XtendWeb cluster architecture
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.. image:: DuEt_cPanel.jpg


Software components
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XtendWeb (Script that generates nginx configuration for all nginx servers in the cluster)
XtendWeb-cluster-slave (A trimmed down version of XtendWeb that is useful on the slave)

csync2 - http://oss.linbit.com/csync2/ - Synchronize the nginx config on the cluster

unison - https://www.cis.upenn.edu/~bcpierce/unison/ - Synchronize /home/user directory

cPanel DNS cluster - DNS clustering and LoadBalancing

MySQL Replication or a Dedicated MySQL server - For providing a redundant MySQL database


LoadBalancing
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This happens at the DNS layer. The DNS server return multiple A records in round-robin fashion
making each successive request to point to different servers in the cluster .

Thus you achieve horizontal scalability as each server is doing request processing on its own

HighAvailability
----------------
High availability for the webstack:

Modern browsers are smart enough to query DNS and redirect to next A record if it is unable
to connect to one . There may be slight delays in connections and this may not
work with old web browsers or commandline http clients. But Our tests with modern
Chrome/Safari/FireFox etc returned positive results

High availability for MySQL:

Each nginx server is configured to connect to one or more upstream mysql servers
thus creating a failover for scripts accessing MySQL

High availability for Email:

PostFix server configured on the slave acts as a queue and forward backupMX for the exim MTA on the
cPanel master and thus provide uninterrupted SMTP for the domains

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