# Tutorial: Writing policies for a simple photo-sharing service

This documentation is for an as-yet unreleased version of Cerbos PDP. Choose 0.53.0 from the version picker at the top right or navigate to [https://docs.cerbos.dev](https://docs.cerbos.dev/) for the latest version.

## Getting started

- We will use Docker to run the server and the compiler.

```shell
docker pull ghcr.io/cerbos/cerbos:0.54.0-prerelease
```

- Create a file named `.cerbos.yaml` with the following contents:

```yaml
---
server:
  httpListenAddr: ":3592"

storage:
  driver: "disk"
  disk:
    directory: /photo-share/policies
```

- Create a directory named `policies` to hold the policies.

## The Apatr application

Apatr is a simple photo-sharing service that allows users to upload their photos and optionally share them with the rest of the world. Users sign-up to the service either by creating their own user account on the website or by signing-in with an identity provider (IDP) like Google or Facebook. Apatr uses a third-party identity management tool to manage these accounts and authenticating users to the site. Once they are logged-in, users can do the following:

- Create albums to organize their photos
- Upload photos to albums
- Share albums or individual photos with other Apatr users
- Share albums or individual photos with the internet

Apatr also employs a team of moderators to investigate complaints and remove any illegal or offensive items from the site. To respect users privacy, moderators are only allowed to view photos or albums that are public or those that have been flagged as inappropriate by another user.

Apatr’s identity provider allows defining roles for users. The roles currently defined in this system are:

- `moderator`: Member of the moderator team
- `user`: Authenticated users

## Resources and actions

In the Apatr application, the most obvious resource hierarchy is the following:

- Album

- Photo
    - Caption
    - Comment
  - Description
- User Profile

| Resource       | Action     | Allowed role | Condition                                       |
|----------------|------------|--------------|--------------------------------------------------|
| **album:object**| `create`  | `user`       |                                                  |
|                 | `delete`  | `user`       | - If user owns the album                         |
|                 |            | `moderator`  | - If the album is flagged as inappropriate       |
|                 | `share`   | `user`       | - If user owns the album                         |
|                 | `unshare`  | `user`      | - If user owns the album                         |
|                 | `view`    | `user`       | - If user owns the album<br><br>- If the album is public |
|                 |            | `moderator`  | - If the album is flagged as inappropriate<br><br>- If the album is public |
|                 | `flag`    | `user`       | - If the album is public                         |

## Derived roles

There are some recurring themes in the above permissions matrix.

- People who have the `user` role can be either owners or viewers depending on the resource they are trying to access
- Moderators get extra capabilities when the content is flagged as inappropriate

These capabilities are determined based on contextual information. Let’s codify them so that they can be reused.

```yaml
---
apiVersion: "api.cerbos.dev/v1"
description: |-
  Common dynamic roles used within the Apatr app
derivedRoles:
  name: apatr_common_roles
  definitions:
    - name: owner
      parentRoles: ["user"]
      condition:
        match:
          expr: request.resource.attr.owner == request.principal.id

- name: abuse_moderator
      parentRoles: ["moderator"]
      condition:
        match:
          expr: request.resource.attr.flagged == true
```

Save the above definition as `apatr_common_roles.yaml` in the `policies` directory.

Run the compiler to make sure that the contents of the file are valid.

```shell
docker run -it -v $(pwd):/photo-share ghcr.io/cerbos/cerbos:0.54.0-prerelease \
    compile /photo-share/policies
```

## Resource policies

Let’s write a resource policy for the `album:object` resource.

```yaml
---
apiVersion: api.cerbos.dev/v1
resourcePolicy:
  version: "default"
  importDerivedRoles:
    - apatr_common_roles
  resource: "album:object"
  rules:
    - actions: ['*']
      effect: EFFECT_ALLOW
      derivedRoles:
        - owner

- actions: ['view', 'flag']
      effect: EFFECT_ALLOW
      roles:
        - user
      condition:
        match:
          expr: request.resource.attr.public == true

- actions: ['view', 'delete']
      effect: EFFECT_ALLOW
      derivedRoles:
        - abuse_moderator
```

Save the above policy definition as `resource_album_object.yaml` inside the `policies` directory.

Run the compiler to make sure that the contents of the policies directory are valid.

```shell
docker run -it -v $(pwd):/photo-share ghcr.io/cerbos/cerbos:0.54.0-prerelease \
    compile /photo-share/policies
```

Let’s start the server and try out a request.

```shell
docker run -it -v $(pwd):/photo-share -p 3592:3592 ghcr.io/cerbos/cerbos:0.54.0-prerelease \
    server --config=/photo-share/.cerbos.yaml
```

## Writing tests to verify behaviour

It’s not practical to start the server and manually make requests every time a policy is updated. So let’s write some tests instead.

