Tell about database sharding strategies: when to start sharding and what criteria to choose for the sharding key?

Database sharding is the horizontal partitioning of data into parts (shards), each stored separately. You should start sharding when:

• The volume of data becomes too large for a single machine (performance issues, response time, backup).
• The load on the database exceeds the capabilities of one server (CPU, RAM, I/O).
• Horizontal scaling is required to ensure fault tolerance and high availability.

The sharding key is chosen based on:

• Query frequency: the key should evenly distribute the load across shards to avoid "hot" shards.
• Access logic: it is preferable for operations to often be performed within a single shard to minimize cross-shard queries.
• Data size: the key should allow for an even distribution of data across shards.

Example keys: user ID, geographic region, time interval.

It is important to consider that an incorrect choice of key can lead to load imbalance and complicate queries.