First parts: DNS/CDN/LB

1. Domain name system
    
A Domain Name System (DNS) translates a domain name such as www.example.com to an IP address.

DNS is hierarchical, with a few authoritative servers at the top level.  Your router or ISP provides information about which DNS server(s) to contact when doing a lookup.  Lower level DNS servers cache mappings, which could become stale due to DNS propagation delays.  DNS results can also be cached by your browser or OS for a certain period of time, determined by the time to live (TTL).

• NS record (name server) - Specifies the DNS servers for your domain/subdomain.
• MX record (mail exchange) - Specifies the mail servers for accepting messages.
• A record (address) - Points a name to an IP address.
• CNAME (canonical) - Points a name to another name or CNAME (example.com to www.example.com) or to an A record.
  

Services such as CloudFlare and Route 53 provide managed DNS services.  Some DNS services can route traffic through various methods:

• Weighted round robin
  
  • Prevent traffic from going to servers under maintenance
  • Balance between varying cluster sizes
  • A/B testing
    
• Latency-based
• Geolocation-based
  

Disadvantage(s): DNS

• Accessing a DNS server introduces a slight delay, although mitigated by caching described above.
• DNS server management could be complex and is generally managed by governments, ISPs, and large companies.
• DNS services have recently come under DDoS attack, preventing users from accessing websites such as Twitter without knowing Twitter's IP address(es).
  

2. Content delivery network
    
A content delivery network (CDN) is a globally distributed network of proxy servers, serving content from locations closer to the user.  Generally, static files such as HTML/CSS/JS, photos, and videos are served from CDN, although some CDNs such as Amazon's CloudFront support dynamic content.  The site's DNS resolution will tell clients which server to contact.

Serving content from CDNs can significantly improve performance in two ways:

• Users receive content at data centers close to them
• Your servers do not have to serve requests that the CDN fulfills
  

Push CDNsPush CDNs receive new content whenever changes occur on your server.  You take full responsibility for providing content, uploading directly to the CDN and rewriting URLs to point to the CDN.  You can configure when content expires and when it is updated.  Content is uploaded only when it is new or changed, minimizing traffic, but maximizing storage.
Sites with a small amount of traffic or sites with content that isn't often updated work well with push CDNs.  Content is placed on the CDNs once, instead of being re-pulled at regular intervals.
Pull CDNsPull CDNs grab new content from your server when the first user requests the content.  You leave the content on your server and rewrite URLs to point to the CDN.  This results in a slower request until the content is cached on the CDN.
A time-to-live (TTL) determines how long content is cached.  Pull CDNs minimize storage space on the CDN, but can create redundant traffic if files expire and are pulled before they have actually changed.
Sites with heavy traffic work well with pull CDNs, as traffic is spread out more evenly with only recently-requested content remaining on the CDN.
Disadvantage(s): CDN

• CDN costs could be significant depending on traffic, although this should be weighed with additional costs you would incur not using a CDN.
• Content might be stale if it is updated before the TTL expires it.
• CDNs require changing URLs for static content to point to the CDN.
  

3. Load balancer
    
Load balancers distribute incoming client requests to computing resources such as application servers and databases.  In each case, the load balancer returns the response from the computing resource to the appropriate client.  Load balancers are effective at:

• Preventing requests from going to unhealthy servers
• Preventing overloading resources
• Helping eliminate single points of failure
  

Load balancers can be implemented with hardware (expensive) or with software such as HAProxy.
Additional benefits include:

• SSL termination - Decrypt incoming requests and encrypt server responses so backend servers do not have to perform these potentially expensive operations
  
  • Removes the need to install X.509 certificates on each server
    
• Session persistence - Issue cookies and route a specific client's requests to same instance if the web apps do not keep track of sessions
  

To protect against failures, it's common to set up multiple load balancers, either in active-passive or active-active mode.
Load balancers can route traffic based on various metrics, including:

• Random
• Least loaded
• Session/cookies
• Round robin or weighted round robin
• Layer 4
• Layer 7
  

Layer 4 load balancingLayer 4 load balancers look at info at the transport layer to decide how to distribute requests.  Generally, this involves the source, destination IP addresses, and ports in the header, but not the contents of the packet.  Layer 4 load balancers forward network packets to and from the upstream server, performing Network Address Translation (NAT).
Layer 7 load balancingLayer 7 load balancers look at the application layer to decide how to distribute requests.  This can involve contents of the header, message, and cookies.  Layer 7 load balancers terminates network traffic, reads the message, makes a load-balancing decision, then opens a connection to the selected server.  For example, a layer 7 load balancer can direct video traffic to servers that host videos while directing more sensitive user billing traffic to security-hardened servers.
At the cost of flexibility, layer 4 load balancing requires less time and computing resources than Layer 7, although the performance impact can be minimal on modern commodity hardware.
Horizontal scalingLoad balancers can also help with horizontal scaling, improving performance and availability.  Scaling out using commodity machines is more cost efficient and results in higher availability than scaling up a single server on more expensive hardware, called Vertical Scaling.  It is also easier to hire for talent working on commodity hardware than it is for specialized enterprise systems.
Disadvantage(s): horizontal scaling

• Scaling horizontally introduces complexity and involves cloning servers
  
  • Servers should be stateless: they should not contain any user-related data like sessions or profile pictures
  • Sessions can be stored in a centralized data store such as a database (SQL, NoSQL) or a persistent cache (Redis, Memcached)
    
• Downstream servers such as caches and databases need to handle more simultaneous connections as upstream servers scale out
  

Disadvantage(s): load balancer

• The load balancer can become a performance bottleneck if it does not have enough resources or if it is not configured properly.
• Introducing a load balancer to help eliminate single points of failure results in increased complexity.
• A single load balancer is a single point of failure, configuring multiple load balancers further increases complexity.