wsgi-shim 0.0.3

WSGI shim for when you have limited web log access

wsgi-shim

This repository contains files to set up and configure a Python-based website using the WSGI protocol for infrastructure that uses Phusion Passenger. While the anticipated audience are folks using "project webspace" in the Department of Computer Science at Princeton University, this package may have broader utility.

This package provides a shim between Phusion Passenger and the WSGI application that performs some file system checks at start-up. If any checks fail, a page request to the website will return an error page with feedback rather than a generic message from the webserver with error details going to a system log. The benefit is that in some system configurations, developers do not have direct real-time access to server logs.

Additionally, this package provides a wsgi-shim command that can set up a correct directory structure.

The following sections describe the package in general. Additional details specific to Princeton CS can be found at CS Guide: Python-based Websites Using WSGI.

Infrastructure Assumptions

• Developer has no direct access to the webserver machine and no direct access to webserver logs.
• Files are stored on a shared file system
• Developer has shell access to a shared machine that can access the shared file system
• Multiple virtual hosts are served by the webserver. Files are segregated using POSIX permissions
• The webserver runs Passenger Phusion and can run processes as you
• The webserver initially runs under a private username and group

⚠️ Security Considerations

File Permissions

The webserver will run your Python code with your UID/NetID (i.e., "as you") and with a GID/group (to which you belong) of your choosing. This has the benefit that you can include secrets/credentials in files that are not world-readable. A consideration to remember is that because the webserver will run "as you", your running code will have access to any project space files (in any project) that you have access.

Note that before running "as you," the webserver is running as a user/group that can only read certain configuration/control files if they are world-readable. These files are noted in the documentation below.

Environment Variables

If your shell is bash, all the environment variables you set up in your .bashrc and .bash_profile will be available to your running Python code. (See: the default "on" configuration of PassengerLoadShellEnvvars.) This can obviously be handy; however, there are some downsides. First, from a security perspective, environment variables containing secrets can be exposed to other running processes on the shared webserver. Second, from a configuration management perspective, having some of the configuration for your site be in your personal .bashrc file in your home directory and the rest of the configuration in the project space for the site may lead to issues when you are ready to hand the reins over to another person.

When the webserver runs your .bashrc and .bash_profile, it will have set the environment variable IN_PASSENGER. If you want the webserver to effectively skip your normal Bash configuration, you can put this at the top of your .bashrc and .bash_profile files:

if [ -n "$IN_PASSENGER" ]; then
    return 0
fi

As an alternative, you can pass environment variables and secrets to the webserver application, you can use the [environment] and [secret_files] sections in the config.toml file as described below.

Requesting Resources / Initial Set-up

The step-by-step instructions in this section assume you are starting from scratch. If you already have some of the resources that you wish to use, adapt these instructions as appropriate.

📝 Note: You won't have direct access to web server logs for debugging. Therefore, it's important to be methodical and test along the way. The wsgi-shim package can catch common configuration mistakes and give feedback without access to the web server logs.

1. Request "project disk space"

From your sysadmin, request project disk space and a new POSIX group for the files.

In these steps, we will assume the disk space is at /n/fs/myproject, the POSIX group is mygroup, and your username is mynetid.

2. Verify/set permissions on the project space

After receiving notification that the disk space has been created, verify that it exists with the correct group and permissions.

Verify project space permissions:

ls -ld /n/fs/myproject

The output should look like something like this:

drwxr-sr-x. 4 mynetid mygroup 59 Sep 20 13:24 /n/fs/myproject

If the owner is not mynetid (i.e., your username), contact support.

The group should not be your default group (i.e., the one displayed when you run the id -gn command). If it is your default group, contact support.

If the permissions are not drwxr-sr-x (note the lowercase "s"), change them with chmod 2755 /n/fs/myproject. The lowercase "s" in this position is the SETGID bit and ensures that files and subdirectories created under /n/fs/myproject will belong to the mygroup group by default.

3. Build Python

Build a local-to-your-website version of Python 3.11 or greater. You will build this in the same directory that is shared with the webserver. Note that the webserver machine and your access machine must be configured such that Python built on the access machine will work on the webserver (same architecture, same OS, same shared libraries, etc.).

