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Can someone explain to me how high-availability ("HA") works for a web application ... because I assume HA means that there exist no single-point-of-failure.
However, even if a load balancer is used- isn't that the single point of failure?
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We recommend trying to keep virtual nodes on different hosts where possible. If you only have one host and this goes down, the host becomes your single point of failure. Having two virtual hosts, with a load balancer on each, again ensures redundancy through the whole network chain.
Although RAID eliminates the actual storage medium of the data as a SPOF, the path to storage (and also the RAID controller for hardware RAID) still is a SPOF. The simplest way to eliminate this (applying to both software and host based raid) is to employ two controllers and two separate SCSI buses as in figure 2.
As you may have already noticed, the HA cluster configuration is exactly in line with this principle, and a single point of failure is eliminated by making the important server and its resources redundant to the active system (production system).
I have found this article on the subject: http://www.tenereillo.com/GSLBPageOfShame.htm
Basically if you do not require long lasting sticky sessions you can configure your DNS servers to return multiple A records (IP addresses) for your website.
Web browsers are smart enough to try all the addresses until they find one that works.
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In simple words high availability can be defined as running a system 24*7 without a downtime even if there are hardware and software failures. In other way a fault tolerance application. This helps ensure uninterrupted use of the application for it’s intended users.
Read more on High Availability Deployment Architecture
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It works the following way that you setup two HA Proxy servers with heartbeat, so when one fails (stops responding to queries), it's being removed from the cluster. Requests from HA Proxy can be forwarded to web servers in round robin fashion, and if one web server fails, HA Proxy servers do not try to contact it until it's alive. Web servers are storing all dynamic information in database, which is replicated across two MySQL instances. As you can see, HA Proxy and Cluster MySQL (or simply MySQL replication) as well IP Clustering here is the key.
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Sure it is when operated alone. Usual highly available setup includes 2 or more load balancers running in cluster in either active/active or active/passive configuration. To further increase the availability you can have 2 different Internet Service Providers (or geo distributed datacenters) each running a pair of clustered load balancers. Then you configure DNS A record resolving to 2 distinct public IP addresses which guarantees round-robin processing splitting DNS requests evenly (CloudFlare is very fast and reliable at this). There's also possibility to return IP address of datacenter closest to your originating geo location by using something like PowerDNS dnsdist This is what big players do to make their services highly available.
Please read https://docs.oracle.com/cd/E23824_01/html/821-1453/gkkky.html for more clearity. Actually both load balancer uses same vip(Virtual IP Address. https://techterms.com/definition/vip).
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