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Python bindings to a JavaScript environment

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# python-js-host

[![Build Status](https://travis-ci.org/markfinger/python-js-host.svg?branch=master)](https://travis-ci.org/markfinger/python-js-host)

Python bindings to [js-host](https://github.com/markfinger/js-host), enabling you to send requests and data 
from your python process to a performant JavaScript environment.

There are a variety of libraries which provide low-level access to JS engines - PyExecJS et al - but they 
tend to suffer from limited functionality and poor performance. Rather than relying on basic evaluation
of JavaScript, this library enables you to configure a persistent JS processes which responds to data sent
from your python process.

To reduce the cost of integrating yet another technology, a [manager](#jshostmanager) process is provided 
which can automatically spawn js-host instances in the background.

Documentation
-------------

- [Installation](#installation)
- [Quick start](#quick-start)
- [Settings](#settings)
- [Usage in development](#usage-in-development)
- [Usage in production](#usage-in-production)
  - [Logging](#logging)
  - [Caching](#caching)
    - [Caching requests](#caching-requests)
- [API](#api)
  - [Function](#function)
  - [JSHost](#jshost)
  - [JSHostManager](#jshostmanager)
    - [Under the hood](#under-the-hood)
    - [Quirks](#quirks)
    - [Reading logs](#reading-logs)
- [Running the tests](#running-the-tests)

Installation
------------

```bash
pip install js-host
```

js-host requires access to a [Node.js](https://nodejs.org) or [io.js](https://iojs.org/) binary.

On OSX, you can run `brew install node`. On Linux, you can install node with your package manager.

Quick start
-----------

Create a `package.json` file with

```bash
npm init
```

Install the `js-host` JavaScript library with

```bash
npm install --save js-host@0.12
```

Create a file named `host.config.js` and insert

```javascript
module.exports = {
  functions: {
    hello_world: function(data, cb) {
      var name = data.name || 'World';
      cb(null, 'Hello, ' + name + '!');
    }
  }
};
```

Open a python shell and run

```python
from js_host.conf import settings

settings.configure(USE_MANAGER=True)
```

If everything went ok, you should see some output as the manager process spins up and then spawns a host 
which uses your `host.config.js` file.

In the same python shell, run

```python
from js_host.function import Function

hello_world = Function('hello_world')

hello_world.call()

hello_world.call(name='Foo')
```

Settings
--------

Settings can be defined by importing `js_host.conf.settings` and calling the `configure` method
with keyword arguments matching the setting that you want to define. For example

```python
from js_host.conf import settings

settings.configure(
    SOURCE_ROOT='/path/to/your/project',
    USE_MANAGER=True,
)
```

If you are using js-host in a Django project, add `'js_host'` to your `INSTALLED_APPS`

```python
INSTALLED_APPS = (
    # ...
    'js_host',
)
```

And configure js-host by placing a dictionary named `JS_HOST` into your settings files

```python
JS_HOST = {
    'SOURCE_ROOT': '/path/to/your/project',
    'USE_MANAGER': DEBUG,
}
```

### SOURCE_ROOT

An absolute path to the directory which contains your node_modules directory.

Default: `os.getcwd()  # Your current working directory`

### CONFIG_FILE

A path to the default config file used for hosts and managers.

If the path is relative, it is appended to `SOURCE_ROOT`.

Default: `'host.config.js'`

### USE_MANAGER

Indicates that a manager should be used to spawn host instances.

DO *NOT* USE THE MANAGER IN PRODUCTION.

Default: `False`

### PATH_TO_NODE

A path to a `node` binary.

Default: `'node'`

### PATH_TO_BIN

A path to the `js-host` binary used to control hosts and managers.

If the path is relative, it is appended to the SOURCE_ROOT setting

Default: `os.path.join('node_modules', '.bin', 'js-host')`

### FUNCTION_TIMEOUT

Indicates how many seconds `Function` objects will wait for a response before
raising exceptions.

Default: `10.0`

### CONNECT_ONCE_CONFIGURED

Indicates that once this library has been configured, it should attempt to connect to a
host.

This setting enables any config or connection errors to be raised during startup rather 
than runtime. It also enables connections to managed hosts to be preserved between 
restarts of your python process.

If you want to run multiple hosts and/or control the connection process, set this to
`False`, but be aware that managed hosts may not preserve connections when your python
process restarts.

Default: `True`

### ROOT_URL

Overrides the root url which requests are sent to. By default, the root url is inferred from 
the config that the host generates from your config file and the js-host defaults.

If you want to route requests manually to a host, set it to a string such as 
`'http://127.0.0.1:9009'`.

