I have rather question what do to do in scenerio that KafkaStreams service is consuming some message and during processing it uncaught exception is thrown. At the moment I have implemented UncaughtExceptionHandler which is closing and cleanup old streams and starting new one, but it starts to consumes the same message which means that it ends with infinite loop of restarting...
In the end should I check the type of error and commit this message somehow to stop processing this error prone message?
Is it possible from this setUncaughtExceptionHandler method?
Regards
If you like, you can better handle such messages as Poison pills and skip them using the following piece of code, and move forward.
public final class poisionPillEvent {
public static void ignoreEvents(String topic, String poisonedEventMessage, Consumer<?, ?> consumer) {
final Pattern offsetPattern = Pattern.compile("\\w*offset*\\w[ ]\\d+");
final Pattern partitionPattern = Pattern.compile("\\w*" + topic + "*\\w[-]\\d+");
try {
// Parse the event to get the partition number and offset, in order to `seek` past the poison pill.
Matcher mPart = partitionPattern.matcher(poisonedEventMessage);
Matcher mOff = offsetPattern.matcher(poisonedEventMessage);
mPart.find();
Integer partition = Integer.parseInt(mPart.group().replace(topic + "-", ""));
mOff.find();
Long offset = Long.parseLong(mOff.group().replace("offset ", ""));
consumer.seek(new TopicPartition(topic, partition), offset + 1);
} catch (Exception ex) {
LOGGER.error("Unable to seek past bad message. {}", ex.toString());
}
}
}
Related
I am writing a case to test flink two step commit, below is overview.
sink kafka is exactly once kafka producer. sink step is mysql sink extend two step commit. sink compare is mysql sink extend two step commit, and this sink will occasionally throw a exeption to simulate checkpoint failed.
When checkpoint is failed and restore, I find mysql two step commit will work fine, but kafka consumer will read offset from last success and kafka producer produce messages even he was done it before this checkpoint failed.
How to avoid duplicate message in this case?
Thanks for help.
env:
flink 1.9.1
java 1.8
kafka 2.11
kafka producer code:
dataStreamReduce.addSink(new FlinkKafkaProducer<>(
"flink_output",
new KafkaSerializationSchema<Tuple4<String, String, String, Long>>() {
#Override
public ProducerRecord<byte[], byte[]> serialize(Tuple4<String, String, String, Long> element, #Nullable Long timestamp) {
UUID uuid = UUID.randomUUID();
JSONObject jsonObject = new JSONObject();
jsonObject.put("uuid", uuid.toString());
jsonObject.put("key1", element.f0);
jsonObject.put("key2", element.f1);
jsonObject.put("key3", element.f2);
jsonObject.put("indicate", element.f3);
return new ProducerRecord<>("flink_output", jsonObject.toJSONString().getBytes(StandardCharsets.UTF_8));
}
},
kafkaProps,
FlinkKafkaProducer.Semantic.EXACTLY_ONCE
)).name("sink kafka");
checkpoint settings:
StreamExecutionEnvironment executionEnvironment = StreamExecutionEnvironment.getExecutionEnvironment();
executionEnvironment.enableCheckpointing(10000);
executionEnvironment.getCheckpointConfig().setTolerableCheckpointFailureNumber(0);
executionEnvironment.getCheckpointConfig().setPreferCheckpointForRecovery(true);
mysql sink:
dataStreamReduce.addSink(
new TwoPhaseCommitSinkFunction<Tuple4<String, String, String, Long>,
Connection, Void>
(new KryoSerializer<>(Connection.class, new ExecutionConfig()), VoidSerializer.INSTANCE) {
int count = 0;
Connection connection;
#Override
protected void invoke(Connection transaction, Tuple4<String, String, String, Long> value, Context context) throws Exception {
if (count > 10) {
throw new Exception("compare test exception.");
}
PreparedStatement ps = transaction.prepareStatement(
" insert into test_two_step_compare(slot_time, key1, key2, key3, indicate) " +
" values(?, ?, ?, ?, ?) " +
" ON DUPLICATE KEY UPDATE indicate = indicate + values(indicate) "
);
ps.setString(1, context.timestamp().toString());
ps.setString(2, value.f0);
ps.setString(3, value.f1);
ps.setString(4, value.f1);
ps.setLong(5, value.f3);
ps.execute();
ps.close();
count += 1;
}
#Override
protected Connection beginTransaction() throws Exception {
LOGGER.error("compare in begin transaction");
try {
if (connection.isClosed()) {
