What does In-Sync Replicas in Apache Kafka Really Mean?

Written by Lovisa Johansson

Kafka considers that a record is committed when all replicas in the In-Sync Replica set (ISR) have confirmed that they have written the record to disk. The acks=all setting requests that an ack is sent once all in-sync replicas (ISR) have the record. But what is the ISR and what is it for?

Let's start this article by refreshing the memory about ACK values in Apache Kafka, and how to set them.

Apache Kafka ack-value

An acknowledgment (ACK) is a signal passed between communicating processes to signify acknowledgment, i.e., receipt of the message sent. The ack-value is a producer configuration parameter in Apache Kafka and can be set to the following values:

  • acks=0 The producer never waits for an ack from the broker when the ack value is set to 0. No guarantee can be made that the broker has received the message. The producer doesn’t try to send the record again since the producer never knows that the record was lost. This setting provides lower latency and higher throughput at the cost of much higher risk of message loss.
  • acks=1 When setting the ack value to 1, the producer gets an ack after the leader has received the record. The leader will write the record to its log but will respond without awaiting a full acknowledgment from all followers. The message will be lost only if the leader fails immediately after acknowledging the record, but before the followers have replicated it. This setting is the middle ground for latency, throughput, and durability. It is slower but more durable than acks=0.
  • acks=all Setting the ack value to all means that the producer gets an ack when all in-sync replicas have received the record. The leader will wait for the full set of in-sync replicas to acknowledge the record. This means that it takes a longer time to send a message with ack value all, but it gives the strongest message durability.

How to set the Apache Kafka ack-value

For the highest throughput set the value to 0. For no data loss, set the ack-value to all (or -1). For high, but not maximum durability and for high but not maximum throughput - set the ack-value to 1. Ack-value 1 can be seen as an intermediate between both of the above.

In-Sync Replica

Time to address; what is the ISR and what is it for?

What is the ISR?

The ISR is simply all the replicas of a partition that are "in-sync" with the leader. The definition of "in-sync" depends on the topic configuration, but by default, it means that a replica is or has been fully caught up with the leader in the last 10 seconds. The setting for this time period is: replica.lag.time.max.ms and has a server default which can be overridden on a per topic basis.

At a minimum the, ISR will consist of the leader replica and any additional follower replicas that are also considered in-sync. Followers replicate data from the leader to themselves by sending Fetch Requests periodically, by default every 500ms.

If a follower fails, then it will cease sending fetch requests and after the default, 10 seconds will be removed from the ISR. Likewise, if a follower slows down, perhaps a network related issue or constrained server resources, then as soon as it has been lagging behind the leader for more than 10 seconds it is removed from the ISR.

What is ISR for?

The ISR acts as a tradeoff between safety and latency. As a producer, if we really didn't want to lose a message, we'd make sure that the message has been replicated to all replicas before receiving an acknowledgment. But this is problematic as the loss or slowdown of a single replica could cause a partition to become unavailable or add extremely high latencies. So the goal to be able to tolerate one or more replicas being lost or being very slow.

When a producer uses the "all" value for the acks setting. It is saying: only give me an acknowledgment once all in-sync replicas have the message. If a replica has failed or is being really slow, it will not be part of the ISR and will not cause unavailability or high latency, and we still, normally, get redundancy of our message.

So the ISR exists to balance safety with availability and latency. But it does have one surprising Achilles heel. If all followers are going slow, then the ISR might only consist of the leader. So an acks=all message might get acknowledged when only a single replica (the leader) has it. This leaves the message vulnerable to being lost. This is where the min-insync.replicas broker/topic configuration helps. If it is set it to 2 for example, then if the ISR does shrink to one replica, then the incoming messages are rejected. It acts as a safety measure for when we care deeply about avoiding message loss.

Minimum In-Sync Replica

The minimum number of in-sync replicas specify how many replicas that are needed to be available for the producer to successfully send records to a partition. The number of replicas in your topic is specified by you when creating the topic. The number of replicas specified can be changed in the future.

A high number of minimum in-sync replicas gives a higher persistence, but on the other hand, might reduce availability because the minimum number of replicas given must be available before a publish. If you have a 3 node cluster and the minimum in-sync replicas are set to 3, and one node goes down, the other two nodes will not able to receive any data. Only care about the minimum number of in-sync replicas when it comes to the availability of your cluster and reliability guarantees.

The minimum number of in-sync replicas has nothing to do with the throughput. Setting the minimum number of in-sync replicas to larger than 1 may ensure less or no data loss, but throughput varies depending on the ack value configuration.

Default minimum in-sync replicas are set to 1 by default in CloudKarafka. This means that the minimum number of in-sync replicas that must be available for the producer to successfully send records to a partition must be 1.

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