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Apache Camel: Content Enricher Foibles

The expressive power of Camel overcomes all challenges. See it in action as it transforms data between two applications.

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This is the third of three articles on using Apache Camel with its content enricher to handle transforming data on its way between two applications. The first article discussed Camel and its positive attributes; the second provided an example of using content enricher a straightforward way.

To recap, Apache Camel provides a wealth of components and processors to interact with applications and services in all kinds of ways, including publish-subscribe messaging, REST, SOAP, file transfers, email, and many more. Internally, it uses routes to move and transform data, greatly reducing the amount of code required to integrate diverse pieces of a system. The content enricher is one such processor that takes an incoming piece of data, uses it to retrieve extra data from some other resource (specifically some URI), and then passes the resulting enriched data on down the line.

What occasioned the writing of these articles is that the straightforward use of the content enricher did not work for our use case, for two important reasons. First, the REST web service in question provided only its own information; it didn't repeat back any data passed into it. Second, the REST web service expected a call via GET rather than POST.

Aggregation Strategy

The first of these is very easy to deal with via a feature built into the content enricher called an "aggregation strategy". When configuring the enricher, you can specify a Java class that implements AggregationStrategy. This means a single method with this signature:

public Exchange aggregate(Exchange original, Exchange resource);

This class can then combine information from the "original" exchange that was passed to the enricher and from the "resource" exchange that came back from whatever URI the enricher was configured to use.

As part of a larger example, I've created an example aggregation strategy in a GitHub repo; it looks like this:

public Exchange aggregate(Exchange original, Exchange resource) {
    OrderInfo info = (OrderInfo) original.getIn().getBody();
    OrderInfo recd = (OrderInfo) resource.getIn().getBody();
    if (original.getPattern().isOutCapable()) {
    return original;

Note that we use the "incoming" message in both cases and cast to the expected type. The other thing to note is the isOutCapable() section; most Camel routes are InOut, with the "out" message becoming the "in" message of the next step in the route. The final "out" message goes all the way back to the producer of the route if that makes sense. (For example, imagine a Camel route that starts with an incoming REST call. Camel will route the final "out" message back to the REST client.) However, some routes are InOnly, in which case it's not valid to use the "out" message. This is described in more detail here.

So now we have a way to deal with a resource that only supplies some of the data we need, and we have a way in which the content enricher is more than just syntactic sugar (as we saw in the previous article).

Building the Route

To use our aggregation strategy, we create a route that looks like this:

public void configure() throws Exception {
    from("timer://ordergen").bean(new OrderGenerator(), "generate")
            new OrderAggregationStrategy()).marshal().json(JsonLibrary.Jackson)

Note the way that we can use a "simple" expression with enrich() to configure the URL based on properties of the incoming message.

Unfortunately, this is only a partial solution and still won't work with our REST web service. We still need a way to tell Camel to make a GET call to our web service rather than POST.

In turns out this is a little more challenging, partly because of the way that Camel decides whether to use GET or POST. The algorithm is here. It boils down to either making the body of the incoming message null or setting the header Exchange.HTTP_METHOD to GET.

The second of these is obviously easier. We just need to add setHeader() to the route:

public void configure() throws Exception {
    from("timer://ordergen").bean(new OrderGenerator(), "generate")
        .setHeader(Exchange.HTTP_METHOD, "GET")
            new OrderAggregationStrategy()).marshal().json(JsonLibrary.Jackson)

But here's where we ran into an issue. It turns out that the Camel HTTP component will try to use the incoming message body as an InputStream for the HTTP client, even in cases where GET is being used. Unfortunately, in this case, the body is of type OrderInfo, and Camel doesn't know how to use that as an InputStream. So instead of success, we get an exception traceback containing this unpleasant message:

Caused by: org.apache.camel.NoTypeConversionAvailableException: No type converter available to convert from type: org.anvard.introtojava.camel.OrderInfo to the required type: java.io.InputStream with value org.anvard.introtojava.camel.OrderInfo@30140dd0

What this error message is saying is that the HTTP component is expecting an InputStream and doesn't know how to make one of those using our OrderInfo class. If our message body was something simple like a string, instead of being a POJO, Camel would have a converter available in its built-in set of type converters, and the conversion would happen invisibly. But since this is a POJO, nothing doing.

