play framework parallel WSClient calls error management - asynchronous

Question

I have an action in which I make three parallel HTTP calls (to other services), then I merge the contents of the responses into one document and finally I send it back to the client.
This is a working sample of the code:
#Inject
WSClient wsc;
public CompletionStage<Result> getUrlData() throws Exception {
List<CompletionStage<WSResponse>> stages = new ArrayList<>();
stages.add(wsc.url("http://jsonplaceholder.typicode.com/posts/1").get());
stages.add(wsc.url("http://jsonplaceholder.typicode.com/posts/2").get());
stages.add(wsc.url("http://jsonplaceholder.typicode.com/posts/3").get());
return Futures
.sequence(stages)
.thenApply(responses -> {
StringBuilder builder = new StringBuilder("[");
responses.stream().forEach(response -> builder.append(response.getBody()).append(","));
builder.deleteCharAt(builder.length()-1).append("]");
return ok(builder.toString());
})
.exceptionally(ex -> ok("{\"error\": \"An error has occurred\"}"));
If one of services is not available (you can simulate this behavior modifying the domain name of one of the URLs to a non existing one), the page returned contains only the message contained in the exceptionally() part, while I need to return the contents of the correct calls plus the error message of the not succeeded call. Any hint on how to do it?
I'm using Play 2.5.1.
Thanks,
Andrea

Answer 1

Basically you just want to handle the .exceptionally(..) individually for each call. Something like this should work:
create a function that returns a CompletionStage for an individual URL, incorporating your error handling (returning JSON of the error)
convert that to a list of completion stages to pass to Futures.sequence
As an aside, you can make the JSON manipulation a bit nicer by building the objects programatically using Jackson's ObjectMapper.createObjectNode() and ObjectMapper.createArrayNode():
private static final ObjectMapper mapper = new ObjectMapper();
private CompletionStage<JsonNode> getDataFromUrl(String url) {
return wsc.url(url)
.get()
.thenApply(WSResponse::asJson)
.exceptionally(ex -> {
ObjectNode error = mapper.createObjectNode();
error.put("error", ex.getMessage());
return error;
});
}
public CompletionStage<Result> getUrlData() throws Exception {
List<String> urls = new ArrayList<>();
urls.add("http://jsonplaceholder.typicode.com/posts/1");
urls.add("http://jsonplaceholder.typicode.com/posts/2");
urls.add("http://jsonplaceholder.typicode.com/posts/3");
// Convert to a list of promises
List<CompletionStage<JsonNode>> stages = urls
.stream()
.map(this::getDataFromUrl)
.collect(Collectors.toList());
return Futures
.sequence(stages)
.thenApply(responses -> {
ArrayNode arrayNode = mapper.createArrayNode();
responses.stream().forEach(arrayNode::add);
return ok(arrayNode);
});
}