🔑 Kafka Ordering Basics
- Ordering is guaranteed only
within a single partition.
- Kafka ensures that messages
written to a partition are read in the same order by consumers.
- Across multiple partitions,
Kafka does not guarantee global ordering.
- If you spread messages
across partitions, consumers may read them in different sequences.
🛠 How to Maintain Sequential Order
for Consumers
Here are
the key strategies:
1. Single Partition Strategy
- If strict ordering is
critical, produce all messages to one partition.
- Consumers in the same
consumer group will then read sequentially.
- ⚠️ Limitation: This reduces parallelism and
throughput since only one consumer can read from that partition at a time.
2. Partitioning by Key
- Use a partition key
(e.g., customer ID, order ID).
- Kafka ensures that all
messages with the same key go to the same partition → order is preserved per
key.
- This allows parallelism
while maintaining logical ordering for each entity.
3. Consumer Group Design
- In a consumer group, each
partition is consumed by only one consumer.
- To maintain order:
- Ensure one consumer per
partition.
- Avoid multiple consumers
reading the same partition (Kafka won’t allow it anyway).
4. Processing Guarantees
- Use idempotent producers
and exactly-once semantics (EOS) if you need strong guarantees.
- Ensure consumers process
messages synchronously (don’t reorder them in your app logic).
⚡ Example Scenario
Imagine
you’re processing bank transactions:
- If you put all transactions
for a single account into one partition (using account ID as the key),
Kafka guarantees they’ll be consumed in the correct order.
- But transactions across
different accounts may be processed in parallel (different partitions).
✅ Best Practices
- Define ordering scope: Do you need global
ordering or per-entity ordering?
- Partition wisely: Use keys to balance
throughput and ordering.
- Consumer group size: Match number of consumers
to number of partitions for efficiency.
- Avoid rebalancing chaos: Rebalancing can
temporarily disrupt order; design retry logic carefully.
👉 In short:
- Sequential order for all
messages → single partition, single consumer.
- Sequential order per
key/entity → partition by key, one consumer per partition.
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