Running live and demo at the same time

A host that trades needs both at once: a production subtree taking real orders and a demo subtree exercising the same code with test funds. That is a supervision-tree question, not a configuration-file question, and this family is built for it.

Per-process, not per-node

Application.put_env/3 is node-wide. Flipping the node to demo mid-run redirects every other process on it, including whatever is trading with real money.

So environment resolves through DpExchange.Core.Config, which reads in this order:

1. an explicit :environment in the call's options — wins always
2. Core.Config — the calling process, then its $callers ancestor chain, then application env
3. :production

Step 2's ancestor walk is the one that gets omitted in reimplementations, and omitting it makes the seam work in simple cases and fail in exactly the concurrent ones it exists for.

Two subtrees, side by side

children = [
  Supervisor.child_spec(
    {DpExchange.Gemini,
     name: :gemini_live,
     credentials: live_credentials(),
     environment: :production,
     symbols: ["BTC-USD"],
     subscriber: LiveConsumer},
    id: :gemini_live
  ),
  Supervisor.child_spec(
    {DpExchange.Gemini,
     name: :gemini_demo,
     credentials: demo_credentials(),
     environment: :sandbox,
     symbols: ["BTC-USD"],
     subscriber: DemoConsumer},
    id: :gemini_demo
  )
]

Two id:s and two name:s, because they are two independent venues that happen to share a module. The credentials are different accounts — a production key does not work against a demo host and should never be sent to one.

Crossing a process boundary

A GenServer runs in its own process and will not find the caller's dictionary at all — it is not in the caller's $callers chain. Resolve in the caller and put the answer in the message: DpExchange.Core.Config.snapshot/1 on the way in, DpExchange.Core.Config.resolve_snapshot/4 on the way out.

Resolving inside the server is too late, and it fails in the direction that looks like it works: production is unaffected, so only a consumer's async suite breaks.

What each venue actually offers

Demo environments are not a uniform feature, and the differences change what you can test.

Gemini's is the one that changes what this family can honestly offer: order placement, cancellation and balances stop being "never a test, answered only in production" and become testable against real venue machinery with fake money. A consumer can run its whole trading integration there before a single real order.

Webull's half-story, and why it refuses rather than falls back

environment: :uat gives REST and no stream. mqtt-uat.webullbroker.com is NXDOMAIN; there is no UAT broker, and no configuration produces one.

subscribe/2 in UAT refuses. It does not fall back to production, because a consumer testing against UAT while receiving production prices would be reading real market data believing it was fake — which is the failure mode this whole family is built to prevent.

Environment.streaming?/1 exists so a caller can ask before it commits, rather than discovering the gap from a subscription that never delivers.

Production is the default, and that is the safe direction

The wrong default fails in only one direction:

• defaulting to production means a consumer who meant demo sends a real order to a real exchange with real money — silent, and expensive
• defaulting to demo means a consumer who meant production gets test balances and obviously-wrong prices — loud, and free

Gemini's demo book is crossed as often as not; a frame captured 2026-08-28 carried a bid of 68169.88 against an ask of 64886.32, which no real venue would ever show. You will notice.

So: production is the default, and selecting demo is an explicit act. An unrecognised environment value raises rather than resolving to production — a typo must not quietly become a live order.

A documentation defect worth knowing about

Gemini's market-data page names exchange.sandbox.gemini.com as the sandbox base URL. That is the website, not the API. Measured 2026-08-28: https://exchange.sandbox.gemini.com/v1/symbols returns 404 and an HTML page, while https://api.sandbox.gemini.com/v1/symbols returns 391 symbols.

A consumer following the vendor's page gets 404s that look like a broken endpoint rather than a wrong host. This package uses the correct hosts; the note is here so the vendor page does not send you the other way.