These are abbreviated instructions from the CS Guide: Building a Local Version of Python. To build Python 3.11.5 and install it in your project space at /n/fs/myproject/python-3.11.5 use these commands (adjusting PROJECT_DIR and PYVER as appropriate):

PROJECT_DIR="/n/fs/myproject"
PYVER="3.11.5"
cd $PROJECT_DIR
mkdir temp
cd temp
wget https://www.python.org/ftp/python/$PYVER/Python-$PYVER.tar.xz
tar xJf Python-$PYVER.tar.xz
cd Python-$PYVER
./configure --prefix=$PROJECT_DIR/python-$PYVER
make
make install
cd /n/fs/myproject
rm -rf temp

Test the Python installation:

/n/fs/myproject/python-3.11.5/bin/python3 --version

Output should be:

Python 3.11.5

4. Create Website Directory and Build Python Virtual Environment

Create the directory to hold the files associated with your website. This directory must be world-readable but not world-writeable:

mkdir /n/fs/myproject/mywebsitefiles
chmod o=rx /n/fs/myproject/mywebsitefiles

Build the Python virtual environment for your website in venv.

cd /n/fs/myproject/mywebsitefiles
/n/fs/myproject/python-3.11.5/bin/python3 -m venv venv

📝 Note: This is the Python virtual environment that will be used for the running website. This is, for example, where you would "pip install" packages such as Flask or Django.

5. Install wsgi-shim and Create Initial Configuration

Activate your virtual environment, install the wsgi-shim package, and create the initial PassengerAppRoot directory:

cd /n/fs/myproject/mywebsitefiles
source venv/bin/activate
pip install wsgi-shim
wsgi-shim install /n/fs/myproject/mywebsitefiles/www
deactivate

At this point, your project space directory structure will look like this:

/n/fs/myproject
├── python-3.11.5
│   └── ...
└── mywebsitefiles
    ├── venv
    │   └── ...
    └── www
        ├── config.toml
        ├── passenger_wsgi.py
        └── tmp
            └── restart.txt

6. Request "project web space"

With the directory structure created, it is time to request that your sysadmins configure the webserver to recognize your domain (e.g., mywebsite.example.com) as a valid website and route its requests to your Python code via the provided passenger_wsgi.py file.

You will need to provide the key/value pairs in the [passenger] section of the /n/fs/myproject/mywebsitefiles/config.toml file created in the previous step to your sysadmins.

7. Test Initial Configuration

When you get verification from your sysadmins that the project web space is set up, verify that the DNS name for the website is in place:

host mywebsite.example.com

If you get a response indicating "not found," wait an hour and try again. If at that point you are still getting a "not found" indication, follow-up with your sysadmins.

Open a browser and visit https://mywebsite.example.com

You should see a "Hello, World" page. If so, congratulations! You're ready to start installing and developing code as described in the next section.

If you are not getting the "Hello, World" page, carefully review your steps and also refer to the Troubleshooting section, below.

Install / Develop Site-Specific Code

Directory Structure

Other than being readable by the user/group specified in the config.toml file, the webserver doesn't put specific restrictions on where in project space the code implementing your web application must go. One possibility is the following layout:

/n/fs/myproject
├── python-3.11.5
│   └── ...
└── mywebsitefiles
    ├── venv
    │   └── ...
    ├── www
    │   ├── config.toml
    │   ├── passenger_wsgi.py
    │   └── tmp
    │       └── restart.txt
    ├── secrets
    │   └── credentials.env
    ├── log
    │   └── logfile
    └── myapp
        ├── file1.py
        ├── file2.py
        └── ...

In this example, the secrets directory contains credentials and, as such, should not be world-readable; the myapp directory contains the Python code making up the web application. The myapp directory might reasonably be a checked out Git repository of your code.

The next section describes how to edit your config.toml file to point to your application in myapp.

Configuration Settings

The config.toml file created by the wsgi-shim install contains the documentation on available settings. Here is an example using the directory structure above with the application callable defined in file1.py:

[passenger]
user = "mynetid"
group = "projectgroup"
passenger_app_root = "/n/fs/myproject/mywebsitefiles/www"
passenger_python = "/n/fs/myproject/mywebsitefiles/venv/bin/python"
[wsgi]
module = "/n/fs/myproject/mywebsitefiles/www/myapp/file1.py"
app = "app"
[secret_files]
SECRETS = "/n/fs/myproject/mywebsitefiles/secrets/credentials.env"
[environment]
LOG_FILENAME = "/n/fs/myproject/mywebsitefiles/log/logfile"

Logging

Under the assumption that you don't have ready access to the webserver logs, it is important to set up logging as early as possible in your application.