Camel allows registration of custom type converters. So we were tempted to create a type converter that just returned an empty input stream. But we decided this would be too confusing for maintainers since it would be decoupled from the point of use and the reason for needing it wouldn't be obvious.

So instead, we decided we needed to use the other method of choosing GET, which is to null out the incoming message body. But this presents some difficulties, because we need to use the body in the aggregation strategy, and because we need to use it to build the URI for the enricher.

The Solution, Finally

Fortunately, the expressive power of Camel came through for us. Here's what we wound up with:

public void configure() throws Exception {
    from("timer://ordergen").bean(new OrderGenerator(), "generate")
        .enrich("direct:enricher", new OrderAggregationStrategy()).marshal().json(JsonLibrary.Jackson)
        .setHeader(Exchange.HTTP_URI, simple("http://localhost:8680/rest/order/lookup/${body.orderNumber}"))
        .transform().simple("${null}").to("http://ignored").unmarshal().json(JsonLibrary.Jackson, OrderInfo.class);

What we've done here is split things into two separate routes. This allows us to manipulate just the exchange that's used for retrieving data from the REST web service resource without affecting the primary exchange. The flow works like this:

  • The timer fires, kicking off the first route.
  • The method OrderGenerator.generate() is called and returns an OrderInfo object with some fields populated.
  • The enricher starts. It splits the exchange in two. One side is sent to direct:enricher.

The direct endpoint in Camel implements the channel integration pattern. It creates a coupling between two routes in the same Camel context. The coupling is synchronous; it actually takes the form of a method call.

Work then proceeds in the second route:

  • A header is added to the exchange using data from the message.
  • The message body is set to null.
  • The http component is invoked. It uses the header to override the URI. Since the body is null, it uses a GET. It invokes the REST web service.
  • The result comes back in JSON form and is unmarshalled into an instance of OrderInfo.
  • The OrderInfo object comes back to the enricher.

The key insight here is that the enricher doesn't care what kind of URI is passed into it. When an http URI is used, it looks up that scheme in the context and finds the Camel HTTP component. When direct is used, it looks up that scheme instead and ends up invoking another route. Either way, it uses whatever comes back from the route as the "resource" exchange for the aggregation strategy.

Note that this also solves one more issue, which is that the REST web service returns JSON but we would like to write the aggregation strategy as just a simple combining of two POJOs. By writing a separate route for the enricher, we can perform the unmarshal() before the enricher gets the data back. We could similarly build in some transformation here if it made sense rather than doing it in the aggregation strategy.

With all that work done, the rest is simple:

  • The enricher calls the aggregation strategy with the two exchanges and uses the returned exchange.
  • The resulting object is marshalled back to JSON and sent to a log.

Running this route produces output like this:

[Camel (camel-1) thread #0 - timer://ordergen] INFO route1 - {"timestamp":1472081765263,"orderNumber":477,"customerName":"Johannes Smythe","orderTotal":2.214534914131583}
[Camel (camel-1) thread #0 - timer://ordergen] INFO route1 - {"timestamp":1472081766254,"orderNumber":742,"customerName":"Johannes Smythe","orderTotal":43.44691798994265}
[Camel (camel-1) thread #0 - timer://ordergen] INFO route1 - {"timestamp":1472081767259,"orderNumber":282,"customerName":"Johannes Smythe","orderTotal":62.38526390226096}

Wrapping Up

It would be nice if Camel ignored the incoming message body in the HTTP component when using GET. But we would still end up having to create a separate route for the enricher so we could unmarshal to Java. So, in the end, we wound up with the only solution that would have done us any good. And we got some extra practice debugging Camel routes.

I've written three articles on the topic, but the code is probably the best way to understand what's going on. So I invite you to look and ask any questions in the comments. If I missed an even better way to solve this problem, let me know that too.

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