What is an application server?.

Application server is software that provides business logic to client applications through various protocols, most often HTTP. It manages resources such as databases and message queues, ensuring scalability, reliability, and security.

Main functions of an application server:

• Request processing: Accepts requests from clients (browsers, mobile applications), directing them to the appropriate business logic code.
• Session management: Maintains information about the current user interaction with the application.
• Database connectivity: Facilitates interaction between the application and database management systems.
• Transaction management: Ensures atomicity, consistency, isolation, and durability (ACID) of data operations.
• Integration with other systems: Allows the application to interact with other services and APIs.
• Security: Implements authentication and authorization mechanisms, protecting against common web vulnerabilities.
• Scalability: Provides horizontal and vertical scaling capabilities to handle increasing load.

Examples of application servers for Ruby:

• Puma
• Unicorn
• Passenger
• Goliath

In the context of Ruby on Rails, an application server (e.g., Puma) handles HTTP requests, runs Rails code to generate responses, and sends them back to the client. The application server acts as an intermediary between the web server (e.g., Nginx or Apache) and the Rails framework.

# Example of a basic Rack application that can be run on an application server
class MyApp
  def call(env)
    # Handling request from env hash
    status = 200
    headers = { "Content-Type" => "text/plain" }
    body = ["Hello, world!"] # Response body as an array of strings

    [status, headers, body] # Return array [status, headers, body]
  end
end

# The application server will call the #call method on an object implementing the Rack interface.
# Example: puma MyApp.new

Difference between a web server and an application server:

Property	Web server (e.g., Nginx)	Application server (e.g., Puma)
Main function	Serves static files, proxies, load balancing	Executes business logic, works with dynamic content, manages resources
Content type	Static (HTML, CSS, images)	Dynamic (generated by application code)
Protocols	HTTP, HTTPS	HTTP, HTTPS, other protocols (depends on server)
Interaction with application code	Passes requests to application server	Executes application code

In a typical production environment, a web server (e.g., Nginx) is used as a frontend to serve static content and proxy dynamic requests to an application server (e.g., Puma), which executes Ruby on Rails code.