Develop Reference

Run kafka consumer without specifying partition

I am learning Kafka recently, and my consumers can't consume any records unless I specify the --parititon 0 parameter. In other words I can NOT consume records like:
kafka-console-consumer --bootstrap-server 127.0.0.10:9092 --topic first-topic
but works like:
kafka-console-consumer --bootstrap-server 127.0.0.10:9092 --topic first-topic --partition 0
THE MAIN PROBLEM IS, when I moved to java code, my KafkaConsumer class can't fetch records, and I need to know how to specify the partition number in java KafkaConsumer ?!
my current java code is:
public class ConsumerDemo {
public static void main(String[] args) {
Logger logger = LoggerFactory.getLogger((ConsumerDemo.class.getName()));
String bootstrapServer = "127.0.0.10:9092";
String groupId = "my-kafka-java-app";
String topic = "first-topic";
// create consumer configs
Properties properties = new Properties();
properties.setProperty(ConsumerConfig.BOOTSTRAP_SERVERS_CONFIG, bootstrapServer);
//properties.setProperty(ConsumerConfig.PARTITION_ASSIGNMENT_STRATEGY_CONFIG, partition);
properties.setProperty(ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class.getName());
properties.setProperty(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class.getName());
properties.setProperty(ConsumerConfig.GROUP_ID_CONFIG, groupId);
properties.setProperty(ConsumerConfig.AUTO_OFFSET_RESET_CONFIG, "earliest");
// create consumer
KafkaConsumer<String, String> consumer = new KafkaConsumer<String, String>(properties);
// subscribe consumer to our topic
consumer.subscribe(Collections.singleton(topic)); //means subscribtion to one topic
// poll for new data
while(true){
//consumer.poll(100); old way
ConsumerRecords<String, String> records =
consumer.poll(Duration.ofMillis(100));
for (ConsumerRecord<String, String> record : records){
logger.info("Key: " + record.key() + ", Value: "+ record.value() );
logger.info("Partition: " + record.partition() + ", Offset: "+ record.offset());
}
}
}
}

After a lot of inspection, my solution came out to be using consumer.assign and consumer.seek instead of using consumer.subscribe and without specifying the groupId. But I feel there should be a more optimal solution
the java code will be as:
// create consumer
KafkaConsumer<String, String> consumer = new KafkaConsumer<String, String>(properties);
// subscribe consumer to our topic
//consumer.subscribe(Collections.singleton(topic)); //means subscription to one topic
// using assign and Seek, are mostly used to replay data or fetch a specific msg
TopicPartition partitionToReadFrom = new TopicPartition(topic, 0);
long offsetToReadFrom = 15L;
// assign
consumer.assign(Arrays.asList(partitionToReadFrom));
// seek: for a specific offset to read from
consumer.seek(partitionToReadFrom, offsetToReadFrom);

The way you are doing is correct. You don't need to specify the partition when subscribing to a topic. Maybe your consumer group has already consumed all messages in the topic and has committed the latest offsets.
Make sure new messages are being produced when you run your application or create a new consumer group to consume from the beginning (if you keep the ConsumerConfig.AUTO_OFFSET_RESET_CONFIG set to "earliest")

As the name implies, the ConsumerConfig.PARTITION_ASSIGNMENT_STRATEGY_CONFIG property aims to configure a Partition Assignment Strategy and no to set a fixed partition as instructed by the command line.
The default strategy used is the RangeAssignor which can be changed, for example to a StickyAssignor as follows:
properties.setProperty(ConsumerConfig.PARTITION_ASSIGNMENT_STRATEGY_CONFIG,StickyAssignor.class.getName());
You can read more about Kafka Client Side Assignment Proposal.

How to programmatically get latest offset of a Kafka topic in Java

Here is what I am trying
Collection <TopicPartition> partitions = consumer.partitionsFor(topic).stream();
And also how to indicate you've hit the end or there isn't anymore messages to consume. If the offset doesn't match the broker's end offset at the time how to do that.
Any suggestions.

In order to get the latest offset you can either use the command line:
./bin/kafka-run-class.sh kafka.tools.GetOffsetShell \
--broker-list localhost:9092 \
--topic topicName
or programmatically in Java:
KafkaConsumer<String, String> consumer = new KafkaConsumer<>(consumerProperties)) {
consumer.subscribe(Arrays.asList("topicName"));
Set<TopicPartition> assignment;
while ((assignment = consumer.assignment()).isEmpty()) {
consumer.poll(Duration.ofMillis(500));
}
consumer.endOffsets(assignment).forEach((partition, offset) -> System.out.println(partition + ": " + offset));
}
Now if you want to force the consumer to start consuming from the latest offset, you can either use the following property:
props.put("auto.offset.reset", "latest");
// or props.put(ConsumerConfig.AUTO_OFFSET_RESET_CONFIG, "latest");
or force it to consume from latest offset using
consumer.seekToEnd();

