In-sync replicas (ISR) in Kafka - Step-by-Step Execution

Process Flow - In-sync replicas (ISR)

Leader receives write request

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Leader writes to local log

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Leader sends data to followers

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Followers append data to their logs

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Followers send acknowledgment

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Leader updates ISR list

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Leader commits message if ISR quorum

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Next write request

This flow shows how the leader and followers keep data in sync by exchanging writes and acknowledgments to maintain the ISR list.

Execution Sample

Kafka

Leader receives write
Leader writes locally
Leader sends to followers
Followers append and ack
Leader updates ISR
Leader commits message

Shows the step-by-step process of how replicas stay in sync and how the ISR list is updated.

Process Table

Step	Action	Leader Log	Follower Logs	ISR List	Commit Status
1	Leader receives write request	[]	[]	[]	No
2	Leader writes data locally	[msg1]	[]	[]	No
3	Leader sends data to followers	[msg1]	[]	[]	No
4	Followers append data and ack	[msg1]	[msg1]	[]	No
5	Leader updates ISR list	[msg1]	[msg1]	[Leader, Follower1]	No
6	Leader commits message (quorum ack)	[msg1]	[msg1]	[Leader, Follower1]	Yes
7	Next write request arrives	[msg1]	[msg1]	[Leader, Follower1]	Yes

💡 Process stops here waiting for next write or follower lag

Status Tracker

Variable	Start	After Step 2	After Step 4	After Step 5	After Step 6
Leader Log	[]	[msg1]	[msg1]	[msg1]	[msg1]
Follower Logs	[]	[]	[msg1]	[msg1]	[msg1]
ISR List	[]	[]	[]	[Leader, Follower1]	[Leader, Follower1]
Commit Status	No	No	No	No	Yes

Key Moments - 2 Insights

Why does the ISR list update only after followers acknowledge?

Because the ISR list includes only replicas that have confirmed they have the latest data, as shown in step 5 where ISR updates after follower acks in step 4.

What happens if a follower does not acknowledge in time?

The follower is removed from the ISR list, so the leader will not wait for its ack to commit messages, ensuring availability (not shown in this trace but implied after step 5).

Visual Quiz - 3 Questions

Look at the execution table, at which step does the leader commit the message?

A Step 4

B Step 6

C Step 5

D Step 7

💡 Hint

Check the 'Commit Status' column in the execution table where it changes to 'Yes'.

At which step do followers append the message to their logs?

A Step 4

B Step 2

C Step 3

D Step 5

💡 Hint

Look at the 'Follower Logs' column where it changes from empty to '[msg1]'.

If a follower fails to acknowledge, how would the ISR list change after step 5?

A ISR list would be empty

B ISR list would include all replicas

C ISR list would exclude the non-acknowledging follower

D ISR list would not change

💡 Hint

Refer to the key moment about follower acknowledgments affecting ISR list.

Concept Snapshot

In-sync replicas (ISR) track replicas that have fully caught up with the leader.
Leader writes data locally, sends to followers.
Followers append data and send acknowledgments.
Leader updates ISR list with replicas that acked.
Leader commits messages once ISR quorum is reached.
ISR ensures data durability and availability.

Full Transcript

This visual trace shows how Kafka's In-sync replicas (ISR) mechanism works. The leader receives a write request and writes the data to its local log. Then it sends the data to follower replicas. Followers append the data to their logs and send acknowledgments back to the leader. Once the leader receives acknowledgments from followers, it updates the ISR list to include those replicas. When a quorum of ISR members have acknowledged, the leader commits the message, making it durable. If a follower does not acknowledge in time, it is removed from the ISR list to maintain availability. This process repeats for each new write request, ensuring data consistency and fault tolerance.