Case Study: HTTP Request Function and Processor

Engineering | David Turanski | August 17, 2020 | ...


We began this series by introducing the new stream applications based on Java functions, and function composition. The previous entry presented a tutorial for building a simple stream application and running it in Spring Cloud Data Flow. Today we explore the HTTP Request Function and present examples of how to use it.

In case you missed it, the prior posts in this series are:

The HTTP Request Function

This is an updated implementation of the legacy HTTP Client Processor Stream App Starter, based on the reactive Spring WebClient. The function is an all purpose web client that submits HTTP requests to a URL and returns the response. Designed primarily for streaming applications, it is able to extract the URL, HTTP method, request body, desired response type, and contents, using configured SpEL expressions evaluated against each incoming Message. Also, to support efficient stream processing, the function uses reactive streams. Its signature is:

Function<Flux<Message>, Flux>

That is, it accepts a Flux (stream) of Messages and returns a Flux of any type.

Configuration Properties

The HttpRequestFunction is configured through the following configuration properties:

A SpEL expression to derive the request body from the incoming message. (Expression, default: )

The type used to interpret the response. (Class<?>, default: String)

A SpEL expression used to derive the http headers map to use. (Expression, default: )

A SpEL expression to derive the request method from the incoming message. (Expression, default: GET)

Maximum buffer size in bytes allocated for input stream buffers. Defaults to 256k. Increase, as necessary, for posting or getting large binary content. (Integer, default: 256 * 1024)

A SpEL expression used to compute the final result, applied against the whole http {@link org.springframework.http.ResponseEntity}. (Expression, default: ResponseEntity::getBody)

Request timeout in milliseconds. (Long, default: 30000)

A SpEL expression against the incoming message to determine the URL to use. (Expression, default: )

The SpEL expressions are applied to the incoming Message. So fields like body and headers[name] can be used to evaluate message contents. I say "can be…" because sometimes it is more desirable to use static values. In this case, literal values must be enclosed in single quotes, for example:

http.request.url-expression='' http.request.http-method-expression='POST'

Example 1: Using the HTTP Request Function in a Standalone Application

Let’s look at an example of how to use this function in a simple Spring Boot web application. In this example, we will use it in an app that retrieves an image from a URL and renders a thumbnail of the image. The complete code for this example is here.

We will build the application using Spring Boot and Spring Web Flux, along with our function to retrieve the image, and some code to generate a thumbnail.

The relevant dependencies are:

  • - The HTTP request function transitively includes spring-boot-starter-webflux

  • io.spring.example:image-thumbnail-processor - A simple Java function, included in this example, that creates thumbnails. We won’t get into the details here, just note that it is a separate component which we will reuse in a later example.

We first need to set some configuration properties for our function:

http.request.url-expression=payload http.request.expected-response-type=byte[] http.request.maximum-buffer-size=2097152

Thus, the message payload contains the target URL, the image(response body) will be returned as a byte array. And since these images might be fairly large, we will increase the size of the buffer holding the response body to 2GB (2 * 1024 * 1024).

Here is the code:

public class ThumbnailStandaloneApplication {
  private static Logger logger = LoggerFactory.getLogger(ThumbnailStandaloneApplication.class);

  public static void main(String[] args) {, args);

  private ThumbnailProcessor thumbnailProcessor = new ThumbnailProcessor();

  private HttpRequestFunction httpRequestFunction;

  RouterFunction<?> routes() {
    return RouterFunctions.route()
        .GET("/thumbnail", this::createThumbnail)

  private Mono<ServerResponse> createThumbnail(ServerRequest serverRequest) {
    String url = serverRequest.queryParam("url").orElseThrow(
                           () -> new RuntimeException("URL required"));

    return Mono.from(httpRequestFunction.apply(Flux.just(new GenericMessage<>(url)))
        .flatMap(image -> {
          Map<String, Object> model = new HashMap<>();
          byte[] thumbnail = thumbnailProcessor.apply((byte[]) image);
"creating thumbnail for {}", url);
          model.put("url", url);
          model.put("thumb", new String(Base64.getEncoder().encode(thumbnail)));
          Mono<ServerResponse> serverResponse = ServerResponse.ok()
              .render("thumbnail", model);
          return serverResponse;

We apply the HttpRequestFunction to retrieve the image. Then we apply the thumbnailProcessor to the returned byte array and encode it to base 64 so we can render it on the page.


Example 2: Using the HTTP Request Processor in a streaming application

Now that we know how our function works, let’s put together a streaming application, using Spring Cloud Stream, to do something similar. In this case, we will use the pre-packaged HTTP Request Processor and File Source stream applications. This processor wraps the HTTP request function in a Spring Cloud Stream processor application that simply invokes the function, binding the input and output to a message broker destination (a Kafka topic, or a Rabbit MQ exchange, for example). Our application, expressed in stream definition DSL, looks like:

file-source | http-request-processor | image-thumbnail-sink

where the | represents I/O using a message broker.

Here, we are using a user-developed sink that uses the file-consumer function to write each thumbnail to a file. The sink uses Spring Cloud Function’s declarative composition to compose the thumbnail-processor, from the previous example, with a header enricher, and finally the standard fileConsumer. So our composed function is defined by:|filenameEnricher|fileConsumer


Our composite function definition is conceptually and syntactically similar to the above stream definition. But in this case the | represents in-process communication.

We will explore the ins and outs of the File Source in a future post. For now, we will use it to poll a source directory and produce messages whenever a new file is added to the directory. In this case, we want to process a text file with an image URL per line. We will configure the source to produce a message per line, containing the URL in the payload. We already know what the HTTP request processor does. The sink generates a thumbnail and writes it to a file.

The fully configured stream definition is:

file-source --file.consumer.mode=lines --file.consumer.mode=lines | http-request-processor --http.request.url-expression=payload --http.request.expected-response-type=byte[] --http.request.maximum-buffer-size=2097152| image-thumbnail-sink

If we run this and drop a text file into the source directory, we will see the thumbnails written to the target directory:

thumbnail files

If you want to run this on your local machine, complete instructions are here.


We just did a deep dive on the HTTP Request Function, demonstrating how to use it in a standalone web application and in a streaming pipeline to process images. We also used function composition, composing user-written and out of the box functions, to great effect.

Stay Tuned…​

In the coming weeks we will present many more case studies for Spring Cloud Stream and Spring Cloud Data Flow, each will highlight different stream applications and capabilities.

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