Create a new directory named `tests` and create a file named `album_object_test.yaml` with the following contents.

```yaml
---
name: AlbumObjectTestSuite
description: Tests for verifying the album:object resource policy
resources:
  alicia_private_album:
    id: "XX125"
    kind: "album:object"
    attr:
      owner: "alicia"
      public: false
      flagged: false

alicia_public_album:
    id: "XX525"
    kind: "album:object"
    attr:
      owner: "alicia"
      public: true
      flagged: false

alicia_flagged_album:
    id: "XX666"
    kind: "album:object"
    attr:
      owner: "alicia"
      public: true
      flagged: true

principals:
  alicia:
    id: "alicia"
    roles: ["user"]

bradley:
    id: "bradley"
    roles: ["user"]

maria:
    id: "maria"
    roles: ["moderator", "user"]

tests:
  - name: View album
    input: &testInput
      principals:
        - alicia
        - bradley
        - maria
      actions:
        - view
      resources:
        - alicia_private_album
        - alicia_public_album
        - alicia_flagged_album
    expected:
      - &viewExp
        principal: alicia
        resource: alicia_private_album
        actions:
          view: EFFECT_ALLOW

- <<: *viewExp
        resource: alicia_public_album

- <<: *viewExp
        resource: alicia_flagged_album

- <<: *viewExp
        principal: bradley
        resource: alicia_public_album

- <<: *viewExp
        principal: bradley
        resource: alicia_flagged_album

- <<: *viewExp
        principal: maria
        resource: alicia_public_album

- <<: *viewExp
        principal: maria
        resource: alicia_flagged_album

- name: Delete album
    input:
      <<: *testInput
      actions:
        - delete
    expected:
      - &deleteExp
        principal: alicia
        resource: alicia_private_album
        actions:
          delete: EFFECT_ALLOW

- <<: *deleteExp
        resource: alicia_public_album

- <<: *deleteExp
        resource: alicia_flagged_album

- <<: *deleteExp
        principal: maria
        resource: alicia_flagged_album
```

Now run the compiler.

```shell
docker run -it -v $(pwd):/photo-share ghcr.io/cerbos/cerbos:0.54.0-prerelease \
    compile /photo-share/policies
```

## Using schemas to enforce type safety [Optional]

The derived roles and resource policy rules we defined above rely on certain attributes being present in the `attr` sections of the incoming request. To ensure that API requests are strictly-typed and contain required attributes, we can define schemas for the principal and resource attributes sections.

Create a new directory named `_schemas` inside the `policies` directory.

```sh
mkdir policies/_schemas
```

Let’s add a JSON schema defining the data types and required fields for `album:object` resources. Create a file named `album_object.json` inside the `policies/_schemas` directory with the following contents:

```json
{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "type": "object",
  "properties": {
    "owner": {
      "type": "string"
    },
    "public": {
      "type": "boolean"
    },
    "flagged": {
      "type": "boolean"
    }
  },
  "required": [
    "owner"
  ]
}
```

Now update `policies/resource_album_object.yaml` to add the reference to the schema:

- actions: ['view', 'flag']
      effect: EFFECT_ALLOW
      roles:
        - user
      condition:
        match:
          expr: request.resource.attr.public == true

- actions: ['view', 'delete']
      effect: EFFECT_ALLOW
      derivedRoles:
        - abuse_moderator

schemas:
    resourceSchema:
      ref: cerbos:///album_object.json
```

Update `.cerbos.yaml` to enable schema enforcement.

```yaml
---
server:
  httpListenAddr: ":3592"

storage:
  driver: "disk"
  disk:
    directory: /photo-share/policies

schema:
  enforcement: reject
```

Now start the server again and send a request that does not conform to the schema. The server response should contain a list of validation errors.

```shell
docker run -it -v $(pwd):/photo-share -p 3592:3592 ghcr.io/cerbos/cerbos:0.54.0-prerelease \
    server --config=/photo-share/.cerbos.yaml
```

Invalid request

```shell
cat <<EOF | curl --silent "http://localhost:3592/api/check/resources?pretty" -d @-
{
  "requestId": "test02",
  "includeMeta": true,
  "principal": {
    "id": "alicia",
    "policyVersion": "default",
    "roles": [
      "user"
    ]
  },
  "resources": [
    {
      "actions": [
        "view"
      ],
      "resource": {
        "id": "XX125",
        "policyVersion": "default",
        "kind": "album:object",
        "attr": {
          "public": "false",
          "flagged": "false"
        }
      }
    }
  ]
}
EOF
```

```json
{
  "requestId": "test02",
  "results": [
    {
      "resource": {
        "id": "XX125",
        "kind": "album:object",
        "policyVersion": "default"
      },
      "actions": {
        "view": "EFFECT_DENY"
      },
      "validationErrors": [
        {
          "message": "missing properties: 'owner'",
          "source": "SOURCE_RESOURCE"
        },
        {
          "path": "/public",
          "message": "expected boolean, but got string",
          "source": "SOURCE_RESOURCE"
        },
        {
          "path": "/flagged",
          "message": "expected boolean, but got string",
          "source": "SOURCE_RESOURCE"
        }
      ],
      "meta": {
        "actions": {
          "view": {
            "matchedPolicy": "resource.album_object.vdefault"
          }
        }
      }
    }
  ]
}
```