Flask Example

Here is an example Flask application that would work with the example config.toml shown above.

# /n/fs/myproject/mywebsitefiles/www/myapp/file1.py
import logging.config
import os

from flask import Flask

# Configure logging before instantiating Flask app
if LOG_FILENAME := os.environ.get('LOG_FILENAME'):
    logging.config.dictConfig({
        'version': 1,
        'formatters': {'default': {
            'format': '%(asctime)s %(levelname)s %(name)s '
                      '%(threadName)s : %(message)s',
        }},
        'handlers': {'file': {
            'class': 'logging.handlers.RotatingFileHandler',
            'formatter': 'default',
            'filename': LOG_FILENAME,
            'maxBytes': 40000,
            'backupCount': 2,
        }},
        'root': {
            'level': 'INFO',
            'handlers': ['file'],
        }
    })

app = Flask(__name__)


@app.route("/")
def hello_world():
    app.logger.info('Request: /')
    return "<p>Hello, World!</p>"

Operation

The webserver will start your application when it receives a request to a URL in https://mywebsite.example.com. This process will generally handle multiple requests. After a period of inactivity, the webserver will kill the process until there is another request.

Restarting Your Application

Phusion Passenger allows you to force a restart of your application by updating the timestamp of the restart.txt file:

touch /n/fs/myproject/mywebsitewsgi/www/tmp/restart.txt

Note that the restart is not immediate and the file is only checked when a URL is visited on your site but no more often than every 10 seconds (the value of PassengerStatThrottleRate in the webserver global configuration)

Using Maintenance Mode

The provided passenger_wsgi.py file (via uwsgi-shim install) implements a mechanism that, when enabled, will not load your website code but, instead, show a "maintenance" page where all requests to the site return a 503 Service Unavailable response. In this mode, your site-specific code (e.g., based on Django, Flask, etc.) will not be accessed.

To enable maintenance mode, create the sentinel file:

touch /n/fs/myproject/mywebsitewsgi/www/tmp/maint.txt

To disable maintenance mode, delete the sentinel file:

rm /n/fs/myproject/mywebsitewsgi/www/tmp/maint.txt

Note that the existence of this file is only checked when the WSGI server (e.g., Phusion Passenger) restarts the application. Therefore, anytime you enable/disable maintenance mode, you need to force a restart of the application. For example, to update the code for a website, one might follow these steps:

1. Run: touch /n/fs/myproject/mywebsitewsgi/www/tmp/maint.txt
2. Run: touch /n/fs/myproject/mywebsitewsgi/www/tmp/restart.txt
3. Reload the site a few times until it shows the maintenance page. This can take up to 10 seconds.
4. Update the site
5. Run: rm /n/fs/myproject/mywebsitewsgi/www/tmp/maint.txt
6. Run: touch /n/fs/myproject/mywebsitewsgi/www/tmp/restart.txt
7. Reload the site a few times until it shows the updated content.

Note that the restart.txt file must be world-readable. If you delete this file and later recreate it, be sure it is world-readable.

Troubleshooting

• Check that your website host name is resolvable in DNS:

    host mywebsite.example.com
  

• Use the wsgi-shim check command to verify directory structure and permissions:

    cd /n/fs/myproject/mywebsitefiles
    source venv/bin/activate
    wsgi-shim check /n/fs/myproject/mywebsitefiles/www
    deactivate
  

• Verify that your .bashrc and .bash_profile are not exiting when invoked by the webserver (i.e., when IN_PASSENGER is set)?

• If you have set up logging, check your logs.

• If your application writes to the file system, verify that you haven't run out of space.

If you believe there is a key bit of information in the webserver logs, contact your sysadmins and specify the URL that is not working along with a specific timestamp.

Development

For those developing or maintaining the wsgi-shim package itself, be sure to install it with the [dev] option to pull in packages used when developing.

pip install --editable .[dev]

When developing, this package uses pre-commit. After the initial clone of the repository, you will need to set up pre-commit with:

# in the top level of the checked-out repository:
pre-commit install

Changelog

0.0.3 released 2023-10-14

• Fixed disconnect between generated passenger_wsgi.py and this module

0.0.2 released 2023-10-13

• Documentation is more site agnostic
• Additional tests

0.0.1 released 2023-10-05

• Initial Beta Version