Testing Event-Driven Architecture
Tags: eventdriven, kafka, testing, microservices
Published: 2023-06-05 10:55:09

Event-driven architecture (EDA) is a software design pattern that emphasises the production, detection, consumption, and reaction to events that occur within a system or across multiple systems.


_It is a style of architecture where the flow of data and the triggering of actions are driven by events._



![Image description](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/43hobbq356epts5ut2he.jpg)


Event-driven architecture offers several benefits like higher scalability, modularity and resilience due to the loosely coupled services compared to Request-Driven Architecture.


![Image description](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/6mmrfzexskew7geptftb.jpeg)
*Event-Driven vs Request-Driven Architecture*


However, it introduces some unique testing and monitoring challenges compared to traditional architectures:

1. **Event Ordering and Consistency:** Events in an event-driven architecture may be processed asynchronously and can arrive out of order, and might be lost or duplicated. Testing and monitoring should include scenarios to validate how the system handles these situations

2. **Event-driven Workflow Validation:** In event-driven systems, workflows are often driven by events. Coordinating the behaviour of different services, validating event propagation, and ensuring correct data transformation across different workflows make End-to-End and Integration Testing much harder

3. **Event Payload and Schema Evolution:** Events may evolve over time, with changes to their payload structure or schema, making it hard to ensure the compatibility of events across different versions

4. **Testing Event-Triggered Services:** Event-driven architectures often involve services that are triggered by specific events. Generating and managing event simulations can be challenging, especially when dealing with a large number of events and complex workflows.


A great way to test these Event-driven systems is to use a record-replay strategy. Recording and replaying events allows you to capture real or simulated events and later replay them during testing to verify the behaviour and correctness of the system. Here’s an overview of the steps involved in recording and replaying events:

**Event Recording**: Identify the events that need to be recorded for testing purposes. These can be actual events from a live system or simulated events generated specifically for testing and involves setting up a mechanism to capture and store the events in a suitable format for retrieval. This can be achieved by creating a dummy consumer for the topic we want to replay, which can store events in persistent storage like DB

```
import kafka

# yield events from start_time to end_time

def record_events(self, start_time, end_time):
  consumer = kafka.KafkaConsumer(**(self._configs))
  
  #find offset for each partition which is just after start time
  
  seek_points = find_seek_points(start_time)
     
        try:
            # Set up a the consumer to fetch all desired partitions from their seek points
            
            for tp, offset in six.iteritems(seek_points):
                consumer.seek(tp, offset)
            
            while len(partitions) > 0:
                record = next(consumer)
                if not record:
                    partitions = set()
                else:
                    last_timestamp = record.timestamp
                    if last_timestamp > end_time:
                        partitions.discard(record.partition)
                        tp = kafka.TopicPartition(topic=record.topic, partition=record.partition)
                        if tp not in consumer.paused():
                            consumer.pause(tp)
                    elif (record.partition in partitions and last_timestamp >= start_time
                          and last_timestamp <= end_time):
                        # Send the record to the client if it's within the specified time range
                        yield record
```

Source: https://github.com/SiftScience/python-kafka-replayer

**Event Replaying**: Set up the test environment with a message queue and services configured. Add a dummy producer which reads the events from DB and adds them to the queue.

```
from kafka import KafkaProducer

#read events from DB
events = recordedEvents.stream()

#send next event 
def generate_message():
    return events.next()

# Kafka Producer
producer = KafkaProducer(
    bootstrap_servers=['localhost:9092']
)
if __name__ == '__main__':
    # Infinite loop - runs until you kill the program
    while True:
        recorded_event = generate_message()     
        producer.send('messages', dummy_message)
```


You can extend this simple setup by — adding delays, event duplication, and data corruption to analyse the system’s performance, resilience, and behaviour under different event scenarios. By recording and replaying events, you can simulate real-world scenarios, edge cases, or failure conditions, providing a comprehensive testing approach for event-driven architecture. It helps uncover bugs, validate event processing, and ensure the system functions as intended when exposed to various event sequences and scenarios.

If you are interested in this and want a managed solution without the effort of setting up a test environment, DB storage for events, and logic for creating different scenarios talk to us at [CodeParrot](https://www.codeparrot.ai/).

CodeParrot uses tracing behind the scenes to record the events coming to your services and replays them on CI to highlight all differences through an easy-to-use dashboard and it’s free for individuals and small teams.