How do you choose between SQL and NoSQL and scale each (replication, sharding, federation)?

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Accepted Answer

## Step 1: Use Cases, Constraints, Assumptions, Back-of-Envelope

### Use cases
We'll scope to handle only the core flows that expose the bottlenecks; everything else is deferred.

- **User** needs strong transactions — SQL (ACID, joins) for orders, payments, friendships.
- **Service** needs massive write scale — NoSQL (Cassandra/Dynamo wide-column/KV) for timelines, metrics, crawl frontier.
- **Service** scales reads — add read replicas (async), measure lag.
- **Service** scales writes — shard by hash/range, federate by function (users DB vs posts DB).

### Out of scope
- Specific vendor benchmarks, graph DB internals, time-series DB compression beyond overview.

### State assumptions
- SQL single master: ~5k writes/s max (B-tree + WAL), beyond that shard.
- Read replicas: each adds ~0.8x read throughput, lag 5-20ms DC.
- Shards: 16-256 shards, each 500 GB, rebalancing is live migration.
- NoSQL: 100k writes/s per cluster with linear scale via consistent hashing.
- Federation splits n

How do you choose between SQL and NoSQL and scale each (replication, sharding, federation)?

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Step 1: Use Cases, Constraints, Assumptions, Back-of-Envelope

Use cases

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Understand the problem

Attempt it yourself

Study the solution

Step 1: Use Cases, Constraints, Assumptions, Back-of-Envelope

Use cases

Solution ready — 2 min read

Join the discussion

Discussion (0)

KEEP THESTREAK ALIVE.

KEEP THE
STREAK ALIVE.