spaceapi-server/README.md

# SpaceAPI Server

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A lightweight server for [SpaceAPI][spaceapi] endpoints. Includes
support for pluggable templating, so dynamic content, like sensor
values, can be added.

## Dependencies

- Python 3 (>=3.6)
- [Bottle][pypi-bottle]
- [PyYAML][pypi-yaml]

## License

[MIT License][mit]

## Introduction

This project is an attempt to implement a lightweight, yet versatile
SpaceAPI endpoint server.  In its simplest configuration, it just
serves a plain, static, boring JSON document.

In order to provide dynamic content (e.g. whether your space is
currently open), you can replace parts of the YAML document (anything
except object keys) with custom plugin invocations.  These plugins
look up and return your dynamic content.

<table>
<thead>
<tr>
<td>Input</td>
<td>Output</td>
</tr>
</thead>
<tbody>
<tr>
<td>

```yaml
---
api: "0.13"
api_compatibility: ["14"]
space: My Hackerspace
# This is a plugin invocation
# with no arguments
state: !space_state {}
sensors:
  # This is a plugin invocation with
  # arguments.  They are passed to the
  # plugin function as kwargs.
  network_connections: !network_connections
    networks: [ "2.4 GHz", "5 GHz" ]
```

</td>
<td>

```json
{
  "api": "0.13",
  "api_compatibility": ["14"],
  "space": "My Hackerspace",
  "state": {
    "open": true,
    "lastchange": 1575160777,
    "message": "Visitors Welcome!"
  },
  "sensors": {
    "network_connections": [
      {
        "value": 4,
        "type": "wifi",
        "name": "2.4 GHz"
      },
      {
        "value": 7,
        "type": "wifi",
        "name": "5 GHz"
      }
    ]
  }
}
```
</td>
</tr>
</tbody>
</table>

## Usage

### 0. Download

Head over to the [Releases][releases], download and install the
package that suits your needs.  Alternatively, clone the repo and get
started.  There also is a Container Image available through the
[Gitlab registry][registry] tagged as
`registry.gitlab.com/s3lph/spaceapi-server`.

The remainder of this document assumes that you installed the
server as an OS distribution package.

### 1. Overview

The configuration of this server consists of three parts:

- The **main configuration** file, usually located at
  `/etc/spaceapi-server/config.yaml`.  This file controls all the
  internal settings of the server.
- The **response template** file, located at
  `/etc/spaceapi-server/template.yaml`.  This file defines the content
  served by your sever.
- The **plugins** directory, located at
  `/etc/spaceapi-server/plugins/`.  Here you can put your plugins for
  rendering dynamic content.

### 2. Configure the Server

Open the file `/etc/spaceapi-server/config.yaml`.

The following options are currently available:

```yaml
---
# The address to listen on.
address: "::1"
# The TCP port to listen on.
port: 8000
# The Bottle backend server to use.
server: wsgiref
# Path to the SpaceAPI response template file.
template: template.yaml
# Path to the directory containing your plugins.
plugins_dir: plugins
plugins:
  # Plugin-specific configuration should go in here, separated by plugin
  my_plugin:
    my_option: "Hello, World!"
    my_other_option: [ 42, 1337 ]
```

### 3. Configure a Static SpaceAPI Endpoint

Open the file `/etc/spaceapi-server/template.yaml`.  By default it
contains a minimal example response.  If you only want to serve static
content, your `template.json` should simply contain the SpaceAPI JSON
response you want to serve.

The content is served "almost-as-is" (apart from the conversion from
YAML to JSON).

To learn about how a SpaceAPI response should look like, have a look
at the [SpaceAPI Website][spaceapi].

### 4. Add Dynamic Content

This example guides you through adding a dynamic `state` property to
your SpaceAPI endpoint.  We'll use the following (rather simple, and
probably not too useful) data source: Check a certain file, and mark
the space as open depending on its existence.

1. Create a plugin to fetch the data.  Let's name it `filestate.py`
   and put in in our plugins directory:

    ```python
    import os
    from spaceapi_server import config, plugins

    @plugins.template_function
    # The plugin can be invoked by using the !space_state YAML tag
    def space_state():
        # Get the plugin config dict
        conf = config.get_plugin_config('filestate')
        # Get the filename
        filename = conf.get('filename', '/var/space_state')
        try:
            # Get the file's properties
            stat = os.stat(filename)
        except FileNotFoundError:
            # File doesn't exist, aka. space is closed
            return {
                'open': False
            }
        # File exists, aka. space is open.  Also report the mtime as "last changed" timestamp
        return {
            'open': True,
            'lastchange': int(stat.st_mtime)
        }
    ```

    The `@template_function` decorator registers a constructor in
    PyYAML's parser with the function's name as tag
    (e.g. `!space_state).

2. Call the template function in your template:

    ```yaml
    # ...
    state: !space_state
    # ...
    ```

3. Configure the server:

    ```yaml
    # ...
    template: template.yaml
    plugins_dir: plugins
    plugins:
      filestate:
        filename: /var/space_state
    # ...
    ```

### 5. Start the Server

Start the server with e.g.:

```bash
systemctl start spaceapi-server.service
```

To reload the configuration, template and plugins, send a SIGHUP,
e.g. through `systemctl reload`.

If you need to run the server ad-hoc, you can start it with:

```bash
python3 -m spaceapi_server /path/to/config.yaml
```
### 6. Test the Server

```bash
curl http://localhost:8000/
```

You should be greeted with the SpaceAPI endpoint response.

## Plugin API Reference

### Configuration

The following functions provide access to values defined in the
configuration file.

#### `spaceapi_server.config.get_plugin_config(name: str)`

This function returns a plugin's configuration.

The function takes one argument, the name of the plugin.  This name is
used to look up the plugin configuration.

The function returns the content present at the key `.plugins.<name>`
of the global configuration file, or an empty object if absent.

Usage:

```python
from spaceapi_server import plugins

print(plugins.get_plugin_config('my_plugin'))
```

### Templating

The following decorators register a function for use as a PyYAML
constructor, so they become usable from within templates.  They are
invoked when the template is rendered, which happens for each HTTP
request.

If performance is an issue, consider applying caching, either in your
plugins, or by using a caching HTTP reverse proxy.

#### `spaceapi_server.plugins.template_function`

This decorator registers the function's name as a YAML tag in the parser.

The decorated function may take arguments (always passed as
`**kwargs`, so `*args`, or arguments before `*` won't work) that can
be represented in a YAML file.

The decorated function may return any value that can be serialized
into JSON.  This includes objects and arrays.

Usage:

```python
from spaceapi_server import plugins

@plugins.template_function
def lookup_sensor(query, default=None):
    # Do something with the query
    result = ...
    # If the lookup failed, return a default value
    if not result:
        return default or []
    return result
```

```yaml
# ...
state: !lookup_sensor
  query: SELECT timestamp, value FROM people_now_present LIMIT 1
# ...
```


[main]: https://gitlab.com/s3lph/spaceapi-server/commits/main
[releases]: https://gitlab.com/s3lph/spaceapi-server/-/releases
[spaceapi]: https://spaceapi.io/
[pypi-bottle]: https://pypi.org/project/bottle/
[pypi-yaml]: https://pypi.org/project/PyYAML/
[mit]: https://gitlab.com/s3lph/spaceapi-server/blob/main/LICENSE
[jinja]: https://jinja.palletsprojects.com/
[registry]: https://gitlab.com/s3lph/spaceapi-server/container_registry