What is sharding in message storage?

Sharding is a database scaling technique that partitions data across multiple servers. Each server (shard) holds a subset of the total data, enabling storage and performance beyond single-server limits.

How sharding works in email systems:

Data is divided by a shard key (often customer ID, account ID, or domain)

Each shard handles queries for its subset of data

The application layer routes requests to the appropriate shard

Adding shards increases total capacity

What gets sharded:

Subscriber records: Millions of email addresses partitioned across database servers

Tracking data: Opens, clicks, bounces distributed by time period or account

Message logs: Detailed delivery logs too large for single databases

Queue metadata: Information about pending messages in the delivery pipeline

Sharding strategies:

Range-based: Accounts 1-1000 on shard A, 1001-2000 on shard B

Hash-based: Hash the account ID to determine shard (more even distribution)

Directory-based: Lookup table maps each account to its shard

Sharding adds complexity (cross-shard queries, rebalancing) but is essential for operating at email platform scale. You won't see it directly as a user; it's infrastructure that enables the service.