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Availability Patterns
There are three Availability Patterns which are:
- Fail-Over
- Replication
- Availability in Numbers
Fail-Over
Active-passive
With active-passive fail-over, heartbeats are sent between the active and the passive server on standby. If the heartbeat is interrupted, the passive server takes over the active's IP address and resumes service.
Active-active
In active-active, both servers are managing traffic, spreading the load between them. If the servers are public-facing, the DNS would need to know about the public IPs of both servers. If the servers are internal-facing, application logic would need to know about both servers.
Replication
Replication is futher derived in two components:
- Master-Slave Replication - The master serves reads and writes, replicating writes to one or more slaves, which serve only reads.
- Master-Master Replication - Both masters serve reads and writes and coordinate with each other on writes. If either master goes down, the system can continue to operate with both reads and writes.
Availability In Numbers
Availability is often quantified by uptime (or downtime) as a percentage of time the service is available. Availability is generally measured in number of 9s--a service with 99.99% availability is described as having four 9s.
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