Difference between idempotence and exactly-once in Kafka Stream

Question

I was going through document what I understood we can achieve exactly-once transaction with enabling idempotence=true

idempotence: The Idempotent producer enables exactly once for a producer against a single topic. Basically each single message send has stonger guarantees and will not be duplicated in case there's an error

So if already we have idempotence then why we need another property exactly-once in Kafka Stream? What exactly different between idempotence vs exactly-once

Why exactly-once property not available in normal Kafka Producer?

Answer 1

In a distributed environment failure is a very common scenario that can be happened any time. In the Kafka environment, the broker can crash, network failure, failure in processing, failure while publishing message or failure to consume messages, etc. These different scenarios introduced different kinds of data loss and duplication.

Failure scenarios

A(Ack Failed): Producer published message successfully with retry>1 but could not receive acknowledge due to failure. In that case, the Producer will retry the same message that might introduce duplicate.

B(Producer process failed in batch messages): Producer sending a batch of messages it failed with few published success. In that case and once the producer will restart it will again republish all messages from the batch which will introduce duplicate in Kafka.

C(Fire & Forget Failed) Producer published message with retry=0(fire and forget). In case of failure published will not aware and send the next message this will cause the message lost.

D(Consumer failed in batch message) A consumer receives a batch of messages from Kafka and manually commit their offset (enable.auto.commit=false). If consumers failed before committing to Kafka, next time Consumers will consume the same records again which reproduce duplicate on the consumer side.

Exactly-Once semantics

In this case, even if a producer tries to resend a message, it leads to the message will be published and consumed by consumers exactly once.

To achieve Exactly-Once semantic in Kafka, it uses below 3 property

1. enable.idempotence=true (address a, b & c)
2. MAX_IN_FLIGHT_REQUESTS_PER_CONNECTION=5(Producer will always have one in-flight request per connection)
3. isolation.level=read_committed (address d )

Enable Idempotent(enable.idempotence=true)

Idempotent delivery enables the producer to write a message to Kafka exactly once to a particular partition of a topic during the lifetime of a single producer without data loss and order per partition.

"Note that enabling idempotence requires MAX_IN_FLIGHT_REQUESTS_PER_CONNECTION to be less than or equal to 5, RETRIES_CONFIG to be greater than 0 and ACKS_CONFIG be 'all'. If these values are not explicitly set by the user, suitable values will be chosen. If incompatible values are set, a ConfigException will be thrown"

To achieve idempotence Kafka uses a unique id which is called product id or PID and sequence number while producing messages. The producer keeps incrementing the sequence number on each message published which map with unique PID. The broker always compare the current sequence number with the previous one and it rejects if the new one is not +1 greater than the previous one which avoids duplication and same time if more than greater show lost in messages

In a failure scenario broker will compare the sequence numbers with the previous one and if the sequence not increased +1 will reject the message.

Transaction (isolation.level)

Transactions give us the ability to atomically update data in multiple topic partitions. All the records included in a transaction will be successfully saved, or none of them will be. It allows you to commit your consumer offsets in the same transaction along with the data you have processed, thereby allowing end-to-end exactly-once semantics.

The producer doesn't wait to write a message to Kafka whereas the Producer uses beginTransaction, commitTransaction, and abortTransaction(in case of failure) Consumer uses isolation.level either read_committed or read_uncommitted

• read_committed: Consumers will always read committed data only.
• read_uncommitted: Read all messages in offset order without waiting for transactions to be committed

If a consumer with isolation.level=read_committed reaches a control message for a transaction that has not completed, it will not deliver any more messages from this partition until the producer commits or aborts the transaction or a transaction timeout occurs. The transaction timeout is determined by the producer using the configuration transaction.timeout.ms(default 1 minute).

Exactly-Once in Producer & Consumer

In normal conditions where we have separate producers and consumers. The producer has to idempotent and same time manage transactions so consumers can use isolation.level to read-only read_committed to make the whole process as an atomic operation. This makes a guarantee that the producer will always sync with the source system. Even producer crash or a transaction aborted, it always is consistent and publishes a message or batch of the message as a unit once.

The same consumer will either receive a message or batch of the message as a unit once.

In Exactly-Once semantic Producer along with Consumer will appear as atomic operation which will operate as one unit. Either publish and get consumed once at all or aborted.

Exactly Once in Kafka Stream

Kafka Stream consumes messages from topic A, process and publish a message to Topic B and once publish use commit(commit mostly run undercover) to flush all state store data to disk.

Exactly-once in Kafka Stream is a read-process-write pattern that guarantees that this operation will be treated as an atomic operation. Since Kafka Stream caters producer, consumer and transaction all together Kafka Stream comes special parameter processing.guarantee which could exactly_once or at_least_once which make life easy not to handle all parameters separately.

Kafka Streams atomically updates consumer offsets, local state stores, state store changelog topics, and production to output topics all together. If anyone of these steps fails, all of the changes are rolled back.

processing.guarantee: exactly_once automatically provide below parameters you no need to set explicitly

1. isolation.level=read_committed
2. enable.idempotence=true
3. MAX_IN_FLIGHT_REQUESTS_PER_CONNECTION=5

Answer 2

Kafka stream offers the exactly-once semantic from the end-to-end point of view (consumes from one topic, processes that message, then produces to another topic). However, you mentioned only the producer's idempotent attribute. That is only a small part of the full picture.

Let me rephrase the question:

Why do we need the exactly-once delivery semantic at the consumer side while we already have guaranteed the exactly-once delivery semantic at the producer side?

Answer: Since the exactly-once delivery semantic is not only at the producing step but the full flow of processing. To achieve the exactly-once delivery semantically, there are some conditions must be satisfied with the producing and consuming.

This is the generic scenario: Process A produces messages to the topic T. At the same time, process B tries to consume messages from the topic T. We want to make sure process B never processes one message twice.

Producer part: We must make sure that producers never produce a message twice. We can use Kafka Idempotent Producer

Consumer part: Here is the basic workflow for the consumer:

• Step 1: The consumer pulls the message M successfully from the Kafka's topic.
• Step 2: The consumer tries to execute the job and the job returns successfully.
• Step 3: The consumer commits the message's offset to the Kafka brokers.

The above steps are just a happy path. There are many issues arises in reality.

• Scenario 1: The job on step 2 executes successfully but then the consumer is crashed. Since this unexpected circumstance, the consumer has not committed the message's offset yet. When the consumer restarts, the message will be consumed twice.
• Scenario 2: While the consumer commits the offset at step 3, it crashes due to hardware failures (e.g: CPU, memory violation, ...) When restarting, the consumer no way to know it has committed the offset successfully or not.

Because there are many problems might be happened, the job's execution and the committing offset must be atomic to guarantee exactly-once delivery semantic at the consumer side. It doesn't mean we cannot but it takes a lot of effort to make sure the exactly-once delivery semantic. Kafka Stream upholds the work for engineers.

Noted that: Kafka Stream offers "exactly-once stream processing". It refers to consuming from a topic, materializing intermediate state in a Kafka topic and producing to one. If our application depends on some other external services (database, services...), we must make sure our external dependencies can guarantee exactly-once in those cases.

TL,DR: exactly-once for the full flow needs the cooperation between producers and consumers.

References:

• Exactly-once semantics and how Apache Kafka does it
• Transactions in Apache Kafka
• Enabling exactly once Kafka streams