# Component: External System Gateway

## Purpose

The External System Gateway manages predefined integrations with external systems (e.g., MES, recipe managers) and database connections. It provides the runtime for invoking external API methods and executing database operations from scripts at site clusters.

## Location

Site clusters (executes calls directly to external systems, reads definitions from local SQLite). Central cluster (stores definitions in config DB, brokers inbound requests from external systems to sites).

## Responsibilities

### Definitions (Central)
- Store external system definitions in the configuration database: connection details, API method signatures (parameters and return types).
- Store database connection definitions: server, database, credentials.
- Deploy definitions uniformly to all sites (no per-site overrides). Deployment requires **explicit action** by a user with the Deployment role.
- Managed by users with the Design role.

### Execution (Site)
- Invoke external system API methods as requested by scripts (via Script Execution Actors and Alarm Execution Actors).
- Provide raw MS SQL client connections (ADO.NET) by name for synchronous database access.
- Submit cached database writes to the Store-and-Forward Engine for reliable delivery.
- Sites communicate with external systems **directly** (not routed through central).

### Integration Brokering (Central)
- Receive inbound requests from external systems (e.g., MES querying machine values).
- Route requests to the appropriate site via the Communication Layer.
- Return responses to the external system.

## External System Definition

Each external system definition includes:
- **Name**: Unique identifier (e.g., "MES", "RecipeManager").
- **Base URL**: The root endpoint URL for the external system (e.g., `https://mes.example.com/api`).
- **Authentication**: One of:
  - **API Key**: Header name (e.g., `X-API-Key`) and key value.
  - **Basic Auth**: Username and password.
- **Timeout**: Per-system timeout for all method calls (e.g., 30 seconds). Applies to the HTTP request round-trip.
- **Retry Settings**: Max retry count, fixed time between retries (used by Store-and-Forward Engine for transient failures only).
- **Method Definitions**: List of available API methods, each with:
  - Method name.
  - **HTTP method**: GET, POST, PUT, or DELETE.
  - **Path**: Relative path appended to the base URL (e.g., `/recipes/{id}`).
  - Parameter definitions (name, type). Supports the extended type system (Boolean, Integer, Float, String, Object, List).
  - Return type definition. Supports the extended type system for complex response structures.

## Database Connection Definition

Each database connection definition includes:
- **Name**: Unique identifier (e.g., "MES_DB", "HistorianDB").
- **Connection Details**: Server address, database name, credentials.
- **Retry Settings**: Max retry count, fixed time between retries (for cached writes).

## Database Access Modes

### Synchronous (Real-time)
- Script calls `Database.Connection("name")` and receives a raw ADO.NET `SqlConnection`.
- Full control: queries, updates, transactions, stored procedures.
- Failures are immediate — no buffering.

### Cached Write (Store-and-Forward)
- Script calls `Database.CachedWrite("name", "sql", parameters)`.
- The write is submitted to the Store-and-Forward Engine.
- Payload includes: connection name, SQL statement, serialized parameter values.
- If the database is unavailable, the write is buffered and retried per the connection's retry settings.

## Invocation Protocol

All external system calls are **HTTP/REST** with **JSON** serialization:

- The ESG acts as an HTTP client. The external system definition provides the base URL; each method definition specifies the HTTP method and relative path.
- Request parameters are serialized as JSON in the request body (POST/PUT) or as query parameters (GET/DELETE).
- Response bodies are deserialized from JSON into the method's defined return type.
- Credentials (API key header or Basic Auth header) are attached to every request per the system's authentication configuration.

## External System Call Modes

Scripts choose between two call modes per invocation, mirroring the dual-mode database access pattern:

### Synchronous (Real-time)
- Script calls `ExternalSystem.Call("systemName", "methodName", params)`.
- The HTTP request is executed immediately. The script blocks until the response is received or the timeout elapses.
- **All failures** (transient and permanent) return an error to the calling script. No store-and-forward buffering.
- Use for request/response interactions where the script needs the result (e.g., fetching a recipe, querying inventory).

### Cached (Store-and-Forward)
- Script calls `ExternalSystem.CachedCall("systemName", "methodName", params)`.
- The call is attempted immediately. If it succeeds, the response is discarded (fire-and-forget).
- On **transient failure** (connection refused, timeout, HTTP 5xx), the call is routed to the Store-and-Forward Engine for retry per the system's retry settings. The script does **not** block — the call is buffered and the script continues.
- On **permanent failure** (HTTP 4xx), the error is returned **synchronously** to the calling script. No retry — the request itself is wrong.
- Use for outbound data pushes where deferred delivery is acceptable (e.g., posting production data, sending quality reports).

## Call Timeout & Error Handling

- Each external system definition specifies a **timeout** that applies to all method calls on that system.
- Error classification by HTTP response:
  - **Transient failures** (connection refused, timeout, HTTP 408, 429, 5xx): Behavior depends on call mode — `CachedCall` buffers for retry; `Call` returns error to script.
  - **Permanent failures** (HTTP 4xx except 408/429): Always returned to the calling script regardless of call mode. Logged to Site Event Logging.
- This classification ensures the S&F buffer is not polluted with requests that will never succeed.
- **Idempotency note**: `CachedCall` retries may result in duplicate delivery if the external system received the original request but the response was lost. Callers should use `CachedCall` only for operations that are idempotent or where duplicate delivery is acceptable.

## Blocking I/O Isolation

- External system HTTP calls and database operations are blocking I/O. Script Execution Actors (which are short-lived, per-invocation actors) execute these calls, ensuring that blocking does not starve the parent Script Actor or Instance Actor.
- The Akka.NET actor system should configure a **dedicated dispatcher** for Script Execution Actors to isolate blocking I/O from the default dispatcher used by coordination actors.

## Database Connection Management

- Database connections use **standard ADO.NET connection pooling** per named connection. No custom pool management.
- Pool behavior (max pool size, connection lifetime, etc.) can be tuned via connection string parameters in the database connection definition if needed.
- Synchronous failures on `Database.Connection()` (e.g., unreachable server) return an error to the calling script, consistent with external system permanent failure handling.

## Dependencies

- **Configuration Database (MS SQL)**: Stores external system and database connection definitions (central only).
- **Local SQLite**: At sites, external system and database connection definitions are read from local SQLite (populated by artifact deployment). Sites do not access the central config DB.
- **Store-and-Forward Engine**: Handles buffering for failed external system calls and cached database writes.
- **Communication Layer**: Routes inbound external system requests from central to sites.
- **Security & Auth**: Design role manages definitions.
- **Configuration Database (via IAuditService)**: Definition changes are audit logged.

## Interactions

- **Site Runtime (Script/Alarm Execution Actors)**: Scripts invoke external system methods and database operations through this component.
- **Store-and-Forward Engine**: Failed calls and cached writes are routed here for reliable delivery.
- **Deployment Manager**: Receives updated definitions as part of system-wide artifact deployment (triggered explicitly by Deployment role).
