Time and database values

The database value

A peer holds a database value. The value is immutable. A query runs against one value, so the answer cannot change while the query runs.

A time view wraps the same datoms with a different fold policy. A view never copies a fact.

The views

Given a connection whose latest known basis is t-now:

View	Meaning
db()	Current facts at t-now.
as-of(t)	Facts as they stood at basis t.
since(t)	Only facts added after t.
history()	Every assertion and every retraction ever recorded.
sync(t)	Completes when the basis of the peer reaches t.

as-of and since also accept a wall-clock instant. Corium resolves the instant to the last transaction committed at or before it. An instant older than the database resolves to basis 0.

The as-of and history views disable the uniqueness shortcuts of the planner. Uniqueness holds only in the current view.

Naming a view by wall clock

The t to instant correspondence is part of the database value, because every commit asserts :db/txInstant. Resolution in both directions is O(log n).

A derived view keeps the whole correspondence. An instant therefore means the same thing whatever value it starts from.

Five surfaces accept an instant.

• Db::as_of_instant and Db::since_instant in Rust.
• d/as-of and d/since in the Clojure API.
• as_of_instant and since_instant in DbViewSpec on the wire.
• :as-of <timestamp> and :since <timestamp> in the console.
• \as-of <timestamp> and \since <timestamp> in the SQL shell.

The cost of a view today

Partly implemented. The design opens a view by descent through the segment tree. The implementation folds the view in memory from the recorded log. The costs below are what a peer pays now.

• as-of folds the log up to its basis.
• history folds the whole log.
• since folds the whole log and then filters. It narrows before it projects, so the floor is applied while the index is built.

The result is cached in the database value, and it is shared by the clones of that value. A view that selects exactly the datoms of an already-folded view reuses that fold. A genuinely distinct view pays a full fold on first read.

The operational rule: a report that opens many distinct historical views is expensive on a peer. Reuse one view where possible.

Transaction reports

A peer receives a stream of transaction reports. Each report holds the basis before, the basis after, the datoms, and the tempid map for the submitting peer.

The peer applies the datoms to its live index and then offers the report to registered listeners. One stream serves three needs: keeping the peer basis current, sync, and application change feeds.

Reports arrive in t order with no gaps for a connected peer. After a reconnect the peer declares its basis, and the transactor backfills the gap.

Reports are not durable per consumer. A consumer that needs exactly-once delivery must track its own high-water t. tx-range replays the gap.

Reading the log directly

tx-range(from-t, to-t) streams transactions from the log tree. It is available on any peer, and it does not touch the covering indexes. Use it for audit, for replay, and for change-data capture.