throw new Exception("mysql connection closed");
}
}catch (Exception e) {
LOGGER.error("mysql connection is error: " + e.toString());
LOGGER.error("reconnect mysql connection");
String jdbcURI = "jdbc:mysql://";
Class.forName("com.mysql.jdbc.Driver");
Connection connection = DriverManager.getConnection(jdbcURI);
connection.setAutoCommit(false);
this.connection = connection;
}
return this.connection;
}
#Override
protected void preCommit(Connection transaction) throws Exception {
LOGGER.error("compare in pre Commit");
}
#Override
protected void commit(Connection transaction) {
LOGGER.error("compare in commit");
try {
transaction.commit();
} catch (Exception e) {
LOGGER.error("compare Commit error: " + e.toString());
}
}
#Override
protected void abort(Connection transaction) {
LOGGER.error("compare in abort");
try {
transaction.rollback();
} catch (Exception e) {
LOGGER.error("compare abort error." + e.toString());
}
}
#Override
protected void recoverAndCommit(Connection transaction) {
super.recoverAndCommit(transaction);
LOGGER.error("compare in recover And Commit");
}
#Override
protected void recoverAndAbort(Connection transaction) {
super.recoverAndAbort(transaction);
LOGGER.error("compare in recover And Abort");
}
})
.setParallelism(1).name("sink compare");
I'm not quite sure I understand the question correctly:
When checkpoint is failed and restore, I find mysql two step commit will work fine, but kafka producer will read offset from last success and produce message even he was done it before this checkpoint failed.
Kafka producer is not reading any data. So, I'm assuming your whole pipeline rereads old offsets and produces duplicates. If so, you need to understand how Flink ensures exactly once.
Periodic checkpoints are created to have a consistent state in case of failure.
These checkpoints contain the offset of the last successfully read record at the time of the checkpoint.
Upon recovery Flink will reread all records from the offset stored in the last successful checkpoint. Thus, the same records will be replayed as have been generated in between last checkpoint and failure.
The replayed records will restore the state right before the failure.
It will produce duplicate outputs originating from the replayed input records.
It is the responsibility of the sinks to ensure that no duplicates are effectively written to the target system.
For the last point, there are two options:
only output data, when a checkpoint has been written, such that no effective duplicates can ever appear in the target. This naive approach is very universal (independent of the sink) but adds the checkpointing interval to the latency.
let the sink deduplicate the output.
The latter option is used for the Kafka sink. It uses Kafka transactions for letting it deduplicate data. To avoid duplicates on consumer side, you need to ensure it's not reading uncommitted data as mentioned in the documentation. Also make sure your transaction timeout is large enough that it doesn't discard data between failure and recovery.
I am using Kafka 2 and I was going through the following link.
https://cwiki.apache.org/confluence/display/KAFKA/KIP-98+-+Exactly+Once+Delivery+and+Transactional+Messaging
Below is my sample code for Transactional producer.
My code:
public void runProducer(final int sendMessageCount) throws Exception {
final Producer<Long, String> producer = createProducer();
producer.initTransactions();
final long time = System.currentTimeMillis();
try {
producer.beginTransaction();
for (long index = time; index < (time + sendMessageCount); index++) {
final ProducerRecord<Long, String> record =
new ProducerRecord<>(TOPIC, index,
"Test " + index);
// send returns Future
producer.send(record).get();
}
producer.commitTransaction();
}
catch (ProducerFencedException | OutOfOrderSequenceException | AuthorizationException e) {
e.printStackTrace();
// We can't recover from these exceptions, so our only option is to close the producer and exit.
producer.close();
}
catch (final KafkaException e) {
e.printStackTrace();
// For all other exceptions, just abort the transaction and try again.
producer.abortTransaction();
}
finally {
producer.flush();
producer.close();
}
}
Questions:
Do we need to call endTransaction after commitTransaction ?
