static SuccessFunction ofDefault() {
        return (ctx, log) -> {
            if (log.responseCause() != null) {
                return false;
            }

            final int statusCode = log.responseHeaders().status().code();
            if (statusCode < 100 || statusCode >= 400) {
                return false;
            }

            final Object responseContent = log.responseContent();
            if (responseContent instanceof RpcResponse) {
                return !((RpcResponse) responseContent).isCompletedExceptionally();
            }

            return true;
        };
    }
Returns the default success classification function which checks
{@link RequestLog#responseCause()} is null, 100 &lt;= {@link HttpStatus} &lt; 400
and {@link RpcResponse#isCompletedExceptionally()} == {@code false}.
core/src/main/java/com/linecorp/armeria/common/SuccessFunction.java