broker uses a send limit to "batch" TCP packets,
which can conflict with this flow strategy and can block a consumer. The
n parameter in the flow strategy must be lesser or equal to the half of initial
credits.

References #843

Author: acogoluegnes

src/main/java/com/rabbitmq/stream/ConsumerFlowStrategy.java

@@ -118,9 +118,6 @@ static ConsumerFlowStrategy creditOnChunkArrival() {
    *
    * <p>Calls to {@link MessageHandler.Context#processed()} are ignored.
    *
-   * <p>Consider using {@link #creditEveryNthChunk(int, int)} instead as it generates less network
-   * traffic.
-   *
    * @param initialCredits number of initial credits
    * @return flow strategy
    * @see com.rabbitmq.stream.ConsumerBuilder.FlowConfiguration#initialCredits(int)
@@ -175,11 +172,12 @@ static ConsumerFlowStrategy creditOnProcessedMessageCount(int initialCredits, do
    * Strategy that provides the specified number of initial credits and <code>n</code> credits every
    * <code>n</code> chunks.
    *
-   * <p>This strategy generates less network traffic than {@link
-   * com.rabbitmq.stream.ConsumerFlowStrategy.CreditOnChunkArrivalConsumerFlowStrategy} and should
-   * be used instead, unless <code>n</code> is equal to 1.
+   * <p>This strategy can improve throughput for streams with small chunks (less than 30 messages
+   * per chunk).
+   *
+   * <p>The number of initial credits must be at least twice as big as <code>n</code>.
    *
-   * <p>A rule of thumb is to set <code>n</code> to half the value of initial credits.
+   * <p>A rule of thumb is to set <code>n</code> to a third of the value of initial credits.
    *
    * <p>Calls to {@link MessageHandler.Context#processed()} are ignored.
    *
@@ -195,9 +193,10 @@ static ConsumerFlowStrategy creditEveryNthChunk(int initialCredits, int n) {
    * Strategy that provides the specified number of initial credits and <code>n</code> credits every
    * <code>n</code> chunks.
    *
-   * <p>This strategy generates less network traffic than {@link
-   * com.rabbitmq.stream.ConsumerFlowStrategy.CreditOnChunkArrivalConsumerFlowStrategy} and should
-   * be used instead, unless <code>n</code> is equal to 1.
+   * <p>This strategy can improve throughput for streams with small chunks (less than 30 messages
+   * per chunk).
+   *
+   * <p>The number of initial credits must be at least twice as big as <code>n</code>.
    *
    * <p>Calls to {@link MessageHandler.Context#processed()} are ignored.
    */
@@ -213,9 +212,9 @@ private CreditEveryNthChunkConsumerFlowStrategy(int initialCredits, int n) {
       if (n <= 0) {
         throw new IllegalArgumentException("The n argument must be greater than 0");
       }
-      if (initialCredits <= n) {
+      if (n * 2 > initialCredits) {
         throw new IllegalArgumentException(
-            "The number of initial credits must be greater than the limit");
+            "The number of initial credits must be at least twice as big as n");
       }
       this.initialCredits = initialCredits;
       this.n = n;

src/test/java/com/rabbitmq/stream/impl/CreditEveryNthChunkConsumerFlowStrategyTest.java

@@ -20,27 +20,26 @@
 import static org.assertj.core.api.Assertions.assertThatThrownBy;
 
 import com.rabbitmq.stream.ConsumerFlowStrategy;
+import com.rabbitmq.stream.ConsumerFlowStrategy.Context;
 import java.util.concurrent.atomic.AtomicInteger;
-import org.junit.jupiter.api.Test;
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.CsvSource;
 
 public class CreditEveryNthChunkConsumerFlowStrategyTest {
 
   AtomicInteger requestedCredits = new AtomicInteger();
 
-  @Test
-  void invalidArguments() {
-    assertThatThrownBy(() -> build(1, 1)).isInstanceOf(IllegalArgumentException.class);
-    assertThatThrownBy(() -> build(1, 0)).isInstanceOf(IllegalArgumentException.class);
-    assertThatThrownBy(() -> build(10, 0)).isInstanceOf(IllegalArgumentException.class);
+  @ParameterizedTest
+  @CsvSource({"10,6", "1,1", "1,0", "10,0"})
+  void invalidArguments(int initialCredits, int limit) {
+    assertThatThrownBy(() -> build(initialCredits, limit))
+        .isInstanceOf(IllegalArgumentException.class);
   }
 
   @ParameterizedTest
   @CsvSource({"10,5", "5,2", "2,1"})
   void test(int initialCredits, int limit) {
     ConsumerFlowStrategy strategy = build(initialCredits, limit);
-
     range(0, limit - 1)
         .forEach(
             ignored -> {
@@ -55,9 +54,9 @@ ConsumerFlowStrategy build(int initial, int limit) {
     return creditEveryNthChunk(initial, limit);
   }
 
-  ConsumerFlowStrategy.Context context() {
+  Context context() {
     requestedCredits.set(0);
-    return new ConsumerFlowStrategy.Context() {
+    return new Context() {
       @Override
       public void credits(int credits) {
         requestedCredits.addAndGet(credits);