Cloud Basics - Availability and Redundancy
High Availability
Your ability for your service to remain available by ensuring there is *no single point Of failure and/or ensure a certain level of performance. High availability is achieved by replicating the images in multiple zones or data centres. So in case of any geo-disaster the alternative systems serve the users, hence it is called highly available. There are three types of redundancies:
- Local Redundancy
- Zone Redundancy
- Global/Geo Redundancy
Local redundancy
The default behaviour is locally replicating the data/resource images in multiple copies in the same datacentre. The resources by default have three copies. so if one copy fail to serve the other two are still available. This mode has less availability comparing to the other two but still it provides some piece of mind.
Zones Redundancy
In this mode three copies of the resources are kept in three different zones or data centres in the same location/city. For example, UK West is a location which may have multiple zones or data centres in one location (for example in London) so the zones are physically different data centres but their geo location is same so in case of disaster/failure in one data centre, the other data centre can serve the network traffic.
Global/Geo Redundancy
In addition to three zone redundant copies, the global copies at different location is kept. This prevents geo-disaster as well and SLA is increased to 99.999%
For more information how the high availability is achieved, read about Load Balancer which is infrastructure behind managing traffic across multiple instances of the services
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