TA-3755: Persist the Internal Architecture document in the repo (#223)

Author: ZgjimHaziri

docs/internal-architecture.md

@@ -0,0 +1,139 @@
+# **Content CLI – Architecture & Inner Workings**
+
+## **Overview**
+
+The Content CLI is a TypeScript based CLI that allows interaction with the Celonis EMS. Its core functionality includes extracting, pushing, and managing EMS content.
+
+**Key Dependencies**
+
+| **Dependency** | **Purpose** |
+| --- | --- |
+| commander | CLI parsing and command configuration and routing |
+| axios | HTTP client for API calls |
+| form-data | Multipart file uploads (e.g. zip files) |
+
+## **Entry Point**
+
+The CLI starts at the `content-cli.ts` file, which:
+
+- Loads and registers all command modules.
+- Parses user input from the command line.
+- Initializes the shared context object.
+- Passes execution to the appropriate command.
+
+When built, it produces a `dist/content-cli.js`, which users execute with: `node content-cli.js [commands] [options]`
+
+The **help** dialog is shown when:
+
+- The user passes -h or --help
+- The user runs an incomplete or invalid command
+
+Help output includes descriptions, options and subcommands (if available) for the called command. It is auto-generated from the Commander configuration.
+
+## **Context Passing**
+
+The context object is passed to all commands and services. It includes:
+
+- **Profile**: The profile which is used for each command execution as authentication.
+- **Logger**: The global logger, with optional debug logging.
+- **HttpClient**: Authenticated EMS API client.
+
+This object is passed into command callbacks, ensuring that state and services are shared cleanly:
+```
+private async commandHandler(context: Context): Promise<void> {
+    await context.httpClient.get('...');
+}
+```
+
+## **Profile management**
+
+Every command in the Content CLI runs within the context of a profile. A profile contains information about the target Celonis team, including the base URL and authentication credentials.
+
+There are two ways a profile is used:
+
+- You can pass it directly to any command using the `--profile` flag.
+- Or you can set a default profile so it's used automatically without passing the flag every time.
+
+To manage profiles, use the content-cli profile command group. This allows you to:
+
+- Create a new profile
+- List existing profiles
+- Set a profile as the default
+- Delete unused profiles
+
+## **API communication**
+
+The **HttpClient** class is a centralized client for interacting with the EMS API. It handles:
+
+- **Auth**: Authenticates by sending the token in the provided profile.
+- **URL building**: Prefixing all API paths with the correct base (e.g. realm), inferred from the profile
+- **Request execution**
+    - Uses axios under the hood, for HTTP API call executions.
+    - Handles GET, POST, PUT, and DELETE methods.
+    - Returns parsed responses to commands.
+- **Error handling**: Mapping error codes to readable messages.
+
+This prevents duplication of HTTP logic in individual modules.
+
+## **Managing Command Modules**
+
+### **Modules**
+
+Each common functionality group is implemented as a separate module, which is represented by a directory in the src/command folder, and is enabled as a module by a module.ts class which implements the IModule.
+
+The implementation consists of a register() method, which utilizes the passed Configurator object, which supports interactions with [Commander’s Command](https://www.npmjs.com/package/commander) API. The API gets used to:
+
+- Register commands
+- Define their description, options and action which will be taken when the command is executed.
+
+### **Module Handler**
+
+These modules get loaded by the **module handler**, which:
+
+- Scans available modules (like configuration-management, studio).
+- Calls each module’s registration function.
+- Registers the command using Commander.js.
+
+This means that the registration of a new module is automatically handled after module creation is done as described above.
+
+## **Configurator Wrapper**
+
+In order to provide more control over what is provided and what happens when interacting with Commander, there is a Configurator abstraction that wraps the underlying CLI command configuration. Its purpose is to:
+
+- Enforce validation rules.
+- Unify help text and usage patterns. (e.g. providing deprecation notice helper)
+
+## **Error Handling**
+
+### **General Error Handling**
+
+- CLI argument parsing logic is wrapped in a try/catch.
+- On failure, a clean message is shown and the process exits with a non-zero code.
+- Unhandled errors are [caught globally](https://github.com/celonis/content-cli/blob/b76187e76e1f50d41a149bfe90d6ddead568c853/src/content-cli.ts#L74) to avoid raw stack traces.
+
+### **HTTP Error Handling**
+
+- All API calls go through a centralized HttpClient.
+- HTTP call failures are caught and wrapped inside the HttpClient.
+- Custom error messages (e.g. "Unauthorized" or "Server failed") are shown to the user.
+
+## **Legacy BaseManager – Deprecated**
+
+The project historically used a BaseManager pattern where each content type extended a shared manager class.
+
+### **Why it's discouraged now:**
+
+- Cluttered many functionalities in one place.
+- Wasn’t reusable for new API endpoints
+    - E.g.. config export/import commands.
+- Harder developer experience and readability.
+
+### **New approach:**
+
+Custom API service classes are built around the HttpClient and context, which get used . These are:
+
+- More testable
+- Easier to extend
+- More readable and explicit
+
+We avoid using BaseManager in new modules and aim to migrate old ones when possible.