Partition ordering guarantees in Kafka - Step-by-Step Execution

Process Flow - Partition ordering guarantees

Producer sends message

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Message assigned to partition

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Partition appends message in order

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Consumer reads messages from partition

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Messages delivered in order per partition

Messages sent by a producer go to a specific partition where they are stored in order. Consumers read from partitions in that same order, ensuring ordering per partition.

Execution Sample

Kafka

producer.send(topic, key='user1', value='msg1')
producer.send(topic, key='user1', value='msg2')
consumer.poll()

Producer sends two messages with the same key to the topic; consumer reads them in the order they were sent within the same partition.

Process Table

Step	Action	Partition Selected	Message Offset	Ordering Guarantee
1	Send msg1 with key 'user1'	Partition 2	0	First message in partition 2
2	Send msg2 with key 'user1'	Partition 2	1	Second message in partition 2, after msg1
3	Consumer polls messages	Partition 2	0	Reads msg1 first
4	Consumer polls messages	Partition 2	1	Reads msg2 second
5	Consumer polls messages	-	-	No more messages, end of partition

💡 All messages read in order from partition 2; ordering guaranteed per partition

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
Partition 2 Offset	-	0 (msg1 stored)	1 (msg2 stored)	0 (msg1 read)	1 (msg2 read)	1 (all read)

Key Moments - 3 Insights

Why do messages with the same key go to the same partition?

Kafka uses the message key to determine the partition, ensuring all messages with the same key are stored in the same partition, preserving order as shown in steps 1 and 2.

Does Kafka guarantee ordering across different partitions?

No, ordering is only guaranteed within a single partition. Messages in different partitions can be processed in parallel and may arrive out of order.

What happens if a consumer reads from multiple partitions?

The consumer reads messages in order per partition, but messages from different partitions may be interleaved without global ordering.

Visual Quiz - 3 Questions

Look at the execution table, what is the offset of the second message sent with key 'user1'?

A 2

B 0

C 1

D Not assigned

💡 Hint

Check Step 2 in the execution table where msg2 is stored with offset 1

At which step does the consumer read the first message from partition 2?

A Step 1

B Step 3

C Step 4

D Step 5

💡 Hint

Look at Step 3 in the execution table where consumer polls and reads offset 0

If messages had different keys, how would the partition assignment change?

A Messages would be assigned to different partitions

B Messages would go to the same partition

C Messages would be lost

D Messages would be reordered within the same partition

💡 Hint

Kafka uses keys to assign partitions; different keys usually mean different partitions

Concept Snapshot

Kafka guarantees message order only within a single partition.
Messages with the same key go to the same partition.
Producers send messages that are appended in order.
Consumers read messages in the order stored per partition.
No ordering guarantee across different partitions.

Full Transcript

In Kafka, when a producer sends messages with the same key, those messages are assigned to the same partition. Each partition stores messages in the order they arrive, assigning them sequential offsets. Consumers read messages from partitions in offset order, preserving the order of messages per partition. This means ordering is guaranteed only within a partition, not across partitions. The execution table shows two messages sent with the same key going to partition 2 with offsets 0 and 1. The consumer reads them in that order. If messages have different keys, they may be assigned to different partitions, and ordering is not guaranteed across those partitions.