Eighteen boxes, none of them lit

This is the first piece of drogna itself — as opposed to the site you are reading — that there is anything to look at. It is an application in which nothing works, and that is the finished state of it.

Eighteen boxes: every component drogna intends to have, arranged as the flow chart the architecture actually is, with its loop — monitor, scheduler, model runner, publisher, back to the monitor — drawn as a loop rather than flattened into a row. Zero of them lit.

Why nothing lights

The rule is one line of the constitution and it is not negotiable. Any display of what exists is driven by observed liveness, never by a configuration file listing what ought to exist, and never by mocked traffic. A component is lit because a message from it arrived inside the liveness window that message itself declared, and for no other reason. There is no enabled: true, no hardcoded list of live components, no demo mode, no fixture mode and no populate-for-the-screenshot path.

The reasoning behind the last of those is the part worth spelling out, because populating a display with plausible traffic is such an ordinary thing to do. A mock asserts the existence of something that does not exist. That is not a small compromise on the way to the real thing; it is precisely and exactly the failure this display exists to prevent. A system whose picture can claim a component is alive when it is not has a picture that is decoration. Everything drogna hopes to be evidence of rests on that picture being unable to lie, and the cheapest way to guarantee it is to never build the code path in the first place.

Which leaves a testing problem: how do you check that something is absent? Bluntly. A test reads the client's own source, every file of it, and asserts that none contains a demo mode, a mock, a stub, a fixture, a build flag, or a read of the URL's query string — and separately that nothing under the client ever publishes on the broker, only subscribes. A second test, living with the deployment rather than the client, asserts that nothing in the client reads the Compose file, the generated environment file, or the list of which services this destination starts. Both are crude instruments. The property being defended is an absence, and the only way to check an absence is to look.

Lit does not mean working

The legend says so in those words, and it is the most important sentence on the page. A component that publishes heartbeats while failing at its job is lit, and reports its own status — starting, ok, degraded, stalled, stopping — beside its box. Lit means heard from. The display is not qualified to say more than that, and does not.

There are four states rather than two, and the third is the interesting one.

Dark means nothing has been heard, or what was heard has expired while the page was listening. Cannot tell means the evidence expired while the page could not listen at all. Those are different claims: a client that has gone deaf cannot honestly report a death, and folding the two together would let a network problem look like a failure. It is not a shade of lit — nothing on this page can produce lit without evidence — it is a refusal to say.

This page is the fourth, and it applies to exactly one box. The browser client cannot hear itself over the broker, so rather than fabricating a heartbeat for itself it is drawn with a dashed outline and labelled as what it is: the page you are looking at, lit by its own presence.

Every state carries a word and a mark as well as a colour, and the word is the primary carrier. That is because these screenshots are meant to be printed in blog posts, possibly in greyscale, and a display distinguished only by hue does not survive the trip. NOT HEARD FROM is written inside every dark box above.

Two panels for the awkward cases

A heartbeat arriving under a component id the layout does not recognise is the most interesting message this page can receive: something is running that the picture does not account for. Discarding it would be the display hiding a genuinely live component in order to keep its own diagram tidy. So it gets its own panel, with the unknown id printed exactly as it arrived.

The other panel handles two processes publishing the same component id, each still inside its own window. The page cannot tell which one the drawing means, and choosing would mean hiding something live, so both are listed by run id and configuration digest and the conflict is left visible for someone to resolve where it can actually be resolved.

Square corners and round corners

The boxes have two shapes. Square corners mark bespoke components — logic written for drogna because it could not be had off the shelf: the divergence rules, the scheduling policy, the sound speed and uncertainty mathematics, the quality flagging. Round corners mark plumbing: a broker, two stores, a query layer, a reverse proxy, and the scaffolding around them.

Ten of the eighteen are round. That distinction is drawn on the diagram deliberately, because a reader ought to be able to see at a glance how little of this system is actually novel, and an architecture drawing that makes every box look equally impressive is doing its author a favour rather than its reader.

What is not built

Almost all of it. Seventeen of those eighteen boxes are components that do not exist yet or do not yet run; the eighteenth is the page itself. The drawing is a statement of intent, and it says so on the page, in words directly above the diagram. Adding a box to this layout adds a grey rectangle and nothing else — no claim, no capability, no change to what the page can report.

The first component to publish a real heartbeat will change exactly one of those boxes, and because of everything above, that change will mean something.