Fix flaky kafka cluster example

examples/kafka-cluster/build.gradle

@@ -13,5 +13,5 @@ dependencies {
     testImplementation 'org.apache.kafka:kafka-clients:2.8.0'
     testImplementation 'org.assertj:assertj-core:3.20.2'
     testImplementation 'com.google.guava:guava:23.0'
-    testImplementation 'org.slf4j:slf4j-simple:1.7.32'
+    testImplementation 'ch.qos.logback:logback-classic:1.2.5'
 }

examples/kafka-cluster/src/test/java/com/example/kafkacluster/KafkaContainerCluster.java

@@ -10,6 +10,7 @@
 import org.testcontainers.containers.KafkaContainer;
 import org.testcontainers.utility.DockerImageName;
 
+import java.time.Duration;
 import java.util.Collection;
 import java.util.concurrent.TimeUnit;
 import java.util.stream.Collectors;
@@ -56,7 +57,8 @@ public KafkaContainerCluster(String confluentPlatformVersion, int brokersNum, in
                     .withEnv("KAFKA_OFFSETS_TOPIC_REPLICATION_FACTOR", internalTopicsRf + "")
                     .withEnv("KAFKA_OFFSETS_TOPIC_NUM_PARTITIONS", internalTopicsRf + "")
                     .withEnv("KAFKA_TRANSACTION_STATE_LOG_REPLICATION_FACTOR", internalTopicsRf + "")
-                    .withEnv("KAFKA_TRANSACTION_STATE_LOG_MIN_ISR", internalTopicsRf + "");
+                    .withEnv("KAFKA_TRANSACTION_STATE_LOG_MIN_ISR", internalTopicsRf + "")
+                    .withStartupTimeout(Duration.ofMinutes(1));
             })
             .collect(Collectors.toList());
     }
@@ -81,8 +83,8 @@ private Stream<GenericContainer<?>> allContainers() {
     @Override
     @SneakyThrows
     public void start() {
-        Stream<Startable> startables = this.brokers.stream().map(Startable.class::cast);
-        Startables.deepStart(startables).get(60, SECONDS);
+        // sequential start to avoid resource contention on CI systems with weaker hardware
+        brokers.forEach(GenericContainer::start);
 
         Unreliables.retryUntilTrue(30, TimeUnit.SECONDS, () -> {
             Container.ExecResult result = this.zookeeper.execInContainer(

examples/kafka-cluster/src/test/java/com/example/kafkacluster/KafkaContainerClusterTest.java

@@ -30,7 +30,7 @@ public class KafkaContainerClusterTest {
     @Test
     public void testKafkaContainerCluster() throws Exception {
         try (
-            KafkaContainerCluster cluster = new KafkaContainerCluster("5.2.1", 3, 2)
+            KafkaContainerCluster cluster = new KafkaContainerCluster("6.2.1", 3, 2)
         ) {
             cluster.start();
             String bootstrapServers = cluster.getBootstrapServers();

examples/kafka-cluster/src/test/resources/logback-test.xml

@@ -0,0 +1,14 @@
+<configuration>
+
+    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
+        <!-- encoders are assigned the type
+             ch.qos.logback.classic.encoder.PatternLayoutEncoder by default -->
+        <encoder>
+            <pattern>%d{HH:mm:ss.SSS} %-5level %logger - %msg%n</pattern>
+        </encoder>
+    </appender>
+
+    <root level="INFO">
+        <appender-ref ref="STDOUT"/>
+    </root>
+</configuration>

modules/kafka/src/main/java/org/testcontainers/containers/KafkaContainer.java

@@ -118,7 +118,7 @@ protected void containerIsStarted(InspectContainerResponse containerInfo) {
             "advertised.listeners=[" + String.join(",", getBootstrapServers(), brokerAdvertisedListener) + "]"
         );
         if (result.getExitCode() != 0) {
-            throw new IllegalStateException(result.getStderr());
+            throw new IllegalStateException(result.toString());
         }
     }