Default: `None`

### VERBOSITY

Indicates how much information the host should print the terminal. By default the library
will print to the terminal whenever processes are started or connected to.

If you want to suppress all output, set it to `js_host.utils.verbosity.SILENT`.

Default: `js_host.utils.verbosity.PROCESS_START`

Usage in development
--------------------

In development, you can take advantage of the manager process to abstract away the overhead of 
starting and stopping processes. To use a manager to spawn hosts, set the `USE_MANAGER` setting
to `True`.

If you are writing functions that are on a host, you are recommended to start hosts manually, as 
it will provide more immediate feedback, as well as easier control of a process. To start a host 
manually, refer to [js-host's CLI usage](https://github.com/markfinger/js-host#cli-usage).

Usage in production
-------------------

In a production environment, you are strongly recommended to **not** use the manager. 
Ensure that the `USE_MANAGER` setting is set to `False` before you start your python process.

In a production environment, you should run your hosts under a supervisor system, such as
[supervisor](http://supervisord.org/) or [PM2](https://github.com/Unitech/pm2). You can refer 
to [js-host's CLI usage](https://github.com/markfinger/js-host#cli-usage) for the necessary 
incantation to spin up a process. It will generally boil down to something like

```bash
node_modules/.bin/js-host host.config.js
```

### Logging

By default, js-host instances will only write their logs to stdout and stderr. To log to files,
refer to js-host's [documentation on logging](https://github.com/markfinger/js-host#logging).

### Caching

js-host does not have its own caching layer, but an upstream caching layer can be implemented 
easily. All communication between the python processes and the js-host processes is performed 
via HTTP, so placing a reverse proxy such as [varnish](https://www.varnish-cache.org/) between 
the two can massively boost the performance of your requests.

By default, the python layer will infer a host's url from the host's config. If you want to route
all requests through another address, define the `ROOT_URL` setting. For example

```python
js_host.conf.settings.configure(
    # ...
    ROOT_URL='http://127.0.0.1:8000',
)
```

The python layer will now send all requests through `http://127.0.0.1:8000`, rather than connecting
directly to the host.

#### Caching requests

Requests are sent to hosts according to js-host's 
[endpoint definition](https://github.com/markfinger/js-host#endpoints).

When the python layer sends requests to functions, it appends a `hash` paramater to the url 
which is a `sha1` hash of the serialized data sent to the function. For example, a request to a 
function named `hello_world` with the data `{'foo': 'bar'}` will be sent as:

```
POST: /function/hello_world?hash=bc4919c6adf7168088eaea06e27a5b23f0f9f9da
```

This url convention enables you to easily cache a specific function's output by the data that 
was sent in.

API
---

### Function

`js_host.function.Function` objects enable you to pass data from your python process to a JavaScript 
environment. Functions must be instantiated with a string which matches a name specified in your config 
file.

If your `host.config.js` file resembled the following

```javascript
module.exports = {
  functions: {
    greeter: function(data, cb) {
      cb(null, 'Hello');
    },
    double: function(data, cb) {
      if (!data.number) {
        return cb(new Error('No number was provided');
      }
      cb(null, data.number * 2);
    }
  }
};
```

You can call those functions from Python with the following

```python
from js_host.function import Function

greeter = Function('greeter')

greeter.call()  # returns 'Hello'

double = Function('double')

double.call(number=20)  # returns '40'

double.call()  # raises a error 'No number was provided'
```

Keyword arguments provided to `call` are passed to the host as JSON. The host deserializes the
data sent, and then passes the data to your function as its first argument. The second argument 
to your function is a callback which allows you to pass data back to the python process.

Your functions can complete their task either synchronously or asynchronously. Once the callback 
has been called, the host assumes that the function has completed and sends a response back to 
the Python process.

Functions will lazily bind to the `js_host.host.host` singleton unless you override the function's `host`
`attribute`.

```python
from js_host.function import Function

greeter = Function('greeter')

greeter.host  # returns `None`

greeter.get_host()  # returns `js_host.host.host`

greeter.host = my_host

greeter.get_host()  # returns `my_host`
```

For more information on the API and behaviour of functions, refer to the js-host's
[documentation on functions](https://github.com/markfinger/js-host#functions).

### JSHost

`JSHost` objects read in your config files and act as bridges to JavaScript environments generated
from your config files.

