Outbox

A simple library to implement Outbox microservice pattern in Java using DB Polling.

Read this medium Post to know about what problem Outbox Pattern solves https://link.medium.com/p1FnyAMjWxb

Usage:

first, make sure to reate table outbox_messages from the file schema.sql

1. Annotate your method that contains code that contains network calls with @Outbox:

   @Slf4j
   @Service
   @RequiredArgsConstructor
   public class UseService {
   
       private final RestTemplate restTemplate;
       private final UserRepository userRepository;
   
       public void saveUser(UserEntity user) {
           userRepository.save(user);
       }
   
       @Outbox  // <---- You put the annotation on the method that communicate with external system (queue, another service, etc.)
       public void syncUser(UserEntity user) {
           Map<?, ?> map = restTemplate.postForObject("https://gorest.co.in/public/v2/users", user, Map.class);
           log.info("response from api: {}", map);
       }
   }

2. Call this method regularly as if you call any regular spring service:

   @RestController
   @RequestMapping("/api")
   @RequiredArgsConstructor
   public class UserController {
   
       private final UseService useService;
   
       @Transactional // <---- Now this can run in a single Transaction (hence both invoked methods will write to the database)
       @PostMapping("/users")
       public void send(@RequestBody UserEntity user) {
          useService.saveUser(user);
          useService.syncUser(user);
       }
   }

   The call to useService.syncUser() method will return immediately, saved to outbox_messages table, and then a background process will make sure that method is being called (will try to execute the method and log the exception to the database in case of error happens).

See the UserController in the example project.

Handling Failures:

Sometimes, the Scheduler failed to successfully invoke the target method, maybe due to some network issue etc. So to overcome such issues, You can call a method in OutboxService called setPending, which will make sure that the status of the failed message is eligibility to be rescheduled by the scheduler.

See the AdminController in the example project.