Added env var reference

Guide/deployment.markdown

@@ -460,3 +460,121 @@ This will build a nix package that contains the following binaries:
 -   a binary for each script in `Application/Script`, e.g. `Welcome` for `Application/Script/Welcome.hs`
 
 The build contains an automatic hash for the `IHP_ASSET_VERSION` env variable, so cache busting should work out of the box.
+
+
+# Env Var Reference
+
+Your IHP app can be configured at runtime with environment variables.
+
+## Recommended Env Vars
+
+
+#### `IHP_ENV`
+
+Switch IHP to production mode like this:
+
+```bash
+$ export IHP_ENV="Production"
+$ ./build/bin/RunProdServer
+```
+
+The production mode has effects on caching behaviour of static files, logging and error rendering.
+
+You can also use `IHP_ENV=Development` to force dev mode.
+
+
+#### `IHP_BASEURL`
+
+Without specifying this env var, the app will always use `http://localhost:8000/` in absolute URLs it's generating (e.g. when redirecting or sending out emails).
+
+It's therefore important to set it to the external user-facing web addresss. E.g. if your IHP app is available at `https://example.com/`, the variable should be set to that:
+
+```bash
+$ export IHP_BASEURL=https://example.com
+$ ./build/bin/RunProdServer
+```
+
+#### `IHP_SESSION_SECRET`
+
+In production setup's you want to configure the `IHP_SESSION_SECRET` env variable. It's a private key used to encrypt your session state. If it's not specified, a new one will generated on each container start. This means that all your users will have to re-login on each container start.
+
+**Note on `Config/client_session_key.aes`:** The `IHP_SESSION_SECRET` env variable is an alternative for placing a `Config/client_session_key.aes` inside the your repository. If IHP detects a `Config/` folder, and no `IHP_SESSION_SECRET` is set, it will automatically create a `Config/client_session_key.aes` file. This is designed for persistent sessions in development mode.
+
+When you start an app without specifying the `IHP_SESSION_SECRET` and no `Config/client_session_key.aes` is found, the app will output the randomly generated one. So you can get a new secret key by starting a new container and copying the value:
+
+```bash
+$ ./build/bin/RunProdServer
+IHP_SESSION_SECRET=1J8jtRW331a0IbHBCHmsFNoesQUNFnuHqY8cB5927KsoV5sYmiq3DMmvsYk5S7EDma9YhqZLZWeTFu2pGOxMT2F/5PnifW/5ffwJjZvZcJh9MKPh3Ez9fmPEyxZBDxVp
+Server started
+```
+
+There we can copy the `IHP_SESSION_SECRET=1J8jtRW331a0IbHBCHmsFNoesQUNFnuHqY8cB5927KsoV5sYmiq3DMmvsYk5S7EDma9YhqZLZWeTFu2pGOxMT2F/5PnifW/5ffwJjZvZcJh9MKPh3Ez9fmPEyxZBDxVp` value and use it as our secret:
+
+```bash
+$ export IHP_SESSION_SECRET="1J8jtRW331a0IbHBCHmsFNoesQUNFnuHqY8cB5927KsoV5sYmiq3DMmvsYk5S7EDma9YhqZLZWeTFu2pGOxMT2F/5PnifW/5ffwJjZvZcJh9MKPh3Ez9fmPEyxZBDxVp"
+$ ./build/bin/RunProdServer
+```
+
+#### `IHP_REQUEST_LOGGER_IP_ADDR_SOURCE`
+
+If the app is running behind a load balancer, set the environment variable `IHP_REQUEST_LOGGER_IP_ADDR_SOURCE=FromHeader` to tell IHP to use the `X-Real-IP` or `X-Forwarded-For` header for detecting the client IP.
+
+```bash
+$ export IHP_REQUEST_LOGGER_IP_ADDR_SOURCE=FromHeader
+$ ./build/bin/RunProdServer
+```
+
+#### `PORT`
+
+Specifies the TCP Port where the application will listen to. Defaults to `8000`.
+
+```bash
+$ export PORT=1337
+$ ./build/bin/RunProdServer
+# App now starts on port 1337 instead of 8000
+```
+
+#### `DATABASE_URL`
+
+You can configure the database your app connects to using the `DATABASE_URL` env variable:
+
+```bash
+$ export DATABASE_URL="postgresql://postgres:mysecretpassword@the-hostname/postgres"
+$ ./build/bin/RunProdServer
+```
+
+
+## Advanced Env Vars
+
+#### `IHP_ASSET_VERSION`
+
+As of IHP v0.16 this env variable is automatically set to a unique build hash.
+
+If you use [`assetPath` helpers](assets.html) in your app, specifiy the `IHP_ASSET_VERSION` env var. Set it e.g. to your commit hash or to the release timestamp.
+
+```bash
+$ export IHP_ASSET_VERSION=af5f389ef7a64a04c9fa275111e4739c0d4a78d0
+$ ./build/bin/RunProdServer
+```
+
+#### `GHCRTS`
+
+IHP by default sets some default values for the GHC/Haskell Runtime System (RTS) which work well in production with high loads, at the cost of using a bit of memory. If you want your IHP deployment to use less RAM on your machine, try different values for the environment variable `GHCRTS`. The default value (set in IHP's [Makefile.dist](https://github.com/digitallyinduced/ihp/blob/master/lib/IHP/Makefile.dist#L86)) is `"-A128M -N3 -qn3 -G4"`. If you want very low memory usage, at the cost of more frequent garbage collection, try `export GHCRTS="-A16M -N"`. A middle ground is IHPCloud's default of `export GHCRTS="-A128M -N3 -qn3 -G4"`.
+
+For explanations of these values, see GHC's [manual on the RTS](https://downloads.haskell.org/ghc/latest/docs/users_guide/runtime_control.html) and on [RTS options for concurrency]( https://downloads.haskell.org/ghc/latest/docs/users_guide/using-concurrent.html#rts-options-for-smp-parallelism). In short, `-A` is the allocation area, `-G` is number of generations, `-qn` is number of threads to use for parallel GC, `-N` is number of threads and `-n` is the memory chunk area.
+
+#### `IHP_IDE_BASEURL`
+
+Only relevant in dev mode. This defaults to `IHP_IDE_BASEURL=http://localhost:8001`.
+
+#### `IHP_RLS_AUTHENTICATED_ROLE`
+
+A database role used by IHP DataSync. Defaults to `ihp_authenticated`.
+
+#### `IHP_DATASYNC_MAX_SUBSCRIPTIONS_PER_CONNECTION`
+
+The maximum number of subscriptions per websocket connection in IHP DataSync. Defaults to `128`.
+
+#### `IHP_DATASYNC_MAX_TRANSACTIONS_PER_CONNECTION`
+
+The maximum number of database transactions per websocket connection in IHP DataSync. Defaults to `10`.