This post is courtesy of Dirk Fröhner, Principal Solutions Architect.

The first blog in this series introduces asynchronous messaging for building loosely coupled systems that can scale, operate, and evolve individually. It considers messaging as a communications model for microservices architectures. Part 2 dives into fan-out strategies and applies the respective patterns to a concrete use case.

In this post, I look at how to apply messaging patterns to help coordinate distributed requests and responses. Specifically, I focus on a composite pattern called scatter-gather, as presented in the book “Enterprise Integration Patterns: Designing, Building, and Deploying Messaging Solutions” (Hohpe and Woolf, 2004).

I also show how a client can communicate with a backend via synchronous REST API operations while asynchronous messaging is applied internally for processing.

Overview

The use case is for Wild Rydes, a fictional application that replaces traditional taxis with unicorns. It’s used in several hands-on AWS workshops that illustrate serverless development concepts.

Wild Rydes wants to allow customers to initiate requests for quotation (RFQs) for their rides. This allows unicorns to make special offers to potential customers within a defined schedule. A customer can send their ride details and ask for quotations from all unicorns that are within a certain vicinity. The customer can then choose the best offer.

Wild Rydes

The scatter-gather pattern

The scatter-gather pattern can be used to implement this use-case on the server side. This pattern is ideal for requesting responses from multiple parties, then aggregating and processing that data.

As presented by Hohpe and Woolf, the scatter-gather pattern is a composite pattern that illustrates how to “broadcast a message to multiple recipients and re-aggregate the responses back into a single message”. The pattern is illustrated in the following diagram.

Scatter-gather architecture

The flow starts with the Requester to initiate the broadcast to all potential Responders. This can be architected in a loosely coupled manner using pub-sub messaging with Amazon SNS or Amazon MQ, as shown in this blog post.

All responders must send their answers somewhere for aggregation and processing. This can also be architected in a loosely coupled manner using a message queue with Amazon SQS or Amazon MQ, as described in this blog post.

The Aggregator component consumes the individual responses from the response queue. It forwards the aggregate to the Processor component for final processing. Both Aggregator and Processor can be part of the same application or process. If separated, they can be decoupled through messaging. The Requester can also be part of the same application or process as Aggregator and Processor.

Explaining the architecture and API

In this section, I walk through the use-case and explain how it can be architected and implemented. I show how the scatter-gather pattern works in the backend, and the client-to-backend communication.

Submit instant ride RFQ

To initiate such an RFQ, the customer app communicates with the ride booking service on the backend. The ride booking service exposes a REST API. By default, an RFQ runs for five minutes, but Wild Rydes is working on a feature to let a customer individually set that value.

A request to submit an instant-ride RFQ contains start and destination locations for the ride and the customer ID:

POST /<submit-instant-ride-rfq-resource-path> HTTP/1.1
... { "from": "...", "to": "...", "customer": "..."
}

The RFQ is a lengthy process so the client app should not expect an immediate response. Instead, the API accepts the RFQ, creates an RFQ task resource, and returns to the client. The response contains a URL to request an update for the status. It also provides an estimated time for the end of the RFQ:

HTTP/1.1 202 Accepted
... { "links": { "self": "http://.../<rfq-task-resource-path>", "...": "..." }, "status": "running", "eta": "..."
}

The following architecture shows this interaction, excluding the process after a new RFQ is submitted.

Client app interaction

Processing the RFQ

The backend uses the scatter-gather pattern to publish the RFQ to unicorns and collect responses for aggregation and processing.

Backend architecture

1. The ride booking service acts as the requester in the scatter-gather pattern. Following a new RFQ from the client app, it publishes the details into an SNS topic. This topic is related to the location of the ride’s starting point since customers need quotes from unicorns within the vicinity. These messages are the green request messages.

2. The unicorn management service maintains instances of unicorn management resources and subscribes them to RFQ topics related to their current location. These resources receive the RFQ request messages and handle the interaction with the Wild Rydes unicorn app.

3. The unicorns in the vicinity are notified through the Wild Rydes unicorn app about the new RFQ and can react if they are available. Notification options between the unicorn management service and the Wild Rydes unicorn app include push notifications and web sockets.

4. Every addressed unicorn can now submit their quote. All quotes go back through the unicorn management resources and the unicorn management service into the RFQ response queue. They act as the responders in the sense of the scatter-gather pattern.

5. The ride booking service also acts as aggregator and processor in the sense of the scatter-gather pattern. It uses SQS to consume messages from an RFQ response queue that eventually contains the RFQ responses from the involved unicorns. It starts doing so immediately after it publishes the details of a new RFQ into the RFQ topic. The messages from the RFQ response queue relate to the blue response messages.

The ride booking service consumes all incoming responses from that queue. This continues until the deadline or all participating unicorns have answered, whatever occurs first. The aggregator responsibility can be as simple as persisting the details of each incoming RFQ response into an Amazon DynamoDB table.

To match incoming responses to the right RFQ, it uses a fundamental integration pattern, correlation ID. In this pattern, a requester adds a unique ID to an outgoing message and each responder is asked to forward this ID in their response.

Also, responders must know where to send their responses to. To keep this dynamic, there is another fundamental integration pattern: return address. It suggests that a requester adds meta information into outgoing messages that indicate the address for their responses. In this architecture, this is the ARN of the SQS queue that acts as the RFQ response queue. This supports an option to simplify the response management: the RFQ response queue is a dedicated queue per customer.

Lastly, the processor responsibility in the ride booking service reads the RFQ responses from the DynamoDB table. It converts the data to JSON for the Wild Rydes customer app.

Check RFQ status

During the RFQ processing, a customer may want to know how many responses have already arrived, or if the results are already available. After submitting an instant ride RFQ, the client receives a representation of the running task. It can use the self-link to request an update:

GET /<rfq-task-resource-path> HTTP/1.1

While the task is running, a response from the ride booking service comes back with the respective status value and the count of responses that have already arrived:

HTTP/1.1 200 OK
... { "links": { "self": "http://.../<rfq-task-resource-path>", "...": "..." }, "status": "running", "responses-received": 2, "eta": "..."
}

After the RFQ is completed

An RFQ is completed if either the time is up or all unicorns have answered. The result of the RFQ is then available to the customer. If the client requests an update to the task representation, the response indicates this by redirecting to the RFQ result:

HTTP/1.1 303 See Other
Location: <url-of-rfq-result-resource>

Requesting a representation of the results resource, the client receives the quotes of all the participating unicorns. The frontend customer app can visualize these accordingly:

HTTP/1.1 200 OK
... { "links": { ... }, "from": "...", "to": "...", "customer": "...", "quotes": [ ... ]
}

The ride booking service can also use means of active notifications to make the customer app aware once the RFQ result is ready, including the link to the RFQ result. Examples for this include push notifications and web sockets.

Conclusion

In this blog, I present the scatter-gather pattern, which is a composite pattern based on pub-sub and point-to-point messaging channels. It also employs correlation ID and return address. I show how this is implemented in the Wild Rydes example application. You can use this integration pattern for communication in your microservices.

I cover how synchronous API communication between end user client and backend can work along with asynchronous messaging for request processing internally.

To learn more:

For more serverless learning resources, visit https://serverlessland.com.