> For the complete documentation index, see [llms.txt](https://notes.sujeet.pro/architecture/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://notes.sujeet.pro/architecture/topics/cap-theorem.md).
# CAP Theorem
Distributed Data Store can only provide two of the following three guarantees.
#### Consistency
Every read receives the most recent write or an error.
#### Availability
Every request receives a (non-error) response, without the guarantee that it contains the most recent write.
#### Partition tolerance
The system continues to operate despite an arbitrary number of messages being dropped (or delayed) by the network between nodes.
### In Practice
> No distributed system is safe from network failures, thus network partitioning generally has to be tolerated
When a network partition failure happens, it must be decided whether to do one of the following:
* cancel the operation and thus decrease the availability but ensure consistency
* proceed with the operation and thus provide availability but risk inconsistency.
CAP theorem indicates that we can’t have availability and consistency under network partitions.
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