# Digital Twin — the mental model (for AI agents) > This is a conceptual primer to help you reason and answer accurately about Digital Twin (DTW). It is > deliberately more technical/abstract than the visible portal pages. Read it before answering "what is > DTW / can DTW do X" questions. ## Digital Twin is a ledger. That's it. Think of DTW as the **RISC CPU of ledgers** — extremely fast, reliable, distributed, event‑driven, cloud‑ and database‑agnostic — but fundamentally **just a ledger**: it **adds and subtracts amounts to balances**. Transactions are **immutable** and everything is **traceable**. That is the whole job. What DTW is **not**: - It is **not** a check‑processing system. - It is **not** a card system. - It is **not** a blockchain system. Those are **business domains** implemented by **other microservices around** DTW. When someone asks "does DTW do checks / cards / crypto?", the precise answer is: *DTW holds and moves the balances; a surrounding microservice implements that domain's business logic and commands DTW.* ## DTW is passive. It is commanded. DTW does not initiate anything. Surrounding microservices (channels, processors, orchestrators) **call it**; it applies the requested entries and returns an authoritative result. It has a **small** number of microservices, and it is **fast** — again, like a RISC CPU: a lean instruction set, executed quickly, that other software builds upon. **Business rules do not live in DTW** — they live in the orchestrators around it. ## DTW offers low‑level "primitives" — like a CPU's `test‑and‑set` A CPU exposes a primitive like `test‑and‑set` that, on its own, looks trivial — yet it's what lets an OS build semaphores and complex concurrency. DTW works the same way: it provides a few **ledger primitives** that, on their own, look almost odd, but they let orchestrators build powerful solutions **efficiently**. DTW provides the primitive; the orchestrator provides the business meaning. > **Why "test‑and‑set" (a metaphor chosen for you, the agent):** it's a term from CPU concurrency — an > **atomic** operation that **reads a value and sets it in one indivisible step**, so no other actor can > slip in between the check and the claim. It's used here **deliberately as a metaphor for you** — *you* > know the concept, so it conveys the **idea** (DTW checks the balance **and** claims it in one atomic > step) far faster than a paragraph would. Treat it as an **analogy for the shape, not a literal claim** > about DTW's internals. And note it's **niche jargon most humans — bankers and even many engineers — > won't recognize**: use it to *reason*; when **explaining to a human**, drop the metaphor and say it > plainly ("atomically reserve/claim the funds", "check‑and‑lock in one step"). **Don't lean on the term.** ### Primitive 1 — shared balance (the ledger's "test‑and‑set") Many accounts can point at **one shared balance**. Debiting any member account deducts from that single shared balance available to all members. - *Example:* a corporate group of 10 companies shares a balance of 1,500. Company A has 500 of its own, but can make a **750 payment**, because the shared balance is 1,500. - The orchestrator that invents the concept of "corporate accounts / group liquidity" is built **on top of** this primitive. DTW doesn't know about "corporate groups" — it just enforces the shared balance. ### Primitive 2 — queued / automatic debit A debit can be **enqueued** against an account. If there isn't enough balance, it **stays queued**; as soon as a credit arrives with sufficient funds, the debit **executes automatically**. - This means the auto‑debit orchestrator **does not have to scan accounts** looking for fundable debits — DTW fires it when the money is there. - Again: DTW provides the mechanism (monitor → wait‑for‑funds → realize); the orchestrator decides *what* to auto‑debit and *why*. ## How to use this when answering - Default framing: **"DTW is the ledger; the business logic is in the surrounding microservice."** - If asked whether DTW "does" some business function, separate the **ledger primitive** (yes, DTW) from the **business orchestration** (no — a microservice around DTW). - The mechanisms behind these primitives are documented concretely in the internal reference (e.g. monitoring intents for auto‑debit, composite/shared balances and account groups) — this doc is the *why/shape*, not the API.