Storage

The mechanics of storage and the index are in the guide. The design choice worth recording is the strict separation of location from identity.

Location ≠ identity

A run's directory is a location. Its identity is content-addressed (canonical config + predicate) and entirely independent of where the directory sits. This is deliberate, and it enables several things:

• The index is a cache, not the source of truth. Each run directory carries its own model.json (module, config, version, predicate, inherits, context) and, since format v1, an id.json identity header. The SQLite index maps identities to locations for fast lookup and search, but it can always be rebuilt from the directories. Move a results folder, copy it to a cluster, hand it to a collaborator, check it out fresh: reindex() restores every identity and the catalog is whole again.
• Reindex copies; verify re-derives. Identity keys are persisted in id.json at materialization, so a rebuild is a pure copy with no Config import, implementable in any language (the directory format). The schema-consulting re-derivation (which fields are non-identifying, what the defaults are) lives in verify/repair, where consulting the current code as the source of identity semantics is the point: detecting drift, rather than silently re-keying a catalog on a machine where the project happens not to import.
• Data can be elsewhere. A run can be local, in a remote store, or indexed-but-evicted. Separating identity from location lets machinable track availability and fetch on demand, and lets storage backends (mirrors like Globus/Aimstack) be ordinary interfaces you connect as a context.

Why not store identity in the index alone?

Putting identity only in the database would make the database load-bearing: lose it and you lose the meaning of the directories. By writing model.json next to the data, the directories remain self-describing and portable, and the index is a rebuildable convenience. This is the same instinct as content-addressing the config: keep the durable truth in the artifact, not in a mutable side-channel.