a distributed system can deliver only 2 of 3 desired characteristics: consistency, availability and partition tolerance
Consistency
- All nodes see the same data at the same time.
- Consistency is achieved by updating several nodes before allowing further reads
Availability
- Every request gets a response on success/failure.
- Availability is achieved by replicating the data across different servers.
Partition tolerance
- The system continues to work despite message loss or partial failure.
- A system that is partition-tolerant can sustain any amount of network failure that doesn’t result in a failure of the entire network
Example
Distributed Databases
We cannot build a general data store that is all three of continually available, sequentially consistent, and tolerant to any partition failures. To be consistent, all nodes should see the same set of updates in the same order. But if the network suffers a partition, updates in one partition might not make it to the other partitions before a client reads from the out-of-date partition after having read from the up-to-date one. The only thing that can be done to cope with this possibility is to stop serving requests from the out-of-date partition, but then the service is no longer 100% available.