Dataflow Diagram

Dataflow diagram

A diagram of a running program is a claim about that program. Every claim it makes must be one the code can be held to, and every claim the code makes must appear. Both halves fail quietly, which is why this is a generator problem before it is a drawing problem.

What this is, and what it is not yet

The principles — derive rather than draw, fail loudly on the unclassified, treat every channel as a claim, check placement rather than eyeball it — come from failures that were observed, diagnosed, and fixed. They should survive contact with a different domain.

The conventions — the specific palette, rounded-versus-square, the edge styles, the frame grammar — are defaults distilled from one project's diagrams. They are internally consistent and worth adopting wholesale rather than assembling from scratch, but they have not been tried against a dataflow shaped differently.

So say which conventions you are adopting and ask where they fight the domain:

• Does compute-versus-data actually partition this system's nodes, or is there a third kind that fits neither?
• Does the palette survive the medium it will be read in?
• Is there a distinction the reader needs that no channel here carries?

When a default does not fit, that override is the finding this skill is missing. Capture it with its reason and bring it back here: a convention rejected for a stated reason is worth more than one nobody has tested, and this skill has more of the second kind than it should.

Two such contacts have now happened. The architecture-diagram skill carries these principles into system-architecture charts — a build-time structure chart plus runtime thread/request charts — and records where the defaults fought (arrows dropped from the structure chart, provenance demoted from a channel, geometry emitted directly). Use it for that duo; this skill remains the home for dataflow and pipeline charts of a single running program. The second contact was a request-lifecycle chart of an io_uring runtime library, whose reader found the defects the sections on placement and on the perceptual gate below now name.

This skill ships in two forms with one structure: the skill itself is complete working defaults — enough for a single-use chart in a repository you cannot modify, or a figure for a talk, where the generator script travels with the artifact and the chart is stamped with the commit it describes, a dated snapshot. The dataflow-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.

Derive, never draw

The nodes and edges come from the program's own structures — its topic registry, its step declarations, its wiring function — read at generation time. Not from a list beside them.

A hand-maintained diagram is a second source of truth. It is correct on the day it is drawn and wrong on the first refactor, and nothing reports the divergence: the picture keeps rendering. Deriving it means a renamed topic either appears renamed or breaks the generator, and both are better than a drawing that quietly describes last quarter's design. It is also what makes the chart cheap to keep: a generated chart is regenerated, a drawn one renegotiated.

Fail loudly on anything unclassified

A generator maps program elements onto visual properties through tables: this operation is that role, this topic is that category. An element no table covers must stop the run, not be skipped.

Silent omission is the worst failure available here, because the output still looks complete: a reader cannot tell a pipeline with four stages from a five-stage pipeline whose fifth stage nobody classified.

The same applies in reverse: a table entry naming an element the program no longer has is a stale classification, and should fail just as loudly.

Every visual property is a claim

Shape, fill, line style, decoration, position: each is a channel, and a reader who sees two things drawn differently will look for the difference. A channel spent on ornament teaches a distinction that does not exist.

Choose the channel from what the distinction is:

• Shape — what kind of thing it is. A container of history, a unit of computation, a fixed value. Shape is the strongest channel; spend it on the taxonomy the reader most needs.
• Fill — which family within a kind: the role an operation plays, the boundary a topic sits on.
• Line style — conditionality. Solid for always, dashed for sometimes, dotted for a dependency that is not dataflow.
• Decoration — a property layered on a kind, such as whether a run records it.

A glyph asserts its own structure. A six-celled queue asserts a history a reader can look back over; using it for a parameter that is written once asserts something untrue about the parameter. Match the glyph to the claim or pick another glyph.

Absence must be legible. When a property is encoded, the elements lacking it should read as lacking it rather than as unmarked. The one input in a chart with no recording mark is a finding, and it is only a finding because the marked ones are visibly marked.

Never encode a value you had to guess. An unknown that renders as a plausible mark is worse than an unknown that renders as a gap.

Draw the state, do not bury it

Before a shape can distinguish compute from data, both have to be nodes. The failure comes first as a stateful component folded into the label of the thing that reads or writes it: "recorded streams → rings", "writes to the queue", "the cache".

A component that holds data across invocations — a ring, a queue, a cache, a store, a cursor — is not an implementation detail of its reader. Draw it. The properties that make it worth drawing belong to nothing else in that box: what it holds, how much, what happens when it fills, who else writes it, and whether a reader can fall behind and lose records.

