The main goal of this project is to explore basic features of @Async spring annotation and compare results with non-concurrent approach.

References: https://spring.io/guides/gs/async-method/
References: http://www.baeldung.com/spring-async

This project shows how to create asynchronous queries using Spring.

Our approach is to run expensive jobs in the background and wait for the results using Java’s CompletableFuture interface.

1. Enable asynchronous support:

   @Configuration
   @EnableAsync
   class AsyncConfig {
       @Bean
       Executor asyncExecutor() {
           ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
           executor.setCorePoolSize(4);
           executor.setMaxPoolSize(4);
           executor.setQueueCapacity(500);
           executor.setThreadNamePrefix("EmailSender-");
           executor.initialize();
           return executor;
       }
   }
   

   Remark: asyncExecutor() is used to customize default behaviour and is not mandatory.

2. Annotate method with @Async, and set return type to CompletableFuture<XXX> where XXX is a wanted return type, for example:

   String customMethod() {
   ...
   }
   

   should be transformed to:

   CompletableFuture<String> customMethod() {
   ...
   }    
   

3. Consume it with Completable API, for example:

   • CompletableFuture.allOf(completableFutures.toArray(new CompletableFuture[]{})).join();
   • CompletableFuture.allOf(completableFuture1, completableFuture2).join();

4. Extract requested return (note that get() throws checked exception):

   • XXX xxx = completableFuture.join()

• email-service - microservice responsible for sending emails to given users
  • EmailController - REST controller; receives login-message map, then asks slow-user-service for emails and sends messages
  • in EmailService we have the same methods:
    • @Async method - asyncSend - concurrently sends messages
    • non-concurrent method - send
  • in AppRunner we simulate interactions and compare times
• slow-user-service
  • UserController returns User bean (login, name, email...) for given login
  • in UserRepository we sleep thread for user.repository.delay.seconds (configurable in application.properties) and then return requested user

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