The most common mistake I've seen is not agreeing on what arrows represent: control or data. Does A-(customer data)->B mean A asks B for data or A sends customer data to B?

Of course, sequence diagrams make it clear with two separate arrows when control and data flow in different directions, but a lot of diagrams are of the "plain old boxes and arrows" variety.

• HotGarbage

  today at 2:49 PM

  This is why the C4 Model insists on using verbs to label interactions. (e.g. “reads/writes data from”, “sends reports to”, etc).

  Most of the article's diagrams are actually terrible in this regard.

  • kqr

    today at 5:05 PM

    This is an older discoery than that. Expert systems back in the day often modeled knowledge as graphs with the arrows being labeled with the specific relationship between the things.

    It works because (node, edge, node) triplets then form propositions, the fundamental units of knowledge

    Come to think of it, expertise researchers still do this today to make rough sketches of domains of study. The result is called a concept map.

  • cenamus

    today at 3:47 PM

    C4 is great, even if I can't be bothered to model every layer

• sly010

  today at 8:20 PM

  There is in fact a 3rd: build time dependencies.

  Control: Object/module/function A calls object/module/function B.

  Data: The call can either push or pull data.

  Build dependency: The call can be direct (A depends on B) or indirect through an interface/callback/etc (both A and B depends on the interface).

  Ideally every design document includes all 3 as separate diagrams.

  • sly010

    today at 8:33 PM

    Then of course there is the whole fractal nature of software. As you add more detail, the arrows can flip flop around. Polling in a lower level of a stack can very much be used as a push mechanism. (e.g. USB interrupts are in fact the host polling the device)

    This is why communicating architecture is often as hard if not harder than implementing it.

• NalNezumi

  today at 2:59 PM

  The one solution that works for me is to color code each arrow and at the top left of the diagram add a legend that describe what each colored arrow represent.

  This way sometimes the color can describe control, data, and sometimes even teams expected to implement this arrow by color coding teams.

  The latter is very helpful for cross team meetings to make each group focus on the part of the diagram that will affect them the most, and give pointed feedback to assumptions and lack in specs

• pepperoni_pizza

  today at 3:21 PM

  Yup, we had exactly those hangups when diagrams showed data flowing from restricted network system to data lake. The data is generated and owned by the system and the lake has a secondary copy, except the physical implementation is that the lake opens a connection and pulls. Somehow that is forbidden and we spent months fighting firewall people. Fortune 50 is fun.

• zabzonk

  today at 1:53 PM

  In high-level diagrams, which I think is what is being discussed here, I like to think that A --> B means that A "uses" B in some way, and leave it at that.

  • growse

    today at 6:38 PM

    I do similar, but frame it in terms of dependencies.

    The database can live without the web server, but the web server doesn't work without the database.

    Therefore webserver ---> database.

    Key thing in that these deployment / context / container diagrams don't have a temporal axis. If you want to represent a flow, then you want a diagram where time has directionality, like a sequence diagram.

  • segmondy

    today at 2:03 PM

    yup, A interacts with B with the interaction originating from A.

    • chrisweekly

      today at 5:41 PM

      related tangent (outside of diagrams) lt < and gt > symbols are often dangerously ambiguous; does

          A > B > C  
      

      mean "A then B then C"? or "A is superior to B which is superior to C"?

Idk, while system architecture diagrams look cool and feel informative, I generally don't feel like they actually help you get started working somewhere on a project. Mistake #3 in this article, putting too much in, is part of this.

So https://www.jerf.org/iri/post/2025/on_layers_and_boxes_and_l... is an interesting take: put links in your diagram, so it functions as a table of contents. This seems most useful for someone who needs to start working on a project.

Similarly https://haskellforall.com/2026/02/browse-code-by-meaning asks how to show what's in a repo, but maybe file tree is not best and a diagram with links as table of contents is the answer.

That said practically speaking, I'm not sure what tooling easily creates working links in a diagram that looks good in any context, for instance mermaid might render on github but not a text editor.

Of course for other purposes maybe just go crazy with the diagram. I once had a coworker draw this super detailed master diagram, maybe 50-100 things on it, which I was told impressed senior government officials (after my manager recolored all the red to avoid connoting errors). But for the purpose of orienting developers a table of contents with links sounds better.

• zahlman

  today at 4:33 PM

  > Idk, while system architecture diagrams look cool and feel informative, I generally don't feel like they actually help you get started working somewhere on a project.

