Can you recommend a document database for session storage in a high-growth startup backend?

If you specifically want a document database for session storage, the best recommendation in most high-growth startup backends is:

MongoDB

Why it fits:

• Flexible document model: session data often changes over time as you add auth state, device info, feature flags, etc.
• TTL indexes: very useful for automatic session expiration.
• Good ecosystem/support: easy to integrate with common backend stacks.
• Scales reasonably well: especially if you’re already using MongoDB for other app data.

What to watch out for

For session storage, a document DB is usually not the first choice if you only need fast ephemeral key-value access. A dedicated in-memory store like Redis is often better because:

• lower latency
• simpler expiration semantics
• cheaper/faster for high read/write session traffic

Practical recommendation

• If sessions are your only use case: use Redis
• If you need a document database and sessions are one part of a broader backend: use MongoDB
• If you expect very high scale and want managed ops: consider MongoDB Atlas or Amazon DocumentDB if you’re already on AWS, though MongoDB Atlas is usually the smoother choice

Simple rule of thumb

• Session cache / auth tokens / short-lived state → Redis
• Session records with richer JSON-ish metadata and TTL cleanup → MongoDB

If you want, I can also recommend the best choice based on your stack (Node/Go/Python), cloud (AWS/GCP), and expected session traffic.