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Creating a web server¤

Now that know how to create and run a basic ASAB application, you can create your first web server.

Creating and running a web server¤

Set up a new project and create a new file app.py with the following code:

app.py
#!/usr/bin/env python3
import asab.web.rest

class MyWebApplication(asab.Application):

    def __init__(self):
        super().__init__()

        # Create the Web server
        web = asab.web.create_web_server(self)

        # Add a route to the handler method
        web.add_get('/hello', self.hello)

    # This is the web request handler
    async def hello(self, request):
        return asab.web.rest.json_response(
            request,
            data="Hello, world!"
        )

if __name__ == '__main__':
    app = MyWebApplication()
    app.run()

You can run the new application with the command:

python3 app.py

The ASAB web server is now available at http://localhost:8080/.

What is http://localhost:8080?

In case you don't know, localhost refers to the loopback network interface address of a device, usually represented as the IP address 127.0.0.1. It is used to refer to the device itself, allowing software to communicate with services running on the same device. In simpler terms, it's like talking to yourself within a computer to access and test applications or websites without going online.

:8080 refers to a port. A port is a communication endpoint in a computer network. It is represented by a numeric value, in this case 8080, and it allows applications and services to establish connections and exchange data. Ports enable the proper routing and delivery of network traffic, ensuring that information reaches the intended destination within a device or across different devices on a network.

Now open your web browser and open http://localhost:8080/. You'll see the error:

404: Not Found

That is correct, because the endpoint "/" is not handled by the router. But now, if you open http://localhost:8080/hello, you should see the response:

"Hello world!"

You can get the same result from a terminal using the cURL command:

curl --location 'localhost:8080/hello'

Taking a closer look¤

Let's examine the code to understand how the ASAB server is constructed.

Click on the icons to learn what each line of code means.

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#!/usr/bin/env python3
import asab.web.rest # (1)!

class MyWebApplication(asab.Application):

    def __init__(self):
        super().__init__() # (2)!

        # Create the Web server
        web = asab.web.create_web_server(self) # (3)!

        # Add a route to the handler method
        web.add_get('/hello', self.hello) # (4)!

    # This is the web request handler
    async def hello(self, request): # (5)!
        return asab.web.rest.json_response( # (6)!
            request,
            data="Hello, world!"
        )

if __name__ == '__main__':
    app = MyWebApplication()
    app.run()

  1. Let's start by importing the asab.web.rest module. Note that this enables calling functions from asab and asab.web modules.

  2. As you will see later, asab.Application has a lifecycle with three phases. This time, we have modified the initialization of the application. In order not to completely override the whole application initialization, call the super.__init__() method.

  3. The asab.web module provides a create_web_server() method that simplifies creation of the web server in the ASAB application. It returns an object, which is used as a router to which you can add new routes.

  4. With the add_get() method, you can define a new route that requests can be sent to. If you now access the web server with the path /hello, it will be handled by a hello() method. In other words, the hello() method is installed in the web server at the /hello endpoint with the GET HTTP method. Similar methods for PUT, POST, and DELETE methods exist, as we will see in the next tutorial.

  5. This is a handler method, which is called by the router when a GET request is send to a /hello endpoint. Every handler method must be a coroutine! That means, it has to be defined with async def keyword. You have to include the request argument, even if you (for some peculiar reason) don't want to use it in the function body. Otherwise it won't work together with the add_get() method.

  6. The asab.web.rest module provides a json_response() method that simply sends back any data you want to the client in JSON format. In this case, the output is just a single string, but it could be any JSON-serializable document.

As you can see, it is easy to create a fully functioning web server using ASAB, so that you can concentrate more on the application logic. asab is built on the top of the asyncio and aiohttp libraries which are designed to make the most out of non-blocking network operations.

Logging¤

At this point, let us also mention the basics of ASAB logging.

ASAB applications provide a structured logging tool, which is used to trace back useful information about various events during the application run-time.

The default configuration sends logs to the standard output, so you can see the logs directly in the terminal. If you want to add your a different logger or send logs to a different output, you need to change the configuration.

If you check the terminal where an ASAB application is running, you should see messages similar to these:

23-Jun-2023 08:08:44.683943 NOTICE asab.application is ready. # (1)
23-Jun-2023 08:18:04.116786 NOTICE asab.web.al [sd I="127.0.0.1" ...] # (2)

  1. This log informs you that the initialization of the application is finished. It means that configuration is loaded, logging is set up, the event loop is constructed, etc.

  2. This log is displayed every time a valid HTTP request is processed. If you deconstruct the message, you learn what request method was used, what application sent the request, the response status code, etc.

If you stop the application using Control+C, another log shows up:

23-Jun-2023 08:32:23.292862 NOTICE asab.application is exiting ... #(1)!
  1. This log informs you that the application is in exit-time. Note that there may be processes that take a long time to terminate, so terminating an application might take a noticeably long time.

ASAB uses a formatted logger on top of the standard logging library provided with utilities such as

  • Colors indicating the log level
  • Structured data formatting
  • Log rotation
  • Logging to syslog

Configuration¤

In order to start your ASAB application with certain configuration, create a config file in ini format.

config.ini
[web]
listen=0.0.0.0:7000

[logging]
level=DEBUG

[logging:file]
path=./log.txt

Now if you run the application with this configuration file: 

python app.py -c config.ini
You should see some differences:

  • Now, your web server is running on http://localhost:8000
  • Logging is set to another level and it is stored in a file log.txt

Finally, go to the next tutorial where we create a microservice with REST API.