If you want to introspect a host, there are some utils provided

```python
# Import the host generated from the values in `js_host.conf.settings`
from js_host.host import host

# An absolute path to the config file used by the host
host.config_file

# Returns a URL which points to the location of the host on your network
host.get_url()

# Returns a dict containing information that the host reported during startup
host.get_status()

# Connects to a running host, requests information about it, and returns the data as a dict
host.request_status()

# Returns True/False if a host process is running at the expected url
host.is_running()

# Connect to an environment and validate that it matches your config
host.connect()
```

If you are using the manager to control your hosts, the following utils are also available

```python
from js_host.host import host

# the JSHostManager instance that the host uses
host.manager

# An absolute path to a file that the host writes its logs to
host.logfile

# Connect to the manager and ask it to restart the host
host.restart()

# Connect to the manager and ask it to stop the host
host.stop()
```

### JSHostManager

`js_host.manager.JSHostManager` objects provide an interface to a detached process which runs 
on your local network and can spawn [js-host](https://github.com/markfinger/js-host) instances on 
demand.

The primary benefit of using a manager is that it abstracts away the bother of spawning a host 
manually. During development, your primary goal is to build things - issues regarding stability
and logging should be left to production environments.

To allow a manager to spawn instances automatically, set the `USE_MANAGER` setting to `True`.

If you are writing functions to be used on hosts, be aware that manually starting hosts will provide
easier access to the hosts output streams and debugging functionality. Refer to
[js-host's CLI usage](https://github.com/markfinger/js-host#cli-usage) to manually start a host.

Note: managers should only ever be used in development environments. Do **not** use the manager 
in production. Please refer to the [usage in production](#usage-in-production) section before 
configuring your environment.

```python
# The singleton manager and host which are provided by default
from js_host.host import manager, host

host.manager == manager  # True

# Stops the manager and all managed hosts
manager.stop()
```

#### Under the hood

Managers are spun up via a child process of your python process. The child process blocks python
until the manager has been spawned in a third process which is completely detached from python. 
Using a detached process allows the manager and its hosts to persist even when your python process 
has restarted or exited.

Once a manager is running, it starts listening for incoming requests which indicate that your
python process wants it to spawn a new js-host using a particular config file. Once the new 
instance has been spawned, the python process asks the manager to register a new connection 
to the host.

When your python process exits, it sends a disconnect signal to the manager, notifying it 
that any hosts spawned by your python process are no longer required. When a spawned host no 
longer has ony open connections, the manager waits for 5 seconds to allow any new connections 
to be opened. If the time period expires and no new connections have been opened, the manager 
will stop the host's process.

When all of a manager's hosts have been disconnected and stopped, the manager will stop its own 
process. This enables the manager to clean up after itself, and prevents rogue processes from
running in the background.

The connect/disconnect method enables your python process to hook in to persistent JS environments 
which survive even when your python has restarted. Maintaining persistent environments enables 
you to integrate JS technologies which have a high startup cost that should only be incurred 
once. A typical use-case where this optimisation provides massive performance improvements is 
integrating a compiler - such as webpack or browserify - which have a sizable startup overhead 
as they parse all of your files.

#### Quirks

Be aware that managers introduce some behaviour that you should be aware of:

- Python processes will connect to managed hosts based on the path to your config file. If you
  open a python shell which triggers a connection to a host, the manager will not stop the host
  until your shell has exited.

If you are running a server and a python shell which have both connected to the same host, the 
  manager will not stop the host until both processes have exited.
- Managed hosts can continue running after their config file has changed.
  
  To force a restart, call the `restart` method of a managed `JSHost` instance. For example
  ```python
  from js_host.host import host
  host.restart()
  ```
- Managers and managed hosts run in processes detached from your shell, which can make it difficult 
  to inspect their output streams.

To provide some measure of introspection, managed hosts write their output to logfiles which are 
  stored in your temporary directory. The `logfile` attribute of a `JSHost` instance provides an 
  absolute path to the file. For example
  ```python
  from js_host.host import host
  host.logfile
  ```
- If a managed host encounters an unhandled exception, the host will crash and the python process 
  will raise errors until the host is respawned. By design, hosts will not respawn until you have 
  restarted the python process.

Preventing automatic respawning of crashed hosts simply hides the underlying issue, and makes it
  difficult to reason about a system's state.
  
  If a crash is detected, exceptions will be raised indicating that you should consult the host's
  logfile to inspect the stack traces produced during the unhandled exception.

#### Reading logs

If you want to inspect the log output of a managed host, a path to the host's logfile is accessible via

```python
from js_host.host import host
print(host.logfile)
```

The following one liner will start (or connect to) a managed host and then `tail -f` its logfile.

```bash
$ python -c "from js_host.conf import settings; settings.configure(USE_MANAGER=True); from js_host.host import host; import subprocess; subprocess.call(['tail', '-f', host.logfile])"
```

Running the tests
-----------------

```bash
pip install -r requirements.txt
cd tests
npm install
cd ..
./runtests.sh
```

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