Do we need to call sendOffsetsToTransaction? What will happen if I don't include this?
How does it work when we deploy the same code to multiple servers with same transactionId? Do we need to have a separate transactionId for each instance? Say, machine1 crashes after beginTransaction() and after sending few records? How does machine2 with same transactionId recovers.
Machine1 is using transactionId "test" and it crashed after beginTransaction() and after producing few records. When the same instance comes up how does it resume the same transaction? We will actually again start from init & begin transaction.
How does it work for the same topic which was not involving in transaction and involving in transaction now? I am starting a new consumerGroup with transaction_committed, Will it read the messages which were committed before the transaction? Will the consumer with transaction_uncommitted see the messages which were aborted by transaction?
I'm having a hard time figuring simple patterns for handling exceptions in the consumer of a Kafka topic.
Scenario is as follows: in the consumer I call an external service. If the service is unavailable I want to retry a few times and then stop consuming.
The simplest pattern seems a blocking synchronous way of dealing with it, something like this in java:
ConsumerRecords<String, String> records = consumer.poll(100);
for (ConsumerRecord<String, String> record : records) {
boolean processed=false;
int count=0;
while (!processed) {
try {
callService(..);
} catch (Exception e) {
if (count++ < 3) {
Thread.sleep(5000);
continue;
} else throw new RuntimeException();
}
}
}
However, I have the feeling there must be a simpler approach (without using third party libraries), and one that avoids blocking the thread.
Seems like a common thing we would like to have, yet I could not find a simple example for this pattern.
There is no such retrial mechanism provided by Kafka out of the box. With the experience of using RabbitMQ where the MQ provides a retry exchange. These exchanges are called as Dead-Letter-Exchanges in RabbitMQ.
https://www.rabbitmq.com/dlx.html
You can apply the same pattern in the case of kafka.
On message processing failure we can publish a copy of the message to another topic and wait for the next message. Let’s call the new topic the ‘retry_topic’. The consumer of the ‘retry_topic’ will receive the message from the Kafka and then will wait some predefined time, for example one hour, before starting the message processing. This way we can postpone next attempts of the message processing without any impact on the ‘main_topic’ consumer. If processing in the ‘retry_topic’ consumer fails we just have to give up and store the message in the ‘failed_topic’ for further manual handling of this problem. The ‘main_topic’ consumer code may look like this:
Pushing message to retry_topic on failure/exception
void consumeMainTopicWithPostponedRetry() {
while (true) {
Message message = takeNextMessage("main_topic");
try {
process(message);
} catch (Exception ex) {
publishTo("retry_topic");
LOGGER.warn("Message processing failure. Will try once again in the future.", ex);
}
}
}
Consumer of the retry topic
void consumeRetryTopic() {
while (true) {
Message message = takeNextMessage("retry_topic");
try {
process(message);
waitSomeLongerTime();
} catch (Exception ex) {
publishTo("failed_topic");
LOGGER.warn("Message processing failure. Will skip it.", ex);
}
}
}
The above strategy and examples are picked from the below link. The whole credit goes to the owner of the blog post.
https://blog.pragmatists.com/retrying-consumer-architecture-in-the-apache-kafka-939ac4cb851a
For non-blocking way of doing above can be understood by reading the whole blog post. Hope this helps.
I have Kafka commit policy set to latest and missing first few messages. If I give a sleep of 20 seconds before starting to send the messages to the input topic, everything is working as desired. I am not sure if the problem is with consumer taking long time for partition rebalancing. Is there a way to know if the consumer is ready before starting to poll ?
You can use consumer.assignment(), it will return set of partitions and verify whether all of the partitions are assigned which are available for that topic.
If you are using spring-kafka project, you can include spring-kafka-test dependancy and use below method to wait for topic assignment , but you need to have container.
ContainerTestUtils.waitForAssignment(Object container, int partitions);
You can do the following:
I have a test that reads data from kafka topic.
So you can't use KafkaConsumer in multithread environment, but you can pass parameter "AtomicReference assignment", update it in consumer-thread, and read it in another thread.
