10 Load Balancer, Reverse Proxy (Web Server) PDF

10 Load Balancer, Reverse Proxy (Web Server) (GitHub System Design Primer) **~ Load balancer ~** ** Diagram** … ** Intro ** Load balancers ** distribute ** ** incoming client requests to ** **...

10 Load Balancer, Reverse Proxy (Web Server) (GitHub System Design Primer) **~ Load balancer ~** ** Diagram** … ** Intro ** 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 to eliminate a ~~ single point 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 balancing ~** Layer 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 balancing ~** Layer 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 ** terminate 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 scaling ~** Load 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 a single point of failure results in increased ** complexity. ** A ** single load balancer is a ** ** single point of failure, ** configuring ** multiple load balancers further increases ** ** complexity. ** ⠀ Source(s) and further reading [ NGINX architecture ]() [ HAProxy architecture guide ]() [ Scalability ]() [ Wikipedia ]() [ Layer 4 load balancing ]() [ Layer 7 load balancing ]() [ ELB listener config ]() ⠀ **~ Reverse proxy (web server) ~** *[ Source: Wikipedia ]()* A reverse proxy is a ** ** ** web server that ** ** centralizes internal services and provides ** ** unified interfaces to the public. ** ** Requests from clients are ** ** forwarded to a server that can fulfill it ** before the reverse proxy ** returns the server's response to the client. ** ** Additional benefits include: ** ** Increased security - ** ** Hide information about backend servers, ** ** blacklist IPs, ** ** limit number of connections per client ** ** Increased scalability and flexibility - ** ** Clients only see the reverse proxy's IP, ** allowing you to ** scale servers or change their configuration ** ** 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 [ ]() ~~  ** Compression - ** ** Compress server responses ** ** Caching - ** Return the ** response for cached requests ** ** Static content - ** Serve static content directly HTML/CSS/JS Photos Videos Etc ⠀ Load balancer vs reverse proxy Deploying a load balancer is useful when you have ** multiple servers. ** Often, load balancers route traffic to a set of servers serving the ** same function. ** Reverse proxies can be useful even with just ** one web server or ** ** application server, ** opening up the benefits described in the previous section. Solutions such as ** NGINX and ** ** HAProxy can support both ** ** layer 7 reverse proxying and ** ** load balancing. ** ⠀ Disadvantage(s): reverse proxy Introducing a reverse proxy results in increased ** complexity. ** A ** single reverse proxy is a ** ** single point of failure, configuring **  ** multiple reverse proxies ** (ie a failover) further increases [** **]() ** complexity. ** ⠀ Source(s) and further reading [ Reverse proxy vs load balancer ]() [ NGINX architecture ]() [ HAProxy architecture guide ]() [ Wikipedia ]()