def METHOD_NAME(method, callargs, switchover_timestamp=None):
    spec = method_specifications.get(method)
    if spec is None:
        return "redis"  # default backend (possibly invoke base directly instead?)
    if switchover_timestamp is not None and time.time() < switchover_timestamp:
        return "redis"  # snuba does not yet have all data
    operation_type, model_extractor = spec
    backends = {model_backends[model][operation_type] for model in model_extractor(callargs)}
    assert len(backends) == 1, "request was not directed to a single backend"
    return backends.pop()