HA
High Availability (HA) of a system is achieved by incorporating specific features to reduce service downtime, typically: failover, load balancing and replication.
These techniques can be easily incorporated into services deployed on multi-data center clouds. To some extent, HA features are currently available on public cloud providers but seriously limited by networking constraints. This is a horizontal use case that accommodates any cluster, or multi-tier, application.
Failover
Failover occurs when a connection to a primary database server fails and the connector opens up a connection to another database server.
For example, server A has the current connection. After a failure (server crash, network down …) the connection will switch to another server (B).
Load balancing
Load balancing allows load (read and write) to be distributed over multiple servers.
Replication
In a leaderless distributed system, assuming we have n replicas, there will be quorum algorithm to achieve consensus of read & write. if r + w > n, there will be at least 1 node with latest update. In some cases, there are more than n nodes because there may be partition node.
So here comes the sloppy quorum and hinted hand-off.
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