  My reaction to the title was that trying to create the diagram is the mistake. If you can't explain it in prose, simplify.

  • dpark

    today at 4:51 PM

    A picture’s worth a thousand words.

    A diagram is a dense way to express information. The same information in prose would take much longer for a typical human to absorb.

    > If you can't explain it in prose, simplify.

    Simplify what? The system? Usually you can’t just throw things away from the system to make it easier to describe.

    • vanviegen

      today at 5:13 PM

      > A diagram is a dense way to express information.

      I'd say it's a lossy way to express information. I find that architecture diagrams often cannot express the exact concepts I mean to communicate, so you're left trying to shoehorn concepts into boxes that are somewhat similar, and try to make up for the difference using a couple of cryptic words.

      Prose doesn't look as nice, but allows me to describe exactly what I want to say, on any level of detail required. Of course, like with a diagram, you do need to put in significant time and effort to make it comprehensible.

      • dpark

        today at 5:42 PM

        > I'd say it's a lossy way to express information.

        A simplified explanation of the system is by definition lossy. This equally applies to a plain English description.

        I’ve been in many design reviews and similar forums where someone has attempted to present a design through written English and finally someone says “we need a diagram here; this is too much to follow” and everyone in the audience nods because they are all lost.

        One of the problems with trying to communicate system design with prose is that it makes sense to the person who writes it and has full context, but the audience is often left confused. Diagrams are often easier to follow specifically because they look under specified when they are.

        • vanviegen

          today at 8:47 PM

          > finally someone says “we need a diagram here; this is too much to follow” and everyone in the audience nods because they are all lost.

          Yes, that happens. I can't remember any occasions where the diagram actually cleared things up though.

          Coming to think of it, one way that seems to be pretty effective at getting complex designs across is in an interactive presentation with the presenter drawing on a whiteboard, starting simple and adding stuff while explaining what and why. The narrative is very important though. The whiteboard drawings by themselves are absolutely useless.

    • zahlman

      today at 5:26 PM

      > Usually you can’t just throw things away from the system to make it easier to describe.

      You can't throw away requirements, but sometimes there don't need to be as many moving parts behind the curtain as you think in order to implement those requirements.

      • dpark

        today at 5:45 PM

        This is essentially a statement that the system shouldn’t be unnecessarily complex. And sure, but that’s not really relevant to the discussion.

        If you have a complex system, whether due to legacy or due to actual necessity, you aren’t going to redesign the system just for the sake of simpler explanation. Indeed if someone couldn’t explain the system in its current state I would have zero confidence they could successfully simplify it.

        • zahlman

          today at 5:54 PM

          My point was that the attitude of being able to explain systems by drawing them, leads to over-architecting them. If you stick to prose then you can't as easily delude yourself about the complexity by staring at pretty pictures.

          I was not considering the case of documenting already existing systems, just talking about the planning stage. Your point is well taken.

  • stronglikedan

    today at 5:22 PM

    Some stakeholders will gain more understanding from a diagram than any amount of simplified prose, so both are typically helpful.

The biggest mistake is not knowing your audience.

Is the diagram for marketing? A sales proposal? A business person using the product? Technical peer?

If you don't know this, you don't know if you have the right level of detail.

• pinko

  today at 5:12 PM

  Underrated comment in this thread, which is full of asserts of universal abstractions and patterns which are not universal. (And of course this insight applies to all kinds of written communication, diagrammatic or prose...)

This is just an advertisement for their service.

In my 20 years in this field I can easily count on one hand the times a diagram like this has been useful. I’ve seen more cases where they were clearly created to satisfy some exec that wanted to see it and never updated again.

As the resident Diagram Maker at my job I really appreciate any and all discourse on the topic. Knowing the purpose of your diagram is a hugely under-appreciated part of the process. Service flow chart or system architecture? High level system overview or actionable, followable flow-chart? The engineer in me always wants to put All The Things in the chart, to make it maximally "correct". It's never the right move. But how to make it clear what's included or not, and why?

I still struggle with finding the best approach each time; I'd love more discussion of this stuff.

• corstian

  today at 8:48 PM

  Multiple interlinked diagrams? A whole bunch of diagrams at different levels, from different perspectives all designed to answer different questions?

