Web infrastructures

Objectives

• Understand the concepts of web infrastructures
• Understand how HTTP features can help to build web infrastructures
• Understand the concepts of a reverse proxy
• Understand the concepts of load balancing
• Understand the concepts of caching

Access your hosts file

File to map hostnames to IP addresses, just as DNS does, but only for your computer.

• Windows:
  %WinDir%\System32\Drivers\Etc\Hosts
• Linux and macOS:
  /etc/hosts

Traefik

• An open source reverse proxy (more on this later)
• Works well with Docker Compose and Kubernetes
• Issue and renew Let's Encrypt (HTTPS) certificates automatically
• Easy to use with Docker Compose labels

whoami

• A tiny Go webserver that prints os information and HTTP request to output
• Used to demonstrate the use of HTTP with a reverse proxy and a load balancer

Functional and non-functional requirements

• Requirements used to define the scope of a system.
• An abstract representation of the system that will be implemented.
• Can help to define the architecture of the system.
• Functional requirements: features that a system must have to satisfy the needs of its users. It is the "what" of a system.
• Non-functional requirements: constraints on the system. It is the "how" of a system.

Examples of functional requirements:

• User management: Users must be able to register, login, logout, etc.
• Product management: Users must be able to create, read, update, delete products, etc.
• Order management: Users must be able to create, read, update, delete orders, etc.
• Payment management: Users must be able to pay for their orders, etc.

Examples of non-functional requirements:

• Response time: Time between a request and a response (end user)
• Throughput: Number of requests/interval (service provider)
• Scalability: Property of a system to handle a varying amount of work
• Availability: Percentage of time that the system provides a satisfactory service
• Maintainability: How easily the system can be managed
• Security: Confidentiality, authentication, authorization, etc.
• ...and many, many more

Web infrastructure definition

Software and hardware components that are necessary to support:

• the development
• the deployment
• the management

of web applications.

The Host header

• Part of the HTTP request
• Used to specify the domain name of the server
• Can be used to host multiple websites on the same server using a reverse proxy
• The reverse proxy will route the request to the correct website based on the Host header

Forward proxy and reverse proxy

• Proxies are components that intercept requests and responses and filter/forward/change them to another component.
• Forward proxy: used by a client to access external servers
• Reverse proxy: used by an external server to access internal servers

Forward proxy

• Operates between clients and external systems
• Can be used to:
  • Restrict access to external systems
  • Regulate traffic
  • Mask the identity of the client
  • Enforce security policies

Reverse proxy

• Operates between external systems and internal systems
• Can be used to:
  • Restrict access
  • Load balance requests to internal systems
  • Cache responses from internal systems

System scalability

• Capacity of a system to handle a varying amount of work
• Can be achieved by:
  • Vertical scaling (scale up)
  • Horizontal scaling (scale out)
• Can be achieved by:
  • Adding more resources
  • Adding more servers

Vertical scaling

Add more resources to a server:

• More RAM
• More CPU
• etc.

Limited by the hardware: at a certain point, you cannot add more/better resources to a server.

Horizontal scaling

Add more servers to a system and distribute the load between them.

Limited by the software - your software must be able to run on multiple servers:

• Backends/API accessing the same database(s)
• Frontends accessing backends/API

When to use scale up or scale out?

• Determined by the non-functional requirements of the system
• You need metrics to determine when to scale up or scale out to identify bottlenecks
• Once the bottleneck is identified (from monitoring), you can decide to scale up or scale out

How to monitor a system?

Out of scope for this course, but here are some tools you can use:

• Prometheus
• Grafana
• Sentry
• LibreNMS

Load balancing

Process of distributing the load between multiple servers.

This can work thanks to the stateless nature of HTTP and the Host header.

The load balancer must know the pool of servers it can forward the requests to.

Multiple strategies can be used to distribute the load:

• Round-robin: each server in the pool in turn (covered in this course)
• Least connections: least number of active connections
• Least response time: least response time
• Hashing: based on a hash of the request (e.g. the IP address of the client, the URL of the request, etc.)

An issue with load balancing is session management: the load balancer could forward requests from the same client to different servers, loosing their session.

As HTTP is stateless, the load balancer must know how to forward requests from the same client to the same server. A solution is sticky sessions with the help of a cookie.

Caching

Process of storing a copy of a resource to serve it faster.

Caching can improve the performance of a system and reduce the load on the backend.

Caching can be done on the client-side or server-side.

Managing cache with HTTP

Managing chache is challenging because it is difficult to know when to invalidate the cache (the data can be stale).

Two main caching models:

• Expiration model: the cache is valid for a certain amount of time
• Validation model: the cache is valid until the data is modified

Expiration model

• The cache is valid for a certain amount of time
• If the cache is not expired, the cache is used
• Uses the Cache-Control: max-age=<secondes> header
• The cache is invalidated after the expiration time

Validation model

• The cache is valid until the data is modified
• If the cache is not expired, the cache is used
• Two ways to validate the cache:
  • Last-Modified: Last-Modified and If-Modified-Since headers
  • ETag: ETag and If-None-Match headers

CDN

Content Delivery Network (CDN) is a network of servers that are geographically distributed around the world.

Improve performance by serving static content (images, videos, etc.) from the closest server.

Where to cache?

The best would be to cache at each level of the system to ensure the best performance but it is not always possible or faisable:

• Client-side: the cache is stored on the client
• Server-side: the cache is stored on the server
• CDN: the cache is stored on a CDN

Private caches are caches that are only used by one client. Public caches are caches that are used by multiple clients.

What will you do?

• Set up a reverse proxy
• Set up whoami
• Explore the features of the reverse proxy:
  • PathPrefix rule
  • Host rule
  • StripPrefix middleware
  • Sticky sessions

Finished? Was it easy? Was it hard?

Can you let us know what was easy and what was difficult for you during this chapter?

This will help us to improve the course and adapt the content to your needs. If we notice some difficulties, we will come back to you to help you.

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Sources

• Main illustration by Nicolas Picard on Unsplash
• Illustration by Aline de Nadai on Unsplash
• Illustration by Mohammadreza alidoos on Unsplash
• Illustration by Gaurav Dhwaj Khadka on Unsplash
• Illustration by Imre Tömösvári on Unsplash
• Illustration by Kelvin T on Unsplash
• Illustration by Alfred Kenneally on Unsplash
• Illustration by Mikhail Vasilyev on Unsplash

• Illustration by Ibrahim Boran on Unsplash
• Illustration by Elena Mozhvilo on Unsplash
• Illustration by Evan Krause on Unsplash
• Illustration by Markus Spiske on Unsplash
• Illustration by Fermin Rodriguez Penelas on Unsplash
• Illustration by Andrik Langfield on Unsplash
• Illustration by Karen Grigorean on Unsplash
• Illustration by Shubham's Web3 on Unsplash