How to implement a disaster recovery retry mechanism from serverB to serverC in Spring WebFlux?

Spring WebFlux builds LLM Gateway's disaster recovery retry solution

This article explains how to build an efficient disaster recovery and retry mechanism for LLM Gateway under the Spring WebFlux framework. Specific scenario: When the request from Gateway to Server B fails, automatically retry Server C to ensure that the client (Client A) gets the correct response, even if Server B is not available. The scheme supports verbatim data transmission of server-send events (SSE).

challenge

Client A accesses Server B through Gateway. If the connection between Gateway and Server B fails, Gateway needs to automatically switch to Server C and try again. The goal is that Client A can receive the correct results even if Server B fails, as long as Server C is available. In addition, the integrity and sequence of SSE data streams need to be ensured.

Solution: Disaster recovery strategy based on retryWhen and onErrorResume

Use Spring WebFlux's retryWhen operator and onErrorResume operator to build flexible retry logic.

1. Error capture and retry: retryWhen intercepts errors, decide whether to retry based on the error type. If Server C retry still fails, the error message is returned to Client A.
2. Avoid duplicate responses: Use flags (such as AtomicBoolean ) to ensure that only the first successful response is returned, preventing duplicate responses when both Server B and Server C are available.

Code example:

 AtomicBoolean hasRetried = new AtomicBoolean(false);

Flux<response> responseFlux = sseHttp(serverB.getUrl())
    .retryWhen(companion -> companion.flatMap(error -> {
        if (error instanceof GatewayException) {
            // Gateway exception, try to connect to Server C
            return sseHttp(serverC.getUrl())
                .flatMap(serverCResponse -> {
                    hasRetried.set(true);
                    return Flux.just(serverCResponse);
                });
        } else {
            // Other errors directly return Flux.error(error);
        }
    }))
    .onErrorResume(error -> {
        // Server C failed to retry, return an error response to Client A
        return Flux.just(GatewayExceptionHandler.toStreamErrorResponse(
            new GatewayException("Upstream service error.", HttpStatus.INTERNAL_SERVER_ERROR)));
    })
    .doOnNext(response -> {
        if (!hasRetried.get()) {
            // Only process the first successful response// ... your original logic here ...
        }
    });</response>

In this example, retryWhen catches Server B's error and tries to connect to Server C. The hasRetried flag ensures that only the first successful response is processed.

Summarize

Through retryWhen and onErrorResume and combined with flag bit control, we have implemented an efficient LLM Gateway disaster recovery retry mechanism in Spring WebFlux environment to ensure high service availability and ensure the integrity of SSE data flow. This solution is flexible and scalable and is suitable for all types of error handling and retry policies.

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