For example, snipped of working code in project for testing:
private void readAvro(String readFromKafka,
AtomicBoolean needStop,
List<Event> events,
String bootstrapServers,
int readTimeout) {
// print the topic name
AtomicReference<Set<TopicPartition>> assignment = new AtomicReference<>();
new Thread(() -> readAvro(bootstrapServers, readFromKafka, needStop, events, readTimeout, assignment)).start();
long startTime = System.currentTimeMillis();
long maxWaitingTime = 30_000;
for (long time = System.currentTimeMillis(); System.currentTimeMillis() - time < maxWaitingTime;) {
Set<TopicPartition> assignments = Optional.ofNullable(assignment.get()).orElse(new HashSet<>());
System.out.println("[!kafka-consumer!] Assignments [" + assignments.size() + "]: "
+ assignments.stream().map(v -> String.valueOf(v.partition())).collect(Collectors.joining(",")));
if (assignments.size() > 0) {
break;
}
try {
Thread.sleep(1_000);
} catch (InterruptedException e) {
e.printStackTrace();
needStop.set(true);
break;
}
}
System.out.println("Subscribed! Wait summary: " + (System.currentTimeMillis() - startTime));
}
private void readAvro(String bootstrapServers,
String readFromKafka,
AtomicBoolean needStop,
List<Event> events,
int readTimeout,
AtomicReference<Set<TopicPartition>> assignment) {
KafkaConsumer<String, byte[]> consumer = (KafkaConsumer<String, byte[]>) queueKafkaConsumer(bootstrapServers, "latest");
System.out.println("Subscribed to topic: " + readFromKafka);
consumer.subscribe(Collections.singletonList(readFromKafka));
long started = System.currentTimeMillis();
while (!needStop.get()) {
assignment.set(consumer.assignment());
ConsumerRecords<String, byte[]> records = consumer.poll(1_000);
events.addAll(CommonUtils4Tst.readEvents(records));
if (readTimeout == -1) {
if (events.size() > 0) {
break;
}
} else if (System.currentTimeMillis() - started > readTimeout) {
break;
}
}
needStop.set(true);
synchronized (MainTest.class) {
MainTest.class.notifyAll();
}
consumer.close();
}
P.S.
needStop - global flag, to stop all running thread if any in case of failure of success
events - list of object, that i want to check
readTimeout - how much time we will wait until read all data, if readTimeout == -1, then stop when we read anything
Thanks to Alexey (I have also voted up), I seemed to have resolved my issue essentially following the same idea.
Just want to share my experience... in our case we using Kafka in request & response way, somewhat like RPC. Request is being sent on one topic and then waiting for response on another topic. Running into a similar issue i.e. missing out first response.
I have tried ... KafkaConsumer.assignment(); repeatedly (with Thread.sleep(100);) but doesn't seem to help. Adding a KafkaConsumer.poll(50); seems to have primed the consumer (group) and receiving the first response too. Tested few times and it consistently working now.
BTW, testing requires stopping application & deleting Kafka topics and, for a good measure, restarted Kafka too.
PS: Just calling poll(50); without assignment(); fetching logic, like Alexey mentioned, may not guarantee that consumer (group) is ready.
You can modify an AlwaysSeekToEndListener (listens only to new messages) to include a callback:
public class AlwaysSeekToEndListener<K, V> implements ConsumerRebalanceListener {
private final Consumer<K, V> consumer;
private Runnable callback;
public AlwaysSeekToEndListener(Consumer<K, V> consumer) {
this.consumer = consumer;
}
public AlwaysSeekToEndListener(Consumer<K, V> consumer, Runnable callback) {
this.consumer = consumer;
this.callback = callback;
}
#Override
public void onPartitionsRevoked(Collection<TopicPartition> partitions) {
}
#Override
public void onPartitionsAssigned(Collection<TopicPartition> partitions) {
consumer.seekToEnd(partitions);
if (callback != null) {
callback.run();
}
}
}
and subscribe with a latch callback:
CountDownLatch initLatch = new CountDownLatch(1);
consumer.subscribe(singletonList(topic), new AlwaysSeekToEndListener<>(consumer, () -> initLatch.countDown()));
initLatch.await(); // blocks until consumer is ready and listening
then proceed to start your producer.