How to delete data which already been consumed by consumer? Kafka

I am doing data replication in kafka. But, the size of kafka log file is increases very quickly. The size reaches 5 gb in a day. As a solution of this problem, ı want to delete processed data immediately. I am using delete record method in AdminClient to delete offset. But when I look at the log file, data corresponding to that offset is not deleted.
RecordsToDelete recordsToDelete = RedcordsToDelete.beforeOffset(offset);
TopicPartition topicPartition = new TopicPartition(topicName,partition);
Map<TopicPartition,RecordsToDelete> deleteConf = new HashMap<>();
deleteConf.put(topicPartition,recordsToDelete);
adminClient.deleteRecords(deleteConf);
I don't want suggestions like (log.retention.hours , log.retention.bytes , log.segment.bytes , log.cleanup.policy=delete)
Because I just want to delete data consumed by the consumer. In this solution, I also deleted the data that is not consumed.
What are your suggestions?

You didn't do anything wrong. The code you provided works and I've tested it. Just in case I've overlooked something in your code, mine is:
public void deleteMessages(String topicName, int partitionIndex, int beforeIndex) {
TopicPartition topicPartition = new TopicPartition(topicName, partitionIndex);
Map<TopicPartition, RecordsToDelete> deleteMap = new HashMap<>();
deleteMap.put(topicPartition, RecordsToDelete.beforeOffset(beforeIndex));
kafkaAdminClient.deleteRecords(deleteMap);
}
I've used group: 'org.apache.kafka', name: 'kafka-clients', version: '2.0.0'
So check if you are targeting right partition ( 0 for the first one)
Check your broker version: https://kafka.apache.org/20/javadoc/index.html?org/apache/kafka/clients/admin/AdminClient.html says:
This operation is supported by brokers with version 0.11.0.0
Produce the messages from the same application, to be sure you're connected properly.
There is one more option you can consider. Using cleanup.policy=compact If your message keys are repeating you could benefit from it. Not just because older messages for that key will be automatically deleted but you can use the fact that message with null payload deletes all the messages for that key. Just don't forget to set delete.retention.ms and min.compaction.lag.ms to values small enough. In that case you can consume a message and than produce null payload for the same key ( but be cautious with this approach since this way you can delete messages ( with that key) you didn't consume)

Try this
DeleteRecordsResult result = adminClient.deleteRecords(recordsToDelete);
Map<TopicPartition, KafkaFuture<DeletedRecords>> lowWatermarks = result.lowWatermarks();
try {
for (Map.Entry<TopicPartition, KafkaFuture<DeletedRecords>> entry : lowWatermarks.entrySet()) {
System.out.println(entry.getKey().topic() + " " + entry.getKey().partition() + " " + entry.getValue().get().lowWatermark());
}
} catch (InterruptedException | ExecutionException e) {
e.printStackTrace();
}
adminClient.close();
In this code, you need to call entry.getValue().get().lowWatermark(), because adminClient.deleteRecords(recordsToDelete) returns a map of Futures, you need to wait for the Future to run by calling get()

This code will only work if the cleanup policy is "delete" or "compact, delete" else the code will throw a Policy Violation exception.

