# 🧩 Internal Structure of a Method

This page explores how a Method is represented internally in CCP, from its metadata to execution scripts and lifecycle.

---

## 🧭 What Is a Method?

In CCP, a **Method** is a reusable computational unit:  
- It can be an algorithm, a script, a model, or a processing function  
- It can be written in any language (Python, R, Java, Shell, etc.)
- It runs in an isolated container
- It receives inputs and produces outputs

---

## 📘 Method Definition Format

Methods are described in a `method.json` file that follows the [OGC API - Processes](https://ogcapi.ogc.org/processes/) specification.  
This file includes:

- Metadata (title, author, context)
- Input parameters
- Declared outputs
- Deploy/execute/undeploy scripts
- Compatible infrastructures

---

## 🧱 Method JSON Example (Java + Maven)

```json
{
  "title": "JDK-8 Example Maven",
  "id": "408d9dc5-ee37-4123-9f07-4294f13bce19",
  "version": "0.0.1",
  "description": "Runs a Maven-built Java app from GitHub",
  "inputs": {
    "ccpimage": {
      "schema": { "type": "string", "default": "nubisware/ccp-jdk8-jammy:latest" }
    },
    "repository": {
      "schema": { "type": "string", "default": "https://github.com/dcore94/jdk-maven-example" }
    },
    "mainclass": {
      "schema": { "type": "string", "default": "example.HelloWorld" }
    }
  },
  "outputs": {
    "filejson": {
      "schema": { "type": "string", "contentMediaType": "application/json" }
    }
  },
  "additionalParameters": {
    "parameters": [
      {
        "name": "deploy-script",
        "value": [
          "git clone {{ repository }} execution",
          "cd execution",
          "mvn clean package"
        ]
      },
      {
        "name": "execute-script",
        "value": [
          "cd execution",
          "java -cp target/*.jar {{ mainclass }}",
          "cp /tmp/output.json /ccp_data/"
        ]
      }
    ]
  },
  "links": [
    {
      "rel": "compatibleWith",
      "href": "infrastructures/nubisware-docker-swarm-nfs"
    }
  ]
}
```

---

## 🔄 Execution Lifecycle

The figure below shows the phases of a method execution:

```
User → CCP → Launch → Deploy → Execute → Fetch → Destroy
```

Each phase corresponds to:

- **Launch**: Prepares container using `ccpimage`
- **Deploy**: Runs deploy-script (e.g. clone repo, install deps)
- **Execute**: Runs the main algorithm
- **Fetch**: Captures outputs
- **Destroy**: Cleans up environment

If an error occurs in any phase, execution transitions to `FAILED`.

---

## 🧬 Execution Context and Variables

During execution, CCP provides several context values.

### 🧩 Inputs as Placeholders

Inputs are referenced in scripts via:

```bash
--input {{input_file}} --threshold {{threshold}}
```

These values are replaced at runtime with the actual user input.

### 🌐 Execution Variables

| Variable       | Meaning                         |
|----------------|----------------------------------|
| `ccpimage`     | Docker image for runtime         |
| `ccptaskid`    | Index of replica (1-based)       |
| `ccptaskname`  | Unique execution identifier      |
| `ccpnote`      | Optional user note for tracking  |
| `ccpmaxtime`   | Max execution time (in seconds)  |
| `ccpreplicas`  | Number of parallel instances     |

### 🔐 Secure Variables

| Env Variable       | Usage                          |
|--------------------|--------------------------------|
| `ccprefreshtoken`  | Token for API authentication   |
| `ccpclientid`      | OAuth client                   |
| `ccpcontext`       | Execution context / VRE        |


---

## 🔗 Related Pages

- 📜 [Method JSON guide](/04_methods_development/07_method_json.md)

- 🐳 [Runtime architecture](/05_advanced_topics/01_architecture/02_runtime_architecture.md)

- 🔄 [Execution lifecycle](/05_advanced_topics/02_execution_engine/01_execution_lifecycle.md)

- 🧬 [Provenance metadata (provo.xml)](/03_execution/05_metadata_provo.md)