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If you are developing a service intended to go in a level 2 module (for example, a grading service) and you find that your new service needs to call out to a service in the registration module - also a level 2 module - you will need to move your new service to a level 1 module where it can safely aggregate lower-level services.
CSF "Common Legacy" and the Future
One large piece of code that does not currently conform to this architecture is csf-common-legacy. This currently is a level 4 module, but contains code that should really be in levels 1, 2, and 3. We will be working to move code out of common-legacy to the appropriate level as part of CSF version 3. Until then our plan is to deprecate code in CSF 2 csf-common legacy and where appropriate, design and code replacement modules in levels 1, 2, and 3. There will be a future infrastructure project that converts apps from CSF 2 to CSF 3.