Two facts surface the moment it becomes a node, and neither is visible while it is a phrase:

• Where paths converge. Two producers meeting at one buffer is an architectural claim a label cannot make. In one case that fact moved the boundary of a chart's central claim: two paths drawn as separate pipelines turned out to meet at the ring, so what was "identical from the program down" was really identical from the buffer down, one node earlier.
• Where records can be lost. Loss happens at the thing with a capacity. A chart that draws only processes has nowhere to put the loss, so it does not show it, and a reader concludes the path is lossless.

The test: if a box names both a process and something that outlives it, split it. A file being read and the buffer it is read into are two nodes, and only one of them can lap.

Compute and data must not share a silhouette

The first thing a reader resolves is what kind of thing is this, and they resolve it from outline before they resolve fill or text. Give the two halves of a dataflow different corners:

• Compute — rounded. Operations, steps, stages, transforms: anything that runs. shape=box, style="rounded,filled".
• Data — square. Queues, topics, buffers, parameters, stores: anything that holds. Straight corners, whatever the shape otherwise is.

Rounded-versus-square is deliberately quiet. It should register as a texture across the whole chart rather than as a label on each node, which is what lets fill stay free for the distinction within each half.

Then split data by what it promises:

• A history — a segmented glyph, cells in a row, reading as a queue with depth. It asserts that a reader can look back over a window.
• One value — a solid glyph with no segmentation, such as a dog-eared page (shape=note) for a parameter fixed before the run.

Segmentation is the claim. A six-celled queue drawn on a write-once parameter promises a history the parameter does not have.

Vary a shape before adding one

The shape inventory is a vocabulary the reader has to learn, and it is the one channel with no gradient — two shapes are either the same or they are not. Every shape added costs every future reader a lookup, so a new shape asserts a new kind of thing, and nothing less than that earns one.

Where the thing is the same kind carrying a property, annotate the shape it already has:

• Fill for which family it belongs to — an input queue and a state queue are both queues.
• A frame for a property layered on: recorded, tapped, exported.
• Line style on that frame for whether the property always holds.
• Segmentation, size, a corner for structure within the kind.

These compose. One shape plus three annotation channels expresses more distinctions than four unrelated shapes, and the reader decodes them incrementally: queue, so a history — framed, so recorded — dashed, so only sometimes.

The test for whether something has earned a new shape: would a reader who knows the base shape still recognize it? A framed queue is a queue. A tinted queue is a queue. A page with a folded corner is not a queue, and should only appear because a parameter genuinely is not a history — a different kind, not a queue with a property.

Adding a shape is also the change most likely to strand the key, since a key that fakes shapes can approximate a fill but not a silhouette.

A default palette

Adopt this or replace it wholesale, but do not mix: a palette works because its members were chosen against each other.

Operation fills — ColorBrewer Accent (d3.schemeAccent), pastel rather than saturated because these are large filled areas a reader looks at for a long time:

Data tints, lighter than the operation fills so data reads as quieter than compute:

Edge colors — #4D4D4D for ordinary dataflow, then #CC79A7 and #D55E00 for the two paths worth separating. Those two are Okabe–Ito colors, chosen for colorblind-safe separation; keep them if you change everything else, because edge color is the channel with no shape to fall back on.

Panel — #F2F2F2 fill with a #9E9E9E border, for the key.

An edge vocabulary

Drawing deferred reads dashed and unconstrained is what keeps a cyclic program legible as a DAG: the cycle is visible as data without the layout engine trying to satisfy it.

Decoration layers onto a glyph without changing its kind, so give it the same solid/dashed grammar the edges use: a solid frame for a property that always holds, a dashed frame for one that holds conditionally, no frame for absent — three states from one channel. Scale the frame with the glyph — a padding that is a hairline at chart size is a slab on a key's smaller copy.

Position carries meaning too

Lay the graph out along the direction of flow (rankdir=LR for a pipeline) and pin each topological level to one rank. Columns then are levels, and a column's height is how much of that work is independent — which the reader gets for free from a layout they were going to read anyway.

Where a program has a boundary the reader must respect — asynchronous arrival on one side, deterministic evaluation on the other — pin the inputs into a single column so the boundary can be drawn as one rule.

Number the compute nodes with their position in the evaluation order, as a circled digit (U+2460 onward). That is a real fact about the program, and numbering is only honest when order carries information.

Where one chart would carry two materially different implementations of the same flow, give each its own panel on an obvious comparison axis — top and bottom — and repeat the stages they share. Interleaving two backends' operation names inside one set of nodes makes the shared lifecycle look like shared mechanism, which is the opposite of what a comparison is for. Repetition costs a reader less than one node carrying two vocabularies, and what the variants genuinely share belongs in the prose beside the chart.

