• Consistency - Every read receives the most recent write or an error
• Availability - Every request receives a response, without guarantee that it contains the most recent version of the information
• Partition Tolerance - The system continues to operate despite arbitrary partitioning due to network failures

Networks aren't reliable, so you'll need to support partition tolerance. You'll need to make a software tradeoff between consistency and availability.

AP is a good choice if the business needs to allow for eventual consistency or when the system needs to continue working despite external errors.

CP is a good choice if your business needs require atomic reads and writes.

Consistency Pattern

1. Weak consistent :

   After a write, reads may or may not see it. A best effort approach is taken.

   This approach is seen in systems such as memcached.

   Eg: Video Player, phone call, Live gaming

2. Eventual Consistency

   After a write, reads will eventually see it (typically within milliseconds). Data is replicated asynchronously.

   This approach is seen in systems such as DNS and email. Eventual consistency works well in highly available systems.

3. Strong Consistency:

   After a write, reads will see it. Data is replicated synchronously.

   This approach is seen in file systems and RDBMSes. Strong consistency works well in systems that need transactions.

Availablity Pattern

Fail Over

1. Active Passive

   • Heartbeat Mechanism:
     • Heartbeats are sent between the active and passive server.
     • If the heartbeat is interrupted, the passive server takes over the active server's IP address and resumes service.
   • Downtime:
     • Downtime depends on whether the passive server is in 'hot' standby (already running) or 'cold' standby (needs to start).
     • Hot standby reduces downtime, while cold standby increases it.

2. Active-active

   In active-active, both servers are managing traffic, spreading the load between them.

Disadvantages

• Fail-over adds more hardware and additional complexity.
• There is a potential for loss of data if the active system fails before any newly written data can be replicated to the passive.

Replication

1. Master-slave

   The master serves reads and writes, replicating writes to one or more slaves, which serve only reads. Slaves can also replicate to additional slaves in a tree-like fashion. If the master goes offline, the system can continue to operate in read-only mode until a slave is promoted to a master or a new master is provisioned.