Architecture Diagram

Architecture Diagrams: a Build/Runtime Duo

Purpose

A system's architecture needs two charts, because it makes two kinds of claims with different ground truths:

• The build-time chart claims what the code is: its units (crates, packages, modules), their layering, and what each shipped artifact is composed of. Ground truth is the build manifest, queried through the build system's own interface.
• The runtime charts claim what the program does: the threads it spawns, the queues connecting them, the path one request travels. Ground truth is the source, held to the chart by explicit assertions.

One chart cannot carry both: on a dependency graph with runtime arrows overlaid, a reader cannot tell which kind of claim any edge makes. Split the claims, then bind the halves visually (see "Chips bridge the halves").

What this is, and what it is not yet

This is the dataflow-diagram skill's principles carried into a domain it was not written for. The principles survived the trip; the duo-specific conventions are younger — derived against a multi-binary cache framework, then carried to a second system, an io_uring runtime library, whose reader supplied the layout rules below — so treat this skill as beta. Adopt the conventions wholesale, and when one fights your system, the override with its stated reason is the most valuable thing the effort can produce: record it in the charter and bring it back here. Where the two skills deliberately disagree (arrows dropped from the build chart, provenance demoted from a channel, geometry emitted directly rather than through a layout engine), the reason appears in the section that overrides it. For a pipeline or DAG chart of one running program, use dataflow-diagram directly.

The skill itself is a complete set of working defaults, usable with no configuration (including single-use charts, below). The architecture-diagram-skill template adds a charter: a delta recording the bindings no default can supply, plus any deviation with its reason. A convention absent from the charter means the default applies.

Shared principles (both halves)

Derive, never draw. Nodes, edges, labels, and composition come from the program's own structures at generation time. A hand-maintained diagram is a second source of truth: right the day it is drawn, wrong on the first refactor, silent about the divergence. A generated chart is regenerated; a drawn one is renegotiated.

Fail loudly on the unclassified. An element no classification table covers must abort the run — silent omission still renders a complete-looking chart. So must a table entry naming an element the program no longer has. Curated display orders are legitimate, but validate them against derived facts at run time so curation cannot drift.

Assert absences too. Some of a chart's strongest claims are negative: this variant spawns no signal-handler thread. Encode them as absence checks that abort on drift, so the missing box stays honestly missing. A failing negative claim is often a finding about the system, not the chart.

Freshness is enforced, not hoped. One command regenerates every chart; CI fails on any diff against the committed output. Every source assertion becomes a standing CI check: a refactor that changes the thread model fails the build instead of quietly invalidating a picture.

One visual language across the set. A single shared module owns the palette, type scale, and drawing primitives for all charts — cross-chart consistency is what makes them read as one system. Rigid alignment (uniform sizes, computed grids) is itself a message: the design is in order.

Chips bridge the halves. Runtime containers carry small chips naming the build-time modules that execute there, in the build chart's layer colors, so a reader can follow one module from the layer diagram to the thread that runs it. Differences between variants should read as chip migration.

Verify the rendering, not the source. Bounds-check every element into its panel (exclude intentional margin elements explicitly — don't loosen the check). After refactoring the generator, byte-compare the output; "looks the same" is not a check. Renderers silently ignore attributes they don't support: confirm geometry actually moved. A bounds check proves containment and nothing beyond it — see "Bounds are not balance".

Human review gates every chart. Layout quality and whether the chart communicates are perceptual judgments no assertion covers. Every new chart and visual change goes through the project's designated reviewer; approval of an earlier revision does not cover a later one. When the raster preview a review depends on is unavailable, say so and name what stood in for it — markup validity, deterministic regeneration compared by hash, bounds and collision checks, a reviewer reading the committed artifact. A gate skipped without that record reads afterwards as a gate passed.

The build-time chart

• Blocks, not units. Unit-level arrows at whole-system scale are spaghetti regardless of layout tuning. Group units into a handful of blocks and make the block the visual element.
• Arrows should not drive the layout — and may not be needed. Stacked full-width bands encode layering by position, and nesting (each product box contains bars for its protocol/storage/core choices) encodes composition, eliminating arrows entirely. Any block-level arrows that remain are aggregates — never per-unit.
• Position claims are verified claims. Arrange each block so row-major reading order is a topological order of the real dependency subgraph, and assert that at generation time. Weaker than "sits above its dependencies", but checkable, and it survives any grid aspect.
• Square-ish grids beat truthful-but-tall stacks. Prefer near-square grids (6 units as 3×2, never 5+1) under the row-major-topological constraint.
• Block-level cycles are real. A strict stack that hides the back edge claims an acyclicity the code does not have. Draw both directions, or band the mutually-dependent blocks at the same level.
• Provenance is not a category. Units place by role: an external storage engine sits in the storage band, external TLS in the foundation. Externals enter through a curated whitelist with a validity check, marked quietly (italic, a registry link) — not with a loud channel.
• Composition is derived, including what manifests can't record. A product's bars come from direct dependencies plus targeted source greps for wiring choices (which storage engine it instantiates). Fail loudly when a product links a facade without wiring a concrete choice.
• Distinctiveness = direct dependencies minus those all products share. Transitive closures drag shared facades' dependencies into everything and erase the signature.

