AWS Compute Blog

Building ad-hoc consumers for event-driven architectures

This post is written by Corneliu Croitoru (Media Streaming and Edge Architect) and Benjamin Smith (Principal Developer Advocate, Serverless)

In January 2022, the Serverless Developer Advocate team launched Serverlesspresso Extensions, a program that lets you contribute to Serverlesspresso. This is a multi-tenant event-driven application for a pop-up coffee bar that allows you to order from your phone. In 2022, Serverlesspresso processed over 20,000 orders at technology events around the world. The goal of Serverlesspresso extensions is to showcase the power and simplicity of evolving an event-driven application.

Event-driven architecture is a design pattern that allows developers to create and evolve applications by responding to events generated by various parts of the system. For modern applications, the need for flexible and scalable approaches is critical, and event-driven architecture can provide a powerful solution.

This blog post shows how to build and deploy an extension to an event-driven application. It describes the benefits and challenges of evolving event-driven applications. It also walks through a real-life example that was created in under 24 hours.

Decoupled integrations

A key benefit of event-driven architecture is its ability to decouple different parts of the system, making it easier to manage changes and evolve the application. In traditional, monolithic applications, changes to one part of the system can affect the entire application.

With event-driven architecture, you can change individual parts of the system without affecting the rest of the application. Event-driven architecture also makes it easier to integrate new functionality into an existing application by creating new event handlers to respond to existing events. This way, you can add new functionality without affecting the existing system, making it easier to test and deploy.

The following diagrams illustrate how to add and remove consumers and producers without affecting the core application.

Adding and removing event-driven extensions

Extension 2 is consuming events from the event bus and emitting events back onto the bus. It can be added to the core application without creating any dependencies. When extension 2 is removed, the core application remains unchanged.

In monolithic applications, additional features can create dependencies on the core application. Removing those features keeps those dependencies in place, making it more complex to remove them.

Adding and removing monolithic extensions


In a traditional monolithic application, it can be difficult to collaborate with multiple developers on a single code base. It can lead to conflicts, bugs, and other issues that must be resolved. Integrating new features and components into these applications can be challenging, especially when multiple developers are using different technologies. Deploying updates can also be complex when multiple developers are involved and different parts of the application must be updated simultaneously.

With event-based applications, these challenges are often less significant. A well-designed consumer contains well-defined permissions boundaries. Its resources should not need permission to interact with resources outside the extension definition. This means you can deploy and delete them independently of other extensions and of the core application. This makes it easier to collaborate with multiple developers across different languages, runtimes, and deployment frameworks.

Near real-time feedback

Another characteristic of event-driven architecture is the ability to provide real-time feedback to users. This is because consumers can process events as they occur, making it possible to provide immediate feedback. This can be useful in applications that handle high volumes of data or interact with multiple users, as they can provide real-time updates and ensure that the application remains responsive.

An alternative approach for near real-time feedback is to use batching. This involves grouping multiple events or data points into a batch and processing them. Choosing between batching and processing data in real-time with events depends on the amount of data being processed, the latency requirements, and the complexity of the processing logic. Batching can be more efficient for large volumes of data as it reduces the overhead of processing each event individually, while processing data with events can be better suited for real-time applications that require low latency.

The newest Serverlesspresso extension uses an event-driven approach to gain real-time insight into the application.

The average wait time extension

A new extension was created by Corneliu Croitoru that calculates the average wait for each drink at the Serverlesspresso coffee bar. This extension uses AWS Step Functions, DynamoDB, and AWS Lambda. The app displays the results in near real-time, allowing customers to see how long they may need to wait for their order. The extension uses the AWS Cloud Development Kit (CDK) for deployment.

The extension uses the existing Amazon EventBridge event bus to start a Step Functions workflow. The workflow is triggered by the order submission and order completion events and calculates the average wait time for each type of drink (for example, Caffe Latte). This information is then sent back to the Serverlesspresso event bus.

The following diagram illustrates the Step Functions workflow:

When a new order submission event is emitted, the Step Functions workflow persists the event timestamp to a DynamoDB table, a key/value data store. It uses the unique order ID as the key. When an order completion event is emitted, the workflow persists the completion timestamp to DynamoDB. The workflow then invokes a Lambda function to calculate the average duration of that specific drink by using the last 10 orders stored in the DynamoDB table.

This is the DynamoDB table structure:

The workflow sends an event to the Serverlesspresso event bus with the calculated duration and drink type. A rule on the event bus routes this event to an IoT topic, which publishes it to the front-end application via an existing open WebSocket connection. The result appears on the front end:

Alternative approaches

There are a number of alternative approaches that you could use to build a real-time “average wait” extension without using events.

One such approach might be to use DynamoDB as a cache for the event-driven data. This way it would be possible to query the database periodically to check for updates. This approach can be implemented by adding a timestamp field to the database records and querying for records that have been updated since the last time you checked.

Alternatively, you could use DynamoDB streams to capture changes as they occur instead of subscribing to new events directly. However, these approaches may face several challenges. The extension would require permission to read data from the DynamoDB table or stream. Since the DynamoDB table resource is defined in the application’s core template, this presents challenges of ownership, permissions boundaries and dependencies. It adds additional complexity to the application as the extension would not be decoupled from the core.

The challenges

The biggest challenge in building this extension is the required shift in developer mindset. Despite understanding the principles of decoupled event-driven architecture, it was not until building an event-driven architecture extension that the concept became clear.

For example, you may think it necessary to deploy the existing application to submit orders, emit events onto the application event bus, and interact with various core resources. The development team had discussions about the degree to which the extension should interact with existing application components. This was not an event-driven mindset.

Each new extension must be based entirely on events. This means it can only interact with the core application through the shared event bus by consuming and emitting events. It also means that you could write the extension in any runtime, with any infrastructure as code (IaC) framework, and that it should be possible to deploy and destroy the extension stack with no effect on the core application.

Once you understand this, the next challenge is discoverability. Finding the right events to consume may prove harder than expected. This is why documenting events as you build your application is important. The event schema, producer and consumer should be documented, and evolve with each version of the event. The Serverlesspresso Events Catalog helps to overcome this in this example.

Finally, the event player can emit realistic Serverlesspresso events onto the event bus. This replaces the need to deploy the core application stack.


The Serverlesspresso Extensions program shows the simplicity of developing event-driven applications. Building event-driven architectures allows for decoupled integrations, making it easier to manage changes and develop the application. It also simplifies collaboration among multiple teams as consumers of events can come and go independently without affecting the procedure or core application.

Using these principles, the average wait time extension was built and deployed within 24 hours, using a different IaC framework to the core application.

Use the Serverlesspresso extensions GitHub repository to read how to build more Serverlesspresso extensions.

For more serverless learning resources, visit Serverless Land.