Event choreography vs orchestration in Kafka - CLI Comparison

Introduction

When multiple services need to work together, they can either coordinate themselves by listening and reacting to events, or be controlled by a central manager that tells each service what to do. Event choreography lets services talk directly by sending events, while orchestration uses a central controller to manage the flow.

When you want services to be loosely connected and react independently to events without a central controller.

When you need a central system to control the order and logic of service interactions clearly.

When building a microservices system where services publish and subscribe to Kafka topics to communicate.

When you want to avoid a single point of failure by letting services coordinate themselves.

When you want clear visibility and control over the workflow by using a central orchestrator.

Commands

Create a Kafka topic named 'order-created' where services will publish events when an order is created.

Terminal

kafka-topics --create --topic order-created --bootstrap-server localhost:9092 --partitions 1 --replication-factor 1

Expected Output

Created topic order-created.

→

--topic - Name of the Kafka topic to create

→

--partitions - Number of partitions for the topic

→

--replication-factor - Number of copies of the topic for fault tolerance

Start a producer to send an event message to the 'order-created' topic, simulating an order creation event.

Terminal

kafka-console-producer --topic order-created --bootstrap-server localhost:9092

Expected Output

No output (command runs silently)

→

--topic - Topic to send messages to

→

--bootstrap-server - Kafka server address

Consume the first message from the 'order-created' topic to verify the event was published.

Terminal

kafka-console-consumer --topic order-created --bootstrap-server localhost:9092 --from-beginning --max-messages 1

Expected Output

{"orderId": "12345", "status": "created"}

→

--from-beginning - Read messages from the start of the topic

→

--max-messages - Stop after reading specified number of messages

Create a Kafka topic 'order-processor' to simulate an orchestrator sending commands to process orders.

Terminal

kafka-topics --create --topic order-processor --bootstrap-server localhost:9092 --partitions 1 --replication-factor 1

Expected Output

Created topic order-processor.

→

--topic - Name of the Kafka topic to create

→

--partitions - Number of partitions for the topic

→

--replication-factor - Number of copies of the topic for fault tolerance

Send a command message to the 'order-processor' topic to simulate orchestration controlling the order processing.

Terminal

kafka-console-producer --topic order-processor --bootstrap-server localhost:9092

Expected Output

No output (command runs silently)

→

--topic - Topic to send messages to

→

--bootstrap-server - Kafka server address

Key Concept

If you remember nothing else from this pattern, remember: choreography lets services react to events independently, while orchestration uses a central controller to direct service actions.

Common Mistakes

Trying to use orchestration without a central controller service.

Orchestration requires a central system to send commands; without it, services cannot be coordinated properly.

Set up a dedicated orchestrator service that listens and sends commands to other services via Kafka topics.

Publishing events without consumers listening in choreography.

Events will be lost or ignored if no service subscribes to the Kafka topics, breaking the workflow.

Ensure all relevant services subscribe to the correct Kafka topics to react to events.

Summary

Create Kafka topics to represent events or commands for services to communicate.

Use producers to send messages representing events or orchestration commands.

Use consumers to listen and react to events in choreography or follow commands in orchestration.

Practice

1. What is the main difference between event choreography and event orchestration in Kafka-based systems?

easy

A. Choreography lets services react independently; orchestration uses a central controller.

B. Choreography requires a central controller; orchestration lets services react independently.

C. Both use a central controller but differ in message format.

D. Choreography is synchronous; orchestration is asynchronous.

Solution

1. Step 1: Understand event choreography

   Event choreography means each service listens and reacts to events on its own without a central manager.

2. Step 2: Understand event orchestration

   Event orchestration means a central controller tells services what to do and when, managing the flow.

3. Final Answer:

   Choreography lets services react independently; orchestration uses a central controller. -> Option A

4. Quick Check:

   Choreography = independent reaction, Orchestration = central control [OK]

Hint: Choreography = no boss; orchestration = boss controls flow [OK]

Common Mistakes:

• Confusing which uses a central controller
• Thinking both require central control
• Mixing synchronous and asynchronous concepts

2. Which Kafka component is typically used in event orchestration to coordinate services?

easy

A. Kafka Topics with independent consumers

B. A central orchestrator service producing commands

C. Kafka Connect for data integration

D. Kafka Streams for independent event processing

Solution

1. Step 1: Identify orchestration role

   Orchestration needs a central service to control and send commands to other services.

