When a single database cannot handle the load (CPU, memory, disk, IOPS) or data size. Vertical scaling (bigger server) is simpler — try it first. Shard when you hit vertical scaling limits or need geographic data distribution. Sharding adds significant complexity — do not shard prematurely.

How do I choose a shard key?

Choose a key that: distributes data evenly (high cardinality), is present in most queries (enables shard routing), rarely changes (minimizes resharding), and groups related data (avoids cross-shard joins). Common choices: user_id, tenant_id, geographic region. Avoid hot keys that concentrate traffic on one shard.

Utiliser base de données sharding pour horizontal scaling

Sharding distributes données across multiple databases par un shard clé. Apprendre sharding strategies, cross-shard queries, et trade-offs.

1

Sharding splits a large database into smaller shards, each holding a subset of data. The shard key determines which shard holds each row. Strategies: range (A-M on shard 1, N-Z on shard 2), hash (hash(key) % num_shards), directory (lookup table). Sharding enables horizontal scaling but complicates cross-shard queries, joins, and transactions.
2

Calculatrice de hash →
Use a hash function to determine the shard: shard = hash(shard_key) % num_shards. This distributes data evenly. Use a consistent hash (see consistent hashing) to minimize data movement when adding/removing shards. The hash tool computes the shard assignment for a given key.
3

Formateur SQL →
Formater queries that target a specific shard. Each shard has the same schema — the application routes queries to the correct shard based on the shard key. Cross-shard queries (without the shard key) must query all shards and merge results — avoid these for performance.

Questions fréquentes

When should I shard?: When a single database cannot handle the load (CPU, memory, disk, IOPS) or data size. Vertical scaling (bigger server) is simpler — try it first. Shard when you hit vertical scaling limits or need geographic data distribution. Sharding adds significant complexity — do not shard prematurely.
How do I choose a shard key?: Choose a key that: distributes data evenly (high cardinality), is present in most queries (enables shard routing), rarely changes (minimizes resharding), and groups related data (avoids cross-shard joins). Common choices: user_id, tenant_id, geographic region. Avoid hot keys that concentrate traffic on one shard.