I've just noticed that the WarmupTimer module is dependent on the Warmup module. The latter is an end-user module, whereas the former should be a core module, according to the terminology of the 4 layer canvas. As the WarmupTimer is dependent on the Warmup module, we get a reference violation (see: Validating your Application Architecture), more specifically, an upward reference. My suggestion to solve this issue is to bring all server actions in the Warmup module into the WarmupTimer module. Then we don't have the upward reference anymore. In addition, I would suggest to rename the WarmupTimer module to CS_Warmup to make clear that it belongs to the core layer.