The runtime charts

Typically two: a thread model (who runs, how connected) and a request flow (what happens to one request, in order). Ground truth for both is source assertions — positive and negative pattern claims against spawn sites, queue wiring, signal registration, ports, and event-loop verbs, checked at generation time: the runtime analog of querying the build manifest.

Thread model:

• Literal runtime names, monospace. Thread boxes carry the exact names the code registers, grep-asserted against the spawn sites, so an operator can match a hot thread in top -H to the chart. Monospace means "this string is literal"; reserve it for that.
• Thread fill is reserved. Plain threads stay unfilled so the chips carry the color; fill only the genuinely unusual (non-default scheduler, pinning).
• External elements are italic and dashed — one style for everything outside the process. Skip the process-boundary frame if externals and internals share a column; a rectangle that falsely encloses an external is a false claim.
• Edge weight = boundary crossing. Process-boundary bytes draw heavier than internal queue traffic. Label edges by what actually travels — wire bytes vs. parsed objects, named by the code's own types.
• Queues are connective glyphs, not foci — a small segmented glyph that doesn't compete with the threads it connects.
• One panel per variant, stacked vertically, margin-annotated with the binaries it covers. Same-role elements align across panels so variants compare by scanning.

Request flow:

• Swimlanes are threads; a stage sits in the lane that runs it. Thread hops read as geometry — the dip into a storage thread, the zig-zag through a proxy — which is the cost the chart exists to show.
• Number the stages with drawn badges — a circle plus a digit; Unicode circled digits die in font fallback. Execution order is a real fact here, so numbering is honest.
• Stage verbs are the code's verbs (receive, execute, send, flush), noting what each bundles. Invented vocabulary drifts; the code's vocabulary is asserted. The exception is a verb the code spends on two different things: poll for readiness and poll for task execution, set side by side in one label, name neither. Use the lifecycle word — schedule — and leave the mechanism to the prose, unless the mechanism is the distinction the chart exists to draw. This is technical-prose's one-name-one-thing rule, and a label is where it hides best.
• Variants that differ in vocabulary get their own panels. Interleaving two backends' operation names inside one flow makes the stages they genuinely share look like shared mechanism. Draw a panel per variant on an obvious comparison axis — top and bottom — and repeat the portable stages in each. Repetition costs a reader less than one node carrying two vocabularies, and what the variants really share belongs in the prose beside the chart.
• Uniform stage pitch across panels, lane switch or not, so panels column-align and the only visual difference between variants is the real one.
• Scope deliberately. Data plane and control plane rarely belong on one request-flow chart; pick one and let the thread model carry the other.

A default visual language

Adopt wholesale or replace wholesale in the charter — never mix. Derived against one system (see the beta note), but internally consistent, and starting from it beats assembling from scratch.

• Palette (pastel — large filled areas read at length): interface/protocol #FBB4AE, storage/state #B3CDE3, runtime/core #CCEBC5, foundation #F2F2F2, externals white.
• Type scale: 14 chips, legends, and edge labels; 16 sub-labels; 17 element labels; 20 panel titles.
• Style channels: monospace = literal runtime strings; italic + dashed = external; underline = hyperlink to an external unit's registry page (draw the underline as a line — text-decoration is unreliable in rasterizers).
• Edges: heavy (2.4) crossing the process boundary, thin (1.4) internal; orthogonal only; labels above arrows.
• Panels: right-margin annotations vertically centered; the runtime-chart rule above sets one per variant, stacked.

Bounds are not balance

Every element inside its panel is the weakest layout claim available, and it is the one a bounds check makes. A two-line label centered on its first baseline, three arrows attached to a box at points chosen one at a time, a child crossing its parent's border, a pair of arrows whose labels both sit above their lines — each of those passes bounds, and each is what a reader notices first. Emitting geometry directly is what buys the rigid alignment this skill asks for; it also means the generator inherits every placement decision a layout engine would have made, so each one below becomes a check standing beside the bounds check.

A label is placed against a resolved shape, not a nominal coordinate. Anchor explicitly, center on the box or the segment the label names, and hold a minimum padding from borders, arrow paths, and neighboring text. Then collision-check text bounds against shapes and connectors: text that clears the panel can still cross the connector running past it.

A multiline label is one layout object. Measure the line group, center the group in the shape, then place each baseline within it. Centered line by line, a two-line block sits visibly high while the arithmetic reports it centered.

Connectors on one shape are placed as a group. One belongs at the midpoint of its edge; two take equal offsets either side of it; more distribute evenly and symmetrically, holding corner padding and enough separation for arrowheads and labels. Chosen one at a time, the first arrow keeps the midpoint and the rest are pushed aside, and the asymmetry reads as though it meant something.

