This repository is a visual cheatsheet on the main topics in Backend-development. All the material is divided into topics and subtopics. The structure of the material consists of three parts:
-**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. If possible, the most authoritative sources are indicated, or those that provide information in as simple and comprehensible language as possible.
> 🛠 The repository is under active development, so it is constantly updated and supplemented
> 🤝 If you want to help the project, feel free to send your [pool requesters](https://github.com/cheatsnake/backend-cheats/pulls)
* [Monolithic and microservice architecture](#monolithic-and-microservice-architecture)
* [Horizontal and vertical scaling](#horizontal-and-vertical-scaling)
</details>
[Additional and similar resources](#additional-and-similar-resources)
## Network & Internet
[Internet](https://en.wikipedia.org/wiki/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.
Your computer has never been directly connected to the Internet. Because it can only see its local network to which other devices are connected via wired ([Ethernet](https://en.wikipedia.org/wiki/Ethernet)) or wirelessly (Wi-Fi, Bluetooth). To communicate with the Internet, you have a special minicomputer in your local network - [router](<https://en.wikipedia.org/wiki/Router_(computing)>). It then connects you to [Internet Service Provider](https://en.wikipedia.org/wiki/Internet_service_provider) which in turn connects to other higher-level providers. Thus, your message, transits through the network of several ISPs before reaching the destination network.
The Internet is just a long wire to which a small number of [Tier 1 providers](https://en.wikipedia.org/wiki/Tier_1_network) are directly connected. The ISPs below that are just renting access.
[Domain Names](https://en.wikipedia.org/wiki/Domain_name) 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](https://en.wikipedia.org/wiki/ICANN) 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](https://www.icann.org/en/accredited-registrars?filter-letter=a&sort-direction=asc&sort-param=name&page=1) (you can find them in almost any country).
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<summary>🔗 <b>References</b></summary>
1. 📄 [**What is a Domain Name?** – MDN](https://developer.mozilla.org/en-US/docs/Learn/Common_questions/What_is_a_domain_name)
2. 📺 [**A Beginners Guide to How Domain Names Work!** – YouTube](https://youtu.be/Y4cRx19nhJk)
[IP address](https://en.wikipedia.org/wiki/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_.
> 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](https://en.wikipedia.org/wiki/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](https://en.wikipedia.org/wiki/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.
[DNS (Domain Name System)](https://en.wikipedia.org/wiki/DNS) is a decentralized Internet address naming system that allows you to create human-readable alphabetic names (domain names) corresponding to the numeric [IP addresses](#ip-address) used by computers.
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<summary>🔗 <b>References</b></summary>
1. 📄 [**What is DNS? Domain Name System explained** – freeCodeCamp](https://www.freecodecamp.org/news/what-is-dns/)
2. 📺 [**DNS (Domain Name System) explained. Types of Domain Name Servers** – YouTube](https://youtu.be/JkEYOt08-rU)
3. 📺 [**DNS as Fast As Possible** – YouTube](https://youtu.be/Rck3BALhI5c)