If your policy is set to latest - which takes effect if there are no previously committed offsets - but you have no previously committed offsets, then you should not worry about 'missing' messages, because you're telling Kafka not to care about messages that were sent 'previously' to your consumers being ready.
If you care about 'previous' messages, you should set the policy to earliest.
In any case, whatever the policy, the behaviour you are seeing is transient, i.e. once committed offsets are saved in Kafka, on every restart the consumers will pick up where they left previoulsy
I needed to know if a kafka consumer was ready before doing some testing, so i tried with consumer.assignment(), but it only returned the set of partitions assigned, but there was a problem, with this i cannot see if this partitions assigned to the group had offset setted, so later when i tried to use the consumer it didn´t have offset setted correctly.
The solutions was to use committed(), this will give you the last commited offsets of the given partitions that you put in the arguments.
So you can do something like: consumer.committed(consumer.assignment())
If there is no partitions assigned yet it will return:
{}
If there is partitions assigned, but no offset yet:
{name.of.topic-0=null, name.of.topic-1=null}
But if there is partitions and offset:
{name.of.topic-0=OffsetAndMetadata{offset=5197881, leaderEpoch=null, metadata=''}, name.of.topic-1=OffsetAndMetadata{offset=5198832, leaderEpoch=null, metadata=''}}
With this information you can use something like:
consumer.committed(consumer.assignment()).isEmpty();
consumer.committed(consumer.assignment()).containsValue(null);
And with this information you can be sure that the kafka consumer is ready.
If I have a enable.auto.commit=false and I call consumer.poll() without calling consumer.commitAsync() after, why does consumer.poll() return
new records the next time it's called?
Since I did not commit my offset, I would expect poll() would return the latest offset which should be the same records again.
I'm asking because I'm trying to handle failure scenarios during my processing. I was hoping without committing the offset, the poll() would return the same records again so I can re-process those failed records again.
public class MyConsumer implements Runnable {
#Override
public void run() {
while (true) {
ConsumerRecords<String, LogLine> records = consumer.poll(Long.MAX_VALUE);
for (ConsumerRecord record : records) {
try {
//process record
consumer.commitAsync();
} catch (Exception e) {
}
/**
If exception happens above, I was expecting poll to return new records so I can re-process the record that caused the exception.
**/
}
}
}
}
The starting offset of a poll is not decided by the broker but by the consumer. The consumer tracks the last received offset and asks for the following bunch of messages during the next poll.
Offset commits come into play when a consumer stops or fails and another instance that is not aware of the last consumed offset picks up consumption of a partition.
KafkaConsumer has pretty extensive Javadoc that is well worth a read.
Consumer will read from last commit offset if it get re balanced (means if any consumer leave the group or new consumer added) so handling de-duplication does not come straight forward in kafka so you have to store the last process offset in external store and when rebalance happens or app restart you should seek to that offset and start processing or you should check against some unique key in message against DB to find is dublicate
I would like to share some code how you can solve this in Java code.
The approach is that you poll the records, try to process them and if an exception occurs, you seek to the minima of the topic partitions. After that, you do the commitAsync().
public class MyConsumer implements Runnable {
#Override
public void run() {
while (true) {
List<ConsumerRecord<String, LogLine>> records = StreamSupport
.stream( consumer.poll(Long.MAX_VALUE).spliterator(), true )
.collect( Collectors.toList() );
boolean exceptionRaised = false;
for (ConsumerRecord<String, LogLine> record : records) {
try {
// process record
} catch (Exception e) {
exceptionRaised = true;
break;
}
}
if( exceptionRaised ) {
Map<TopicPartition, Long> offsetMinimumForTopicAndPartition = records
.stream()
.collect( Collectors.toMap( r -> new TopicPartition( r.topic(), r.partition() ),
ConsumerRecord::offset,
Math::min
) );
for( Map.Entry<TopicPartition, Long> entry : offsetMinimumForTopicAndPartition.entrySet() ) {
consumer.seek( entry.getKey(), entry.getValue() );
}
}
consumer.commitAsync();
}
}
}
With this setup, you poll the messages again and again until you successfully process all messages of one poll.
Please note, that your code should be able to handle a poison pill. Otherwise, your code will stuck in an endless loop.