Hydra event store moves from JSON to CBOR in merged PR

What happened.

Hydra PR #2767 changes its persisted event store from JSON to CBOR. On a real 3-node benchmark database, the reported size fell from 27.4 MB to 10.5 MB, or 2.6x smaller. The same record reports migration plus a full replay in 5.3 seconds.

The change includes an automatic, one-shot migration from schema v1 to schema v2. Existing event rows are re-encoded when the database is first opened. Hydra’s client-visible JSON output is described as byte-unchanged, so the storage change is intended to stay behind the existing client-facing format.

Why it matters.

For builders and stake pool operators running Hydra heads, smaller state files could reduce the storage footprint of persisted head events. The reported replay result also points to a potentially smoother startup path for the tested database. This is operationally relevant because the migration changes stored data rather than only an in-memory representation.

The main trade-off is migration risk. The re-encode is one way, with no downgrade path stated for schema v2. Operators considering an upgrade should treat a backup of hydra.db and a test migration as practical safeguards, especially where a rollback may be needed.

What remains uncertain.

This is a merged change on master, not a tagged Hydra release or a mainnet ledger change. The 2.6x result comes from one real 3-node benchmark database, so it does not establish the same reduction or replay time for every Hydra deployment. The supplied record does not provide broader deployment results.