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+# 3. Define directory structure
+
+Date: 2020-12-01
+
+## Status
+
+Accepted
+
+## Context
+
+There will be many services contributed by multiple people. We need a common way of structuring the services
+so it's easy for the user to pick up any one service.
+
+Some services might want to share some common code. We want to have that code structured so that include path shouldn't
+change if these extensions are included into Mbed OS.
+
+## Decision
+
+### Services
+
+Each service will be its own library. All services will be in the services directory.
+Each service will contain at least an include folder and a mbed_lib.json.
+
+An example service called `example` would look like this:
+
+```
+services/example/include/ble-service-example/example.h
+services/example/source/example.cpp
+services/example/mbed_lib.json
+```
+
+and the mbed_lib.json would contain at least:
+
+```
+{
+    "name": "ble-service-example"
+}
+```
+
+This way a user can clone the repo and include an individual service without getting unwanted includes.
+
+### Extensions
+
+If a service needs to share some common code between multiple services this code can be made into an extension.
+
+Extensions are common code, extending the mbed-os ble feature. They are libraries of their own and each service that
+wants to use them needs a `requires` section that pulls it in as a dependency.
+
+And `example` extension library:
+
+```
+extensions/example/include/ble/gatt
+extensions/example/include/ble/gap
+extensions/example/include/ble/common
+extensions/example/source/...
+extensions/example/mbed_lib.json
+```
+
+and the extension mbed_lib.json would contain at least:
+
+```
+{
+    "name": "ble-extension-example"
+}
+```
+
+and the service needs it's mbed_lib.json expanded with a
+
+```
+    requires: ["ble-extension-example"]
+``` 
+
+### Tests
+
+Unit tests and integration tests go into the tests directory:
+
+```
+tests/UNITTESTS
+tests/TESTS
+```
+
+`TESTS` and `UNITTESTS` are automatically ignored by mbed tools when not building tests. Any other files or directories in
+the tests folder need to be added to `.mbedignore` in order to avoid compilation when the repo is included with the
+intention to use one of the services.
+
+## Consequences
+
+All PRs amended to follow the structure. All future PRs comply with the structure.