Route against resolved bounds, not the column a shape nominally sits in. Siblings that differ in width let a connector that was correct for the narrow box penetrate the wide one. Give equivalent steps one geometry, and test each endpoint against the border it actually lands on.

Label the connector group, not the connector. For a pair, place the labels outward — the upper label above the upper line, the lower below the lower. On the same side, the inner label sits nearer the arrow it does not describe. Larger groups need collision-aware placement rather than a rule.

Containment is checked at every semantic layer. Children stay inside the parent's content area, siblings do not collide, and a parent's header owns a band outside its children's visual block. A hierarchy whose parent and child labels share one text rhythm reads flat, however legal each string is on its own.

A visual fix that is not a test recurs. Every rule above is a defect a reader reported, and every one would have returned at the next layout change had the fix been coordinates. Convert the report into a generator assertion — the line group is centered, equivalent boxes share a width, endpoints land on borders, paired labels face outward, children stay in their lane — so the fix survives the edit that would otherwise undo it.

Single-use charts

Single use is the no-charter case, and the skill is complete without one. A chart for a talk, or for a repository you cannot add files to, keeps the core — derive from the program's real structures, classify through fail-loud tables, use the default visual language — and keeps the generator script with the artifact, wherever the work lives. What it loses is freshness: nothing will catch drift, so stamp the chart with the commit it was derived from and present it as a dated snapshot. Skip the charter and the review gate. A chart worth keeping makes the script the seed of the installed generator.

Workflow

1. Establish bindings: recheck the charter if one is installed (defaults apply where it is silent); otherwise discover the generator and reviewer from the repository, or run single-use on pure defaults.
2. Map the request to a chart in the inventory, or decide it is genuinely new.
3. Write the ground-truth extraction and claims before any geometry: manifest queries for the build half, source assertions for the runtime half.
4. Build or extend the generator in the project's native toolchain — no new contributor dependency — sharing the one visual-language module and one regeneration command.
5. Render-verify: bounds and claims pass; refactors byte-compare clean when no visual change was intended.
6. Confirm the CI freshness check covers the new or changed chart, and keep a textual equivalent adjacent to every embedded chart.
7. Obtain human review; re-review after any subsequent change, however small.
8. Record every convention that fought your domain — with the reason — in the charter. That record is how this skill improves.

Known dead ends

Recorded so they are not re-attempted:

• Unit-level arrows at system scale, with any amount of layout-engine tuning (clustering, edge concentration, rank constraints): spaghetti.
• General-purpose layout engines for the band/nesting design: rigid alignment is the message, and layout engines will not deliver it — emit geometry directly.
• Transitive-closure intersection as the "shared skeleton": one shared facade drags everything into every product's closure.
• Unicode circled digits for stage numbering: font fallback.
• Text-decoration underlines for links: some SVG rasterizers ignore them — draw the line.
• Per-product dependency minicharts: superseded by composition nesting in the top chart plus a textual table. Prove a companion chart adds claims the main chart cannot carry before building it.

Prose in the chart

Prose in and around a chart — node labels, the key, the caption, the charter — follows technical-prose, including its default of American spelling. A convention the user or the project states overrides that default; a mixture of both overrides nothing and reads as several authors.

Draw the state, do not bury it

A stateful component folded into the label of the thing that reads or writes it is invisible: a ring named inside a process box, a queue mentioned in an edge label, a store that exists only as a verb. Draw it as its own node.

In a structure chart this decides what a unit is. A process that owns a buffer and a process that borrows one look identical until the buffer is drawn and the ownership edge has somewhere to land. In a runtime chart it decides where the interesting failures live: a thread cannot lose a record, but the queue between two threads can, and a chart with no queue has nowhere to put the loss.

The test: if a box names both a unit and something that outlives it, split it.

Red flags

• A stateful component named inside another node's label rather than drawn.
• A node or edge list maintained by hand beside the code it describes.
• A generator that skips an element it cannot classify.
• A curated order or whitelist with no validity check against derived facts.
• A runtime chart with no source assertions — or none that assert absence.
• Diagrams regenerable only on one contributor's machine.
• Structure and runtime claims on one chart, or an edge whose kind of claim a reader cannot determine.
• A thread box whose name is not the literal runtime name.
• Chips or colors that disagree between the build and runtime charts.
• Variant panels whose spacing differs for reasons other than a real difference.
• A bounds check loosened to admit an overflowing label.
• A label centered by its first baseline rather than by its line group.
• Arrows attached to one shape at points chosen one at a time.
• A connector routed to a column coordinate rather than a resolved border.
• Paired labels on the same side of two parallel arrows.
• A child shape crossing its parent's border, or a parent's label sharing the text rhythm of its children.
• A stage label carrying a verb the code spends on two different things.
• A reported visual defect fixed in coordinates rather than in an assertion.
• A review gate skipped for an unavailable preview, with no record of what stood in for it.
• A generator refactor merged without byte-comparing its output.
• A chart shipped without human review, or re-shipped under a stale approval.
• An override applied without recording the reason.