@SuppressWarnings("SameReturnValue")
    @Operation(
        summary = "Is the analyzer engine running?",
        description = "The old style AppEngine status endpoint."
    )
    @ApiResponse(
        responseCode = "200", // HttpStatus.OK
        description = "The analyzer is running",
        content = @Content(examples = @ExampleObject("YES"))
    )
    @ApiResponse(
        responseCode = "500", // HttpStatus.INTERNAL_SERVER_ERROR,
        description = "The analyzer is starting up",
        content = @Content(examples = @ExampleObject())
    )
    @GetMapping(
        path = "/_ah/health",
        produces = TEXT_PLAIN_VALUE
    )
    public String isHealthy() {
        parseService.ensureStartedForApis(OutputType.TXT);
        return "YES";
    }
<a href="https://cloud.google.com/appengine/docs/flexible/java/how-instances-are-managed#health_checking">
App Engine health checking</a> requires responding with 200 to {@code /_ah/health}.

@return Returns a non empty message body.
webapp/src/main/java/nl/basjes/parse/useragent/servlet/status/AppEngine.java