# Document and wide-column stores

> Use a document store for data read and written by key with a changing shape, and a wide-column store when writes outgrow one machine, and accept what each gives up.

Move data that a relational table fits badly into a document store or a wide-column store, and keep what needs joins on the Database.

## Move up when

- The schema keeps changing and features ship slowly because every change is a migration.
- The work is documents read by key, such as a profile, a cart, or a settings page.
- Writes outgrow one Database, even after a bigger box, and you can live without joins.

## The Design



The App Server calls a Document Store for documents and a Payment Provider for charges. A checkout writes several documents, so **Multi-document transactions** is on, which keeps the order atomic.



The second Design uses a Wide-column Store with N replicas per key. A write waits for **W** of them and a read asks **R**.

## Why it works

A document store trades joins for speed of change: no migration, no pool to share, and one flat rate for reads and writes. A wide-column store spreads each key over N replicas and has no single primary, so writes scale out. When R plus W is greater than N, a read always asks at least one replica holding the latest write.

## What it costs

A document store has no joins, so a checkout or report that spans documents costs more work, and without transactions a checkout can land half saved. A wide-column store spends N replica operations on every write, so a strong setting costs capacity and lowers the rate. A weak setting is cheap and serves stale reads. Even a strong quorum is not linearizable, so keep checkouts on a relational store.

## In Quorum

The [Document Store](https://wiki.playquorum.dev/components/doc.md) needs [Document databases](https://wiki.playquorum.dev/research/document-db.md). The [Wide-column Store](https://wiki.playquorum.dev/components/wide.md) needs [Sharding](https://wiki.playquorum.dev/research/sharding.md) and then [Leaderless replication](https://wiki.playquorum.dev/research/leaderless.md). Watch for the [Hot key](https://wiki.playquorum.dev/incidents/hot-key.md) incident. See [quorum](https://wiki.playquorum.dev/glossary/quorum.md) and [eventual consistency](https://wiki.playquorum.dev/glossary/eventual-consistency.md), and the pattern on [Quorum reads and writes](https://wiki.playquorum.dev/patterns/quorum-reads-and-writes.md). The next rung keeps several stores apart: [Split the database by feature](https://wiki.playquorum.dev/patterns/split-the-database-by-feature.md).

Source: https://wiki.playquorum.dev/patterns/document-and-wide-column-stores