2. Step 2: Match Kafka components

   Kafka Topics and Streams are used for event flow, but orchestration requires a central orchestrator producing commands.

3. Final Answer:

   A central orchestrator service producing commands -> Option B

4. Quick Check:

   Orchestration = central orchestrator service [OK]

Hint: Orchestration needs a central command sender [OK]

Common Mistakes:

• Choosing Kafka Topics as the controller
• Confusing Kafka Connect with orchestration
• Thinking Kafka Streams alone orchestrates

3. Given this Kafka event flow in choreography style:

Service A publishes event 'OrderCreated' to topic 'orders'.
Service B listens to 'orders' and publishes 'PaymentProcessed' to topic 'payments'.
Service C listens to 'payments' and updates inventory.

What happens if Service B fails to publish 'PaymentProcessed' event?

medium

A. Service C will not update inventory because it never receives the event.

B. Service C updates inventory anyway because it listens to 'orders'.

C. Service A retries publishing 'OrderCreated' event automatically.

D. A central orchestrator will resend 'PaymentProcessed' event.

Solution

1. Step 1: Trace event flow in choreography

   Service A publishes 'OrderCreated', Service B reacts and publishes 'PaymentProcessed', Service C listens to 'payments'.

2. Step 2: Analyze failure impact

   If Service B fails to publish 'PaymentProcessed', Service C never receives that event and cannot update inventory.

3. Final Answer:

   Service C will not update inventory because it never receives the event. -> Option A

4. Quick Check:

   Missing event means downstream service does not act [OK]

Hint: In choreography, missing event stops downstream actions [OK]

Common Mistakes:

• Assuming Service C listens to 'orders' directly
• Thinking Service A retries automatically
• Expecting a central orchestrator in choreography

4. You have this orchestration code snippet controlling Kafka services:

orchestrator.sendCommand('ServiceB', 'ProcessPayment')
orchestrator.sendCommand('ServiceC', 'UpdateInventory')

But Service C updates inventory before Service B processes payment. What is the likely problem?

medium

A. Service C is ignoring commands from the orchestrator.

B. Service B is not subscribed to the correct Kafka topic.

C. Commands are sent asynchronously without waiting for Service B to finish.

D. Kafka topics are misconfigured causing message loss.

Solution

1. Step 1: Understand orchestration command flow

   Orchestrator sends commands to services in order, expecting sequential execution.

2. Step 2: Identify asynchronous issue

   If commands are sent asynchronously without waiting for Service B to complete, Service C may act too early.

3. Final Answer:

   Commands are sent asynchronously without waiting for Service B to finish. -> Option C

4. Quick Check:

   Orchestration needs ordered, awaited commands [OK]

Hint: Orchestration requires waiting for each command to finish [OK]

Common Mistakes:

• Blaming Kafka topic subscription
• Assuming Service C ignores commands
• Thinking message loss causes order issue

5. You want to design a Kafka system for an online store where payment processing must strictly happen before inventory update, but you want to avoid a central orchestrator. Which approach fits best and how to ensure order?

hard

A. Use choreography but rely on manual retries to fix order issues.

B. Use event orchestration with a central controller sending commands in order to services.

C. Use orchestration but allow services to process events independently without waiting.

D. Use event choreography with services emitting events and consumers processing in sequence using Kafka partitions and consumer groups.

Solution

1. Step 1: Identify requirement

   Payment must happen before inventory update, but no central orchestrator allowed.

2. Step 2: Choose choreography with ordering

   Event choreography fits because services react independently. To ensure order, use Kafka partitions and consumer groups to process events in sequence.

3. Final Answer:

   Use event choreography with services emitting events and consumers processing in sequence using Kafka partitions and consumer groups. -> Option D

4. Quick Check:

   Choreography + Kafka ordering = correct sequence without central control [OK]

Hint: Choreography + Kafka partitions ensure order without central control [OK]

Common Mistakes:

• Choosing orchestration despite no central controller allowed
• Ignoring Kafka partition ordering
• Relying on manual retries for order