doc: serverless functions doc

serverless_functions/README.md

@@ -1,3 +1,42 @@
 # Serverless Functions
 
 Backend for the mobile application.
+
+## How it works
+
+By adding a file in the `api` folder, it will create an endpoint of the name of the file.
+For instance, the file `directions-wrapper.ts` is linked to the endpoint `/api/directions-wrapper`.
+When targeting the endpoint, the function defined in the related file will be executed.
+
+To have a small example of a serveless function, check out [`directions-wrapper.ts`](./api/directions-wrapper.ts)
+
+If you need to use secrets, define them as environment variables on the [vercel site](https://vercel.com/teamprojectlodz-gmailcom/physigo/settings/environment-variables).
+
+You have to run `npm i` each time the `package.json` is modified to install the new dependencies.
+It also includes each time you pull changes from others.
+
+## Login to Vercel
+
+1. Run `npx vercel login`
+2. Choose Email auth
+3. Email: teamprojectlodz@gmail.com
+4. Password: see teams
+
+## Test the functions
+
+To locally test the functions, run `npx vercel dev`.
+It will start a local server that you can target for your tests.
+
+## Deploy new functions
+
+Once authenticated, run `npx vercel --prod`
+
+### Deploy for the first time
+
+For the first time, you have to link to the existing project.
+1. For `Set up and deploy ...`, answer `y` (default answer)
+2. For the scope, answer `teamprojectlodz-gmailcom` (default answer)
+3. For `Link to existing project`, answer `y`
+4. For the name of the existing project, answer `physigo`
+
+It will generate a `.vercel` folder, which links to the project on vercel.