How Does Kafka's ACK Mechanism Work?

What is the ACK Mechanism?

Think of Kafka's ACK (acknowledgment) mechanism as a delivery receipt system. When a producer sends a message, it can wait for confirmation that the message was received - just like tracking a package delivery. This helps ensure messages aren't lost in transit.

Three ACK Levels

Kafka offers three ACK levels, each striking a different balance between reliability and speed. Let's look at each one:

1. acks=0 (No Acknowledgment)

acks=0

Characteristics:

Fire-and-forget: Send and move on
Lightning-fast throughput
Messages might get lost
Best for metrics or logs where losing a few data points is okay

2. acks=1 (Leader Acknowledgment)

acks=1

Characteristics:

Waits for the leader's confirmation
Decent speed with basic reliability
Some risk if the leader crashes
Good for most everyday use cases

3. acks=-1 or all (All Acknowledgments)

acks=-1

Characteristics:

Waits for all replicas to confirm
Slower but bulletproof
Maximum reliability
Perfect for financial transactions

Configuration Example

Producer Configuration

# ACK level setting
acks=all    # or acks=0, acks=1

ACK Level Comparison and Use Cases

Feature	acks=0	acks=1	acks=all
Message Reliability	Low	Medium	High
Response Latency	Lowest	Medium	Highest
Throughput	Highest	Medium	Lowest
Message Loss Risk	Can lose messages	Only loses if leader fails	Won't lose messages
Use Cases	Metrics & Logging Performance monitoring	Regular updates Analytics streams	Payments Critical data
CPU Overhead	Lowest	Medium	Highest
Network Overhead	Lowest	Medium	Highest

Summary

Pick your ACK level based on what matters most: speed or reliability. For critical data, use acks=all. For metrics and logs where speed is key, acks=0 might be your best bet. Most applications do fine with acks=1, which hits the sweet spot between reliability and performance.