backend-cheats

This repository is a visual cheatsheet on the main topics in Backend-development. All material is divided into chapters that include different topics. There are three main parts to each topic:

• Visual part - various images/tables/cheatsheets for better understanding (may not be available). All pictures and tables are made from scratch, specifically for this repository.
• Summary - A very brief summary with a list of key terms and concepts. The terms are hyperlinked to the appropriate section on Wikipedia or a similar reference resource.
• References to sources - resources where you may find complete information on a particular issue (they are hidden under a spoiler, which opens when clicked). If possible, the most authoritative sources are indicated, or those that provide information in as simple and comprehensible language as possible.

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Contents

Network & Internet

Internet is a worldwide system that connects computer networks from around the world into a single network for storing/transferring information. The Internet was originally developed for the military. But soon it began to be implemented in universities, and then it could be used by private companies, which began to organize networks of providers that provide Internet access services to ordinary citizens. By early 2020, the number of Internet users exceeded 4.5 billion.

• How the Internet works

  

  Your computer does not have direct access to the Internet. Instead, it has access to your local network to which other devices are connected via a wired (Ethernet) or wireless (Wi-Fi) connection. The organizer of such a network is a special minicomputer - router. This device connects you to your Internet Service Provider (ISP), which in turn is connected to other higher-level ISPs. Thus, all these interactions make up the Internet, and your messages always transit through different networks before reaching the final recipient.

  • Host

    Any device that is on any network.

  • Server

    A special computer on the network that serves requests from other computers.

  

  • Network topologies

    There are several topologies (ways of organizing a network): Point to point, Daisy chain, Bus, Ring, Star and Mesh. The Internet itself cannot be referred to any one topology, because it is an incredibly complex system mixed with different topologies.

• What is a domain name

  

  Domain Names are human-readable addresses of web servers available on the Internet. They consist of parts (levels) separated from each other by a dot. Each of these parts provides specific information about the domain name. For example country, service name, localization, etc.

  • Who owns domain names

    The ICANN Corporation is the founder of the distributed domain registration system. It gives accreditations to companies that want to sell domains. In this way a competitive domain market is formed.

  • How to buy a domain name

    A domain name cannot be bought forever. It is leased for a certain period of time. It is better to buy domains from accredited registrars (you can find them in almost any country).

• IP address

  

  IP address is a unique numeric address that is used to recognize a particular device on the network.

  • Levels of visibility

    • External and publicly accessible IP address that belongs to your ISP and is used to access the Internet by hundreds of other users.
    • The IP address of your router in your ISP's local network, the same IP address from which you access the Internet.
    • The IP address of your computer in the local (home) network created by the router, to which you can connect your devices. Typically, it looks like 192.168.XXX.XXX.
    • The internal IP address of the computer, inaccessible from the outside and used only for communication between the running processes. It is the same for everyone - 127.0.0.1 or just localhost.

  • Port

    One device (computer) can run many applications that use the network. In order to correctly recognize where and which data coming over the network should be delivered (to which of the applications) a special numerical number - a port is used. That is, each running process on a computer which uses a network connection has its own personal port.

  • IPv4

    Version 4 of the IP protocol. It was developed in 1981 and limits the address space to about 4.3 billion (2^32) possible unique addresses.

  • IPv6

    Over time, the allocation of address space began to happen at a much faster rate, forcing the creation of a new version of the IP protocol to store more addresses. IPv6 is capable of issuing 2^128 (is huge number) unique addresses.

• What is DNS

  

  DNS (Domain Name System) is a decentralized Internet address naming system that allows you to create human-readable alphabetic names (domain names) corresponding to the numeric IP addresses used by computers.

  • Structure of DNS

    DNS consists of many independent nodes, each of which stores only those data that fall within its area of responsibility.

  • DNS Resolver

    A server that is located in close proximity to your Internet Service Provider. It is the server that searches for addresses by domain name, and also caches them (temporarily storing them for quick retrieval in future requests).

  • DNS record types

    • A record - associates the domain name with an IPv4 address.
    • AAAA record - links a domain name with an IPv6 address.
    • CNAME record - redirects to another domain name.
    • and others - MX record, NS record, PTR record, SOA record.

• Web application design

  Modern web applications consist of two parts: Frontend and Backend. Thus implementing a client-server model.

  The tasks of