# 🔣 CCP Method Descriptor: method.json

Every method in CCP is defined by a structured JSON file called `method.json`. This file fully describes the method’s behavior, interface, and execution context, and is essential for running, sharing, and reproducing the method.

You can download it using the **Download** button (![Download Icon](/_static/imgs/method_button_download.png)) in the method list.

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## Purpose

The `method.json` file is the definitive contract for a method’s definition. It:

- Specifies inputs, outputs, runtime, and logic
- Enables sharing and importing/exporting across VREs
- Supports method versioning and provenance tracking

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## Structure Overview

### `id` (required)

```json
"id": "af3e3771-5b1f-402b-89b8-4ecdb121af92"
```

A globally unique identifier for the method. It must remain consistent to ensure traceability of executions and metadata.

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### `title`, `description`, `version`

Basic metadata used for display and documentation.

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### `metadata`

Contains roles like:

- `author`: person or group creating the method
- `category`: classification in the UI
- `context`: VRE or infrastructure where the method is defined

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### `inputs`

Defines the method’s parameters. Each includes:

- `id`, `title`, `description`
- `schema` with type, format, and MIME type
- `minOccurs` / `maxOccurs`

#### `ccpimage`: Runtime Docker Image (required)

```json
"ccpimage": {
  "id": "ccpimage",
  "title": "Runtime",
  "description": "The image of the runtime to use for method execution.",
  "schema": {
    "type": "string",
    "format": "url",
    "default": "hub.dev.d4science.org/test/python-gdal:latest"
  }
}
```

This input defines the Docker image used to run the method logic inside the container. It must be compatible with the target infrastructure.

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### 📦 Output `outputs`

Standard and custom outputs, including `stdout`, `stderr`, or generated files.

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### `additionalParameters`

Includes:

- `deploy-script`: prepares the environment
- `execute-script`: runs the method
- `undeploy-script`: optional cleanup
- `lifecycle`: timestamps and modifications

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### `links` → `compatibleWith` (required)

```json
"links": [
  {
    "rel": "compatibleWith",
    "title": "D4Science production Infrastructure",
    "href": "infrastructures/d4science-prod-swarm"
  }
]
```

This field is **mandatory** and declares which infrastructure(s) the method is **compatible with**. If not present, the method cannot be scheduled for execution.

It ensures that:

- The method can run on the selected infrastructure
- The Docker image is accessible from that infrastructure
- Runtime features (e.g., network, storage) match execution requirements

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### `keywords` (optional)

Used for search and discovery in the CCP interface.

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## 💡 Note Notes

- The `id` and `compatibleWith` fields are **mandatory**
- The method is executed only if the infrastructure matches `compatibleWith`
- The file can be uploaded again to update, clone, or transfer the method across VREs

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## 🔗 Related Pages

- 🔧 [Method development workflow](./01_method_development_flow.md)

- 🐍 [Python-based methods](./03_python_method.md)

- 🛠 [Deploy and execute scripts](./06_scripts_and_placeholders.md)

- 🐧 [Docker runtime (ccpimage)](./02_defining_ccpimage.md)