Users are sharded by user_id, and you add a global secondary index on email, itself partitioned by email. What do you pay for fast email lookups?

Options

• ✗ Every email lookup must ask all shards
• ✓ A write can touch two shards, so the index is often updated asynchronously and may lag Correct
• ✗ Emails must be unique across shards
• ✗ Users can no longer be found by user_id

Why

The alternative, a local index on each shard, keeps writes on one shard but makes every lookup by email a scatter-gather over all shards.

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