# Kafka: Replications and ISR

In **Apache Kafka**, the **replication factor** refers to the number of copies (replicas) of a topic partition that are maintained across the Kafka cluster.

**ISR** stands for **In-Sync Replicas.** It is the set of all replicas (including the leader replica) that are **in sync** with the leader of a Kafka partition.

For example, if the replication factor is set to `3`, Kafka will maintain 3 copies of the data for that partition across different brokers. (1 leader + 2 followers)

![](https://cdn.hashnode.com/res/hashnode/image/upload/v1733304343168/eb003c29-54d3-40c3-b7d8-36b2895d288b.png align="center")

One replica is designated as the **leader**. All read and write operations for the partition are handled by this leader. Other replicas are **followers**, which replicate data from the leader to stay in sync.

### Importance of ISR

**High Availability**: If the leader replica fails, one of the replicas in the ISR can take over as the new leader, ensuring no data loss or downtime.

**Durability Guarantees**: Kafka can be configured to acknowledge writes only when data is written to all replicas in the ISR (`acks=all`), which ensures stronger durability.

**Replication Lag Monitoring**: Kafka continuously monitors replicas to ensure they remain in sync. If a replica falls behind for too long, it is removed from the ISR.

### **Trade-Offs**

**Increased Fault Tolerance vs. Resource Usage:** Each replica consumes additional disk space on the brokers. For example, a replication factor of 3 triples the storage requirement for the same data.

**High Availability vs. Performance Overheads:** A higher replication factor increases the time to propagate writes across all replicas. This is especially significant when using `acks=all`, as the leader waits for acknowledgments from all in-sync replicas (ISR) before confirming the write.

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Hope this helps to get some understanding of How Replication work in Kafka. Let me know if you have any other questions.

### **Resources**

* [**kafka.js.org**](https://kafka.js.org/)
    
* [**kafka.apache.org**](https://kafka.apache.org/)