The key is a graph, not a picture of one

Draw the key with the same node shapes the chart draws, from the same generator. A key assembled out of table cells or text can only approximate a shape, and an approximated shape teaches the reader a glyph the chart never uses.

That usually means the key is its own graph, merged with the chart at render time so the artifact stays one file. Order its entries by channel — shapes, then line styles, then symbols — so a reader looking up a mark searches one band rather than the whole panel.

Show the glyph, never its color's name. "Lilac means input" asks the reader to translate, fails a colorblind reader entirely, and is inconsistent with every other entry that shows the thing itself.

A key earns its place once the chart carries more than about two orthogonal channels — the point at which a reader stops inferring the encodings from context and starts guessing. Below that it is furniture: a three-node chain with one edge style needs no legend, and adding one implies distinctions the chart does not draw.

Keep it subordinate. Smaller type than the chart, tight rows, and seated in the chart's own whitespace rather than stacked beneath it if the layout has room — a key that reads as a second diagram competes with the first.

Placement is computed, not eyeballed

An inset key must be checked against the laid-out graph, not positioned by looking at it. Check against node boxes and edge splines both: an edge routed through otherwise-empty space is invisible to a box-only check, which will pass a layout it never looked at.

Make the check fail the build rather than warn. The free region moves whenever an element is added, and a warning about a diagram nobody is currently looking at is a warning nobody reads.

A layout engine is doing more placement work than the inset key makes visible, and a generator that emits geometry itself inherits all of it — anchoring, centering a multiline label as one line group, distributing the connectors on one shape, routing to a resolved border, keeping children inside their parent. architecture-diagram carries that set, because emitting geometry directly is its default. Bounds are the weakest check of the group and the only one most generators run: every one of those defects sits comfortably inside the viewport.

Verify the rendering, not the source

Layout engines accept attributes they ignore. A setting on the wrong graph, an attribute the shape does not honor, a size that excludes the border it draws — each fails by doing nothing, and a generator that emitted the attribute looks correct.

After any change meant to alter layout, confirm the rendered geometry actually moved: the bounding box, the node positions, the measured gap. If the number is identical, the change did not land, however plausible the source looks.

Keep the output a text format whose source is the artifact — a .dot or .d2 that renders to SVG, not a pasted raster. A text source diffs and greps; an image is opaque to every tool.

The perceptual gate

Assertions catch drift, not confusion. A chart whose every claim is derived and whose every element is in bounds can still label the wrong thing, name one concept twice, or lay a group out so that the eye reads a difference the program does not have. Nothing in the generator will report that, so a human reads every new chart and every visual change, and approval of one revision does not carry to the next.

When the raster preview that 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.

Then convert what the reader found into an assertion. A visual defect fixed in coordinates comes back at the next layout change; the same defect fixed as a check on the generator's output does not. This is the only mechanism that turns one reader's afternoon into a property of the chart.

One thing readers catch that no check will: a label that spends one word on two meanings. A stage named for poll in a chart comparing readiness polling with task polling names neither, and the reader cannot tell which one the node is about. Reach for 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, with no sentence around it to recover in, is where it hides best.

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.

Red flags

• A node list or edge list maintained by hand beside the code it describes.
• A generator that continues past an element it cannot classify.
• The legend built by a different mechanism than the chart.
• A color named in words in the key.
• A glyph whose structure asserts more than the thing it marks possesses.
• An inset element positioned by adjusting a number until it looked right.
• A collision check that inspects nodes but not edges, or that silently matched fewer elements than the graph contains.
• A multiline label centered by its first baseline rather than by its line group.
• Connectors attached to one shape at points chosen one at a time.
• A child shape crossing the bounds of the cluster that contains it.
• Two implementations interleaved in one flow rather than drawn as panels.
• A label carrying a word that means two things in the chart it sits in.
• A reported visual defect fixed in coordinates rather than in an assertion.
• A chart shipped without a human reading it, or a review gate skipped for an unavailable preview with no record of what stood in for it.
• A layout attribute added without confirming the rendered size changed.
• A diagram checked in as an image with no source beside it.
• Two visual channels carrying the same distinction, or one channel carrying two.
• Presenting the conventions here as settled when applying them, rather than as defaults the domain may override.
• Finishing a diagram without asking which defaults fought the domain.
• Compute and data sharing a silhouette, so kind can only be read from the label.
• A segmented glyph on something with no history to segment.
• A stateful component named inside another node's label rather than drawn.
• A box naming both a process and something that outlives it.
• A new shape introduced for what is really a property of an existing one.
• More shapes in the chart than kinds of thing in the program.
• Palette members borrowed from two different palettes.
• A legend on a chart with one edge style and one node kind.
• Numbered nodes whose numbers encode no order.