# Distributed SQL and leaderless stores

> Replace hand-made shards with a store that replicates by itself, either leaderless with tunable quorums or SQL on Raft, and pick by whether you need every read correct.

Let the store handle replication and splitting: a leaderless store for speed, or distributed SQL for correctness.

## Move up when

- Resharding by hand costs Dev Hours you do not have, and cross-shard transactions fail.
- A feed or log workload writes far more than a single primary can take.
- A checkout must be right everywhere, and a replica that lags is not acceptable.

## The Design



The leaderless Wide-column Store replicates each key to N nodes. A read asks R of them and a write waits for W. A Database stays beside it for the kinds the store does not take.



Distributed SQL puts a SQL store on Raft. Each write waits for a majority of its replicas, so every read sees it.

## Why it works

When R plus W is above N, a read always meets a node that took the latest write. Lower R or W makes it faster and lets stale reads through. Distributed SQL gives that guarantee for transactions and costs more latency per write.

Even a strong quorum is not a transaction. A checkout on the wide-column store still goes wrong now and then, so use distributed SQL for money and a leaderless store for feeds.

## What it costs

Both bill several nodes at once, and distributed SQL bills the most. Neither takes searches or uploads, so a Database stays for them. A split can lose a replica, and a quorum that needs every replica then refuses. Both can also **Spread** across regions, which the multi-region rung uses. The game does not model a lightweight transaction on the wide-column store.

## In Quorum

Place a [Wide-column Store](https://wiki.playquorum.dev/components/wide.md) or [Distributed SQL](https://wiki.playquorum.dev/components/dsql.md). Research [Leaderless replication](https://wiki.playquorum.dev/research/leaderless.md) or [Consensus databases](https://wiki.playquorum.dev/research/consensus.md). Read [quorums](https://wiki.playquorum.dev/glossary/quorum.md), [Raft](https://wiki.playquorum.dev/glossary/raft.md) and [The data tier](https://wiki.playquorum.dev/explanation/the-data-tier.md). The next rung, [logs, metrics and traces](https://wiki.playquorum.dev/patterns/logs-metrics-and-traces.md), shows where these stores spend their time.

Source: https://wiki.playquorum.dev/patterns/distributed-sql-and-leaderless-stores