Debugging an Unexpected Behavior in a Loop When Fetching Data from Microservices
I'm sure I'm missing something obvious here, but While debugging a microservices architecture, I ran into an odd situation where data fetched from multiple services inside a loop isn't producing the expected results. My current setup uses Spring Boot (version 2.5.4) to manage the back-end services, and I am calling these services sequentially in a for-loop. Here’s the relevant snippet: ```java for (String id : userIds) { ResponseEntity<User> response = restTemplate.getForEntity("http://user-service/api/users/" + id, User.class); if (response.getStatusCode() == HttpStatus.OK) { users.add(response.getBody()); } } ``` Despite the loop iterating through all the `userIds`, the `users` list often ends up missing entries. I suspect there’s a timing issue or that some responses are taking longer than expected, highlighting the need for a more synchronous approach. I tried implementing a CompletableFuture to handle asynchronous calls, but that complicated things further: ```java List<CompletableFuture<User>> futures = new ArrayList<>(); for (String id : userIds) { CompletableFuture<User> future = CompletableFuture.supplyAsync(() -> { ResponseEntity<User> response = restTemplate.getForEntity("http://user-service/api/users/" + id, User.class); return response.getStatusCode() == HttpStatus.OK ? response.getBody() : null; }); futures.add(future); } List<User> users = futures.stream().map(CompletableFuture::join).filter(Objects::nonNull).collect(Collectors.toList()); ``` However, now I'm dealing with NullPointerExceptions when some of the responses are empty. Additionally, the overall response time has increased significantly due to the overhead of managing multiple futures. My goal is to ensure that all valid users are fetched without missing any while keeping performance in check. What strategies or patterns can I apply to optimize this loop, ensure data integrity, and maintain responsiveness? For context: I'm using Java on Linux. I'd really appreciate any guidance on this.