Storage mode

In one line: a per-component, design-time choice of memory layout and ACID guarantees — Versioned, SingleVersion, or Transient.

Set on the [Component] attribute and fixed for a given (component name, revision) — to change the mode, bump the [Component] revision; re-using the same revision with a different mode throws InvalidOperationException on reopen. Because it lives on the component type, one archetype freely mixes all three. The mode decides whether MVCC/isolation exists at all — and what a write costs.

Mode	Isolation	Rollback reverts a write	Durability	Write cost (Zen 4)
Versioned (default)	snapshot isolation, MVCC history	yes	zero loss, full ACID	~250 ns
SingleVersion	none — live, last-writer-wins	no by default; yes under the Commit discipline (O(1), staging discarded)	≤ 1 tick loss (tick-fence WAL)	~40 ns
Transient	none — live	no	none — heap only, gone on crash	~40 ns

Measured on Ryzen 9 7950X (Zen 4), .NET 10 Release, hot cache. A Versioned write is ~6× a fast-mode write (copy-on-write: allocate a chunk, copy the value, append a revision, stamp a TSN); reads follow suit — ~80 ns vs ~15 ns (~5×).

📌 Committed is not a fourth mode. It is the Commit discipline layered on the byte-identical SingleVersion layout — commit-time, zero-loss, atomic durability and O(1) rollback, without a revision chain. It moves three ACID properties at once — isolation, rollback and durability — which is why the enum is named for the commit, not for durability alone.

How it relates

• Snapshot isolation — only Versioned provides it.
• Durability — mode & discipline — the Commit discipline applies only to the SingleVersion layout.
• Transaction — what "transactional" guarantees per mode.
• Tick fence — where SingleVersion durability is realised.
• Cluster storage — the implicit consequence: one SingleVersion/Transient component flips the whole archetype to clustered SoA (~50× faster bulk iteration).

In the API

• StorageMode — the enum (Versioned / SingleVersion / Transient), set via [Component(StorageMode = ...)].
• CommitDiscipline — the SingleVersion TickFence ⇄ Commit escalation.

Learn & use

• Narrative: Guide ch.2 §2 — the decision that matters most
• Reference: Isolation & durability cheat sheet
• Feature detail: Storage modes — Versioned · SingleVersion · Transient · Committed