Web Servers

Examples

Apache/httpd with CGI (Common Gateway Interface)

nginx

Node.js

Web Servers

Web Servers are a part of the internet architecture.

Web Server = server hardware + server software

Server Hardware Computer connecter to the internet / any network

Server Software program that accepts HTTP requests over a specific port and responds

Client ↔\lrarr Server

  1. Client (Browser or other Servers / Devices)

    https://www.google.at → Domain Name System DNS → 172.217.23.227 → HTTP Request for Web Resource

  1. Multiple Layers and Proxies (see traceroute)

    Caching, filtering, load balancing... by routers, modems,...

  1. Server

    HTTP Response with web resource or error

    Can be a collection of servers, sharing the load

Static vs. Dynamic Resources

Web page = hypertext document

parts of displayed text are links to other pages of the Web (nodes in hypertext)

Web Server System output is treated as the complete HTTP response (including headers).

  1. Static Resources

    Sent through the filesystem

    assigned only once during the loading of the page

    Static assets from file system mapped to URL path

    They cannot react to other part of the request (like query parameter)

    MIME-Type is usually inferred (file endings) - But headers contain data about the data types.

    Common static files in web servers

    • HTML, CSS
    • JavaScript (for use in browser)
    • Media (Images, Video, Audio, etc.)

  1. Dynamic Resources

    Executing programs inserver side language

    Can react to complete HTTP request (including header information)

    • Path and Query Parameters
    • HTTP Method (GET, POST, PUT, …)
    • Content Negotiation (Accept: application/json) …

    Links to other websites (hyperlinks)

    does not actually lookup the resource until asked for the value

Hardware

Can be your own computer ( localhost,127.0.0.1 )

Contemporary web servers:

  • Part of large data centres
  • Latency based on location → often geographically distributed (e.g., using DNS)
  • Virtual servers: 1 Physical servers can host many virtualized web servers

Software

Program running on a server

  • Maps path of URL to static asset on the file server
  • dynamically rendered resources
  • usually has some functionality for caching and session handling

Geo DNS

"geo-location based Serving" = Serving resources from geographically closer data centres

Possible through Domain Name System DNS.

DNS:={Hostname,’Location’}↦IP \textsf{DNS} := \{ \textsf{Hostname, 'Location'}\} \mapsto \textsf{IP} 

They determine and return IP addresses based on location.

  1. (local) LDNS

    The lookup table provided by the internet service provider ISP.

    Accessed first, if not found, then request sent to global DNS.

  1. (global) DNS

Virtual Servers and Containers

One physical server can host multiple virtual servers and containers

  1. Virtual Machines VMs

    Virtual (isolated) instances of different OS run

    "Hypervisor" technology divides physical resources

  1. Containers

    Not virtualized

    "cgroups" → process isolation but shared through same the Host OS kernel

    Not as secure as VMs

Cloud

Sets up computational resources over API.

  1. Infrastructure as a service (IaaS) → less restriction, lower abstraction

    retrieving new virtual server capacity can be scripted

    ie. AWS EC2, Google Compute Engine, ...

  1. Platform as a Service (PaaS)

    Application runtimes

    Built on top of IaaS but infrastructre is abstracted away, configurable

    ie. Heroku, CloudFoundry, App Engine, ...

  1. Software as a Service (SaaS) → more restriction, higher abstraction

    The software itself is running on the cloud

    Clients usually continuously pay a fee to get access to the software

    ie. Office365, ...