Apache Kafka System Error Handling

We are trying to implement Kafka as our message broker solution. We are deploying our Spring Boot microservices in IBM BLuemix, whose internal message broker implementation is Kafka version 0.10. Since my experience is more on the JMS, ActiveMQ end, I was wondering what should be the ideal way to handle system level errors in the java consumers?
Here is how we have implemented it currently
Consumer properties
enable.auto.commit=false
auto.offset.reset=latest
We are using the default properties for
max.partition.fetch.bytes
session.timeout.ms
Kafka Consumer
We are spinning up 3 threads per topic all having the same groupId, i.e one KafkaConsumer instance per thread. We have only one partition as of now. The consumer code looks like this in the constructor of the thread class
kafkaConsumer = new KafkaConsumer<String, String>(properties);
final List<String> topicList = new ArrayList<String>();
topicList.add(properties.getTopic());
kafkaConsumer.subscribe(topicList, new ConsumerRebalanceListener() {
#Override
public void onPartitionsRevoked(final Collection<TopicPartition> partitions) {
}
#Override
public void onPartitionsAssigned(final Collection<TopicPartition> partitions) {
try {
logger.info("Partitions assigned, consumer seeking to end.");
for (final TopicPartition partition : partitions) {
final long position = kafkaConsumer.position(partition);
logger.info("current Position: " + position);
logger.info("Seeking to end...");
kafkaConsumer.seekToEnd(Arrays.asList(partition));
logger.info("Seek from the current position: " + kafkaConsumer.position(partition));
kafkaConsumer.seek(partition, position);
}
logger.info("Consumer can now begin consuming messages.");
} catch (final Exception e) {
logger.error("Consumer can now begin consuming messages.");
}
}
});
The actual reading happens in the run method of the thread
try {
// Poll on the Kafka consumer every second.
final ConsumerRecords<String, String> records = kafkaConsumer.poll(1000);
// Iterate through all the messages received and print their
// content.
for (final TopicPartition partition : records.partitions()) {
final List<ConsumerRecord<String, String>> partitionRecords = records.records(partition);
logger.info("consumer is alive and is processing "+ partitionRecords.size() +" records");
for (final ConsumerRecord<String, String> record : partitionRecords) {
logger.info("processing topic "+ record.topic()+" for key "+record.key()+" on offset "+ record.offset());
final Class<? extends Event> resourceClass = eventProcessors.getResourceClass();
final Object obj = converter.convertToObject(record.value(), resourceClass);
if (obj != null) {
logger.info("Event: " + obj + " acquired by " + Thread.currentThread().getName());
final CommsEvent event = resourceClass.cast(converter.convertToObject(record.value(), resourceClass));
final MessageResults results = eventProcessors.processEvent(event
);
if ("Success".equals(results.getStatus())) {
// commit the processed message which changes
// the offset
kafkaConsumer.commitSync();
logger.info("Message processed sucessfully");
} else {
kafkaConsumer.seek(new TopicPartition(record.topic(), record.partition()), record.offset());
logger.error("Error processing message : {} with error : {},resetting offset to {} ", obj,results.getError().getMessage(),record.offset());
break;
}
}
}
}
// TODO add return
} catch (final Exception e) {
logger.error("Consumer has failed with exception: " + e, e);
shutdown();
}
You will notice the EventProcessor which is a service class which processes each record, in most cases commits the record in database. If the processor throws an error (System Exception or ValidationException) we do not commit but programatically set the seek to that offset, so that subsequent poll will return from that offset for that group id.
The doubt now is that, is this the right approach? If we get an error and we set the offset then until that is fixed no other message is processed. This might work for system errors like not able to connect to DB, but if the problem is only with that event and not others to process this one record we wont be able to process any other record. We thought of the concept of ErrorTopic where when we get an error the consumer will publish that event to the ErrorTopic and in the meantime it will keep on processing other subsequent events. But it looks like we are trying to bring in the design concepts of JMS (due to my previous experience) into kafka and there may be better way to solve error handling in kafka. Also reprocessing it from error topic may change the sequence of messages which we don't want for some scenarios
Please let me know how anyone has handled this scenario in their projects following the Kafka standards.
-Tatha

if the problem is only with that event and not others to process this one record we wont be able to process any other record
that's correct and your suggestion to use an error topic seems a possible one.
I also noticed that with your handling of onPartitionsAssigned you essentially do not use the consumer committed offset, as you seem you'll always seek to the end.
If you want to restart from the last succesfully committed offset, you should not perform a seek
Finally, I'd like to point out, though it looks like you know that, having 3 consumers in the same group subscribed to a single partition - means that 2 out of 3 will be idle.
HTH
Edo

Offset missing from Kafka logs - Simple Consumer unable to proceed

I have a 3-node kafka cluster setup. I am using storm to read messages from kafka. Each topic in my system has 7 partitions.
Now I am facing a weird problem. Till 3 days ago, everything was working fine. However, now it seems my storm topology is unable to read specifically from 2 partitions - #1 and #4.
I tried to drill down to the problem and found that in my kafka logs, for both of these partitions, one offset is missing i.e. after 5964511, next offset is 5964513 and not 5964512.
Due to missing offset, Simple Consumer is not able to proceed to next offsets. Am I doing something wrong or is it a known bug ?
What possibly could be the reason for such behaviour ?
I am using following code to read window of valid offsets :
public static long getLastOffset(SimpleConsumer consumer, String topic, int partition,
long whichTime, String clientName) {
TopicAndPartition topicAndPartition = new TopicAndPartition(topic, partition);
Map<TopicAndPartition, PartitionOffsetRequestInfo> requestInfoMap = new HashMap<TopicAndPartition, PartitionOffsetRequestInfo>();
requestInfoMap.put(topicAndPartition, new PartitionOffsetRequestInfo(kafka.api.OffsetRequest.LatestTime(), 100));
OffsetRequest request = new OffsetRequest( requestInfoMap, kafka.api.OffsetRequest.CurrentVersion() , clientName);
OffsetResponse response = consumer.getOffsetsBefore(request);
long[] validOffsets = response.offsets(topic, partition);
for (long validOffset : validOffsets) {
System.out.println(validOffset + " : ");
}
long largestOffset = validOffsets[0];
long smallestOffset = validOffsets[validOffsets.length - 1];
System.out.println(smallestOffset + " : " + largestOffset );
return largestOffset;
}
This gives me following output :
4529948 : 6000878
So, the offset I am providing is well within the offset range.

Sorry for the late answer, but...
I code for this case by having a Long instance var to hold the next offset to read and then checking after the fetch to see if the returned FetchResponse hasError(). If there was an error I change the next offset value to a reasonable value (could be the next offset or the last available offset) and try again.