Sharding

1. English dictionary meaning of shard is fragment.
2. In database terminology it refers to data distribution across multiple machines — a form of horizontal scaling or scale out.
3. Sharding can be applied to databases as well as software architectures.
4. Pros for database sharding:
   Increased Read/Write Throughput.
   Increased Storage Capacity.
   High Availability.
5. Cons for database sharding:
   Query Overhead.
   Complexity of Administration.
   Increased Infrastructure Costs.
6. Some database sharding architectures and types:
   Ranged/Dynamic Sharding.
   Algorithmic/Hashed Sharding.
   Entity-Relationship-Based Sharding.
   Geography-Based Sharding.
7. Sharding can be considered if your core application database contains large amounts of data, requires high read and high write volume, and/or you have specific availability requirements.
8. Some alternatives to database sharding:
   Functional Partitioning.
   Replication.
   Caching and Queueing.
   Supplemental Technologies.

References and further reading:

MongoDB Sharding

Sharding

Database Sharding: Concepts & Examples

To Shard, or Not to Shard - DZone Java