> 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.

<figure><img src="https://2094249939-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FNTN4EdaVH9gV24NJpAjV%2Fuploads%2Fgit-blob-fb0fc17474d104a7ca20b968cfc902ef9785ef05%2FCAP_Theorem_Venn_Diagram.png?alt=media" alt=""><figcaption></figcaption></figure>