What is the right way to implement sync and async methods in a library?

Question

I need to make a library in which I will have synchronous and asynchronous feature.

• executeSynchronous() - waits until I have a result, returns the result.
• executeAsynchronous() - returns a Future immediately which can be processed after other things are done, if needed.

Core Logic of my Library

The customer will use our library and they will call it by passing DataKey builder object. We will then construct a URL by using that DataKey object and make a HTTP client call to that URL by executing it and after we get the response back as a JSON String, we will send that JSON String back to our customer as it is by creating DataResponse object. Some customer will call executeSynchronous() and some might call executeAsynchronous() method so that's why I need to provide two method separately in my library.

Interface:

public interface Client {

    // for synchronous
    public DataResponse executeSynchronous(DataKey key);

    // for asynchronous
    public Future<DataResponse> executeAsynchronous(DataKey key);
}

And then I have my DataClient which implements the above Client interface:

public class DataClient implements Client {

    private RestTemplate restTemplate = new RestTemplate();
    private ExecutorService executor = Executors.newFixedThreadPool(10);

    // for synchronous call
    @Override
    public DataResponse executeSynchronous(DataKey key) {
        DataResponse dataResponse = null;
        Future<DataResponse> future = null;

        try {
            future = executeAsynchronous(key);
            dataResponse = future.get(key.getTimeout(), TimeUnit.MILLISECONDS);
        } catch (TimeoutException ex) {
            PotoLogging.logErrors(ex, DataErrorEnum.TIMEOUT_ON_CLIENT, key);
            dataResponse = new DataResponse(null, DataErrorEnum.TIMEOUT_ON_CLIENT, DataStatusEnum.ERROR);
        } catch (Exception ex) {
            PotoLogging.logErrors(ex, DataErrorEnum.CLIENT_ERROR, key);
            dataResponse = new DataResponse(null, DataErrorEnum.CLIENT_ERROR, DataStatusEnum.ERROR);
        }

        return dataResponse;
    }

    //for asynchronous call
    @Override
    public Future<DataResponse> executeAsynchronous(DataKey key) {
        Future<DataResponse> future = null;

        try {
            Task task = new Task(key, restTemplate);
            future = executor.submit(task); 
        } catch (Exception ex) {
            PotoLogging.logErrors(ex, DataErrorEnum.CLIENT_ERROR, key);
        }

        return future;
    }
}

Simple class which will perform the actual task:

public class Task implements Callable<DataResponse> {

    private DataKey key;
    private RestTemplate restTemplate;

    public Task(DataKey key, RestTemplate restTemplate) {
        this.key = key;
        this.restTemplate = restTemplate;
    }

    @Override
    public DataResponse call() {
        DataResponse dataResponse = null;
        String response = null;

        try {
            String url = createURL();
            response = restTemplate.getForObject(url, String.class);

            // it is a successful response
            dataResponse = new DataResponse(response, DataErrorEnum.NONE, DataStatusEnum.SUCCESS);
        } catch (RestClientException ex) {
            PotoLogging.logErrors(ex, DataErrorEnum.SERVER_DOWN, key);
            dataResponse = new DataResponse(null, DataErrorEnum.SERVER_DOWN, DataStatusEnum.ERROR);
        } catch (Exception ex) {
            PotoLogging.logErrors(ex, DataErrorEnum.CLIENT_ERROR, key);
            dataResponse = new DataResponse(null, DataErrorEnum.CLIENT_ERROR, DataStatusEnum.ERROR);
        }

        return dataResponse;
    }

    // create a URL by using key object
    private String createURL() {
        String url = somecode;
        return url;
    }
}

Customer within our company will use my library like this as shown below by using my factory in their code base -

// if they are calling `executeSynchronous()` method
DataResponse response = DataClientFactory.getInstance().executeSynchronous(dataKey);

// and if they want to call `executeAsynchronous()` method
Future<DataResponse> response = DataClientFactory.getInstance().executeAsynchronous(dataKey);

What is the best way to implement sync and async method for my library? Does implementing sync call as async + waiting is a bad idea? Because it will consume one thread from the thread pool per a call with my currrent setup? If yes, then can anyone explain why it's a bad idea and will it have any performance issue?

