# Common to All Modules

Devices, experiments and analyzers are different kinds of module (see
[The three things you build](../quickstart/index.md#the-three-things-you-build)),
but they share the same ground: how they are configured, how they log what they
do, and the data types they exchange. This chapter covers that shared ground once,
so the chapters on each kind of module do not repeat it.

## Where each kind of setting goes

A module is configured in three places. Choose the place by asking who the setting
belongs to:

| The setting belongs to … | Example | Goes in | Explained in |
|---|---|---|---|
| this computer or this lab, or is secret | an instrument's address, a server's port, a password | `.env` | [The .env File](env_file.md) |
| the module itself | motor counts per degree, a hardware speed limit, which devices an experiment uses | `config.yaml` inside the package | [The module configuration: `config.yaml`](yaml_config.md#the-module-configuration-configyaml) |
| one measurement | the wavelengths to scan, the exposure time | `config/*.yaml` (experiments only) | [The run configuration: `config/*.yaml`](yaml_config.md#the-run-configuration-configyaml) |

Getting this right is what lets the same module run in another lab unchanged:
only `.env` differs.

## In this chapter

| Page | Covers |
|---|---|
| [The .env File](env_file.md) | settings that differ per computer, and the variables plesty-lib reads by itself |
| [YAML Configuration](yaml_config.md) | the module's own `config.yaml`, and an experiment's run settings |
| [Logging System](logging_system.md) | where a module's log messages go, and how to control them |
| [Data & Metadata Schemas](data_schemas.md) | the data types devices produce, experiments record and analyzers read |