How will you implement sync and async method given the above criteria? What is the best way to do this? This library will be used under very heavy load and it has to be fast, meaning it should take time whatever my server is taking to respond.

Should I use AsyncRestTemplate in my code base which will be async non-blocking architecture?

Answer 1

For synchronous call, executing in a separate thread is definitely not a good idea. You are incurring extra costs and resources for a Thread along with the cost of context switch of threads in this case.

If there are lots of synchronous calls then you will be unnecessarily blocking the threads for asynchronous calls as your executor is of fixed size threads. The total throughput of the system will be less in that case.

For example: If there are 10 clients calling each of the synchronous and asynchronous calls, in your implementation only threads will be actually working. However, if you were to utilize the client threads also and not make synchronous call as asynchronous and wait then all the 20 calls will be processed at the same time.

Answer 2

If you are creating a new thread even in case of synchronous operation(when it is actually not required), it will lead to performance hit. You are basically creating new Thread(read as wasting resources) without even getting any benefits of it. That being said I think better way would be to wrap the HTTP part in a different class. That way you would be re-using the code for HTTP access in both synchronous and asynchronous case.

class HTTPAccess{
    private RestTemplate restTemplate;
    private DataKey key;

    public HTTPAccess(DataKey key,RestTemplate restTemplate){
        this.key = key;
        this.restTemplate = restTemplate;

    }


    public DataResponse performRequest() {
        DataResponse dataResponse = null;        
        try {
            String url = createURL();
            response = restTemplate.getForObject(url, String.class);

            // it is a successful response
            dataResponse = new DataResponse(response, DataErrorEnum.NONE, DataStatusEnum.SUCCESS);
        } catch (RestClientException ex) {
            PotoLogging.logErrors(ex, DataErrorEnum.SERVER_DOWN, key);
            dataResponse = new DataResponse(null, DataErrorEnum.SERVER_DOWN, DataStatusEnum.ERROR);
        } catch (Exception ex) {
            PotoLogging.logErrors(ex, DataErrorEnum.CLIENT_ERROR, key);
            dataResponse = new DataResponse(null, DataErrorEnum.CLIENT_ERROR, DataStatusEnum.ERROR);
        }

        return dataResponse;
    }

    // create a URL by using key object
    private String createURL() {
        String url = somecode;
        return url;
    }

}

Now for the Client implementation, simply use this class.

public class DataClient implements Client {

    private ExecutorService executor = Executors.newFixedThreadPool(10);
    private RestTemplate restTemplate;
    private void initRestClient(DataKey key){
    if(restTemplate == null)
        restTemplate = new RestTemplate(clientHttpRequestFactory(key));
    }

    private ClientHttpRequestFactory clientHttpRequestFactory(DataKey key) {
        HttpComponentsClientHttpRequestFactory factory = new HttpComponentsClientHttpRequestFactory();
        factory.setReadTimeout(key.getTimeout());
        factory.setConnectTimeout(key.getTimeout());
        //if you need to set otherparams this is the place we can do it extracting from DataKey obj
        return factory;
    }

    // for synchronous call
    @Override
    public DataResponse executeSynchronous(DataKey key) {
        initRestClient(key);
        DataResponse dataResponse = new HTTPAccess(key).performRequest();
        return dataResponse;
    }

    //for asynchronous call
    @Override
    public Future<DataResponse> executeAsynchronous(final DataKey key) {
        return executor.submit(new Callable<DataResponse>() {
            @Override
            public DataResponse call() throws Exception {
                return executeSynchronous(key);
            }
        });
    }
}

This way your HTTP implementation is completely separate and in future if you need to change the way to receive DataResponse (maybe from DB call ), then you have to change the HTTPAccess class only and other part will not be affected.

Answer 3

I think this is better:

@Override
public DataResponse executeSynchronous(DataKey key) {
    Task task = new Task(key, restTemplate);
    return task.call();
}

It performs the same job, is clear, shorter, and has no overhead.

Notice that his also cleans up the duplicate Exception handling you currently have.

If the timeout is a must, an option is to use the underlying timeouts for the RestTemplate class, as explained in Spring RestTemplate timeout

Then the timeout will cause a RestClientException that you or the library client can handle.