• exogenousdata

  today at 8:19 PM

  Just because you said that you were interested in some Opinions, one of the least appreciated aspects of any documentation (but especially diagrams) is defining who the stakeholders are at the start of the document. It’s the difference between having frustrated users who can’t understand things to happy users that understand limitations.

  The corollary to this is that the best diagram that boundaries are often along communication lines between teams. This is Conway’s law all the way down. And the reason is that most often people use diagrams to get a spatial sense of where ‘they’ fit into things. I have only anecdotal evidence for this, but the most helpful and lasting diagrams I’ve ever made are when 1) they define (and stick to) specific stakeholders, and b) they are delineated by groups/teams.

Couple of comments:

> This can be as simple as adding a type suffix to a resource name (e.g. Orders Table, Results Bucket)

Don't encode types in names. And I disagree somewhat that the names are really needed at all.

> Making a “master” diagram

I think such a diagram is useful but obviously each top-level "box" in it doesn't need to contain all sub-components.

• gruez

  today at 1:34 PM

  >Don't encode types in names.

  Why? Hungarian notation probably is probably going too far, but in cases where a single word is heavily overloaded encoding types is helpful (eg. image file, image table, image bucket).

  • zabzonk

    today at 1:49 PM

    I don't think the type needs to be in the name because it is displayed elsewhere in the diagram, possibly as the object's icon. Plus of course the reasons no-one uses Hungarian anymore - types change.

    And for your naming, I would probably have something like "Unnormalized orders", "normalised orders", "queued orders", but obviously I can't tell without much more information.

• tremon

  today at 2:05 PM

  And I disagree somewhat that the names are really needed at all

  You want a diagram containing only icons? You will still need a legend somewhere that explains what each icon means, otherwise you will end up with at least as many interpretations of the diagram as there are readers of it.

  And I'd say that that first image as shown is virtually useless anyway. There is little value in just laying out resource components without linking them to system operation in some way -- which means that that diagram can only be understood in its larger context, and that's typically not how diagrams are used: they end up being the main focus of discussions.

Diagrams are communication tools, and are best done with a target and goal in mind. The C4 framework is good for addressing multiple levels of abstraction and different types of viewers. The business execs don't need the level of detail that someone debugging the system does.

I generally have given up on diagrams. Systems and flows I work with are too convoluted to be mapped out. Only the simplest of flows can be diagrammed and it usually leaves out important facts. When dealing with non technical people, I have found out through trial and error that excel works best. I start out with sample data on one sheet and walk them through the various transformations in sheet2, sheet3 etc. I even create a table of contents that has links to the different sheets. In a phrase, seeing data is believing.

> Meaningless animations

As someone who usually hates animations, in the example given I actually find them useful, assuming that they are representative of the actual flow. They are also unobtrusive because they are steady-state.

• RaftPeople

  today at 8:08 PM

  I agree. I think it gives a quicker view of what is happening and for some subset of diagrams I have wanted to use tools that do more of this to help people build a mental model.

Architecture diagrams are the bane of my existence right now. I sit there and listen to engineers opine on the different types, levels, details, etc. And all I can think is...

What a TERRIBLE way to store information in an AI era. Diagrams are so...human.

Their master diagram example in #3 contains a #2 mistake with an unconnected resource (the stripe account). Maybe a double validation of why the master diagrams can be hard to maintain.

The worst ones are diagrams that look clean but hide all the decisions that actually matter. A messy diagram that shows the real tradeoffs is more useful than a pretty one that lies

• chaps

  today at 1:27 PM

  Once worked with a systems architect who intentionally disorganized their flow diagrams by just moving nodes in their flow to random places (hi Dan!). The only reason I can think of why he'd do that is to maintain job security by keeping the junior apps folk confused.

• 01HNNWZ0MV43FF

  today at 2:28 PM

  The Slack notification flowchart is an old favorite: https://slack.engineering/reducing-slacks-memory-footprint/

  • raw_anon_1111

    today at 2:56 PM

    It amazes me that they are spending all of this time reducing the memory footprint and not do the most obvious thing - just stop using fucking Electron

My thought process is that a diagram should stand on its own and should be understandable by non technical business people. I always have callout notes as stickies on the diagram explaining what it does.

Route53 being off on the side but unconnected is still valuable info...

The main one i would add is a lack of symmetrical alignment. the point of a diagram is to create a shared abstraction for reasoning about a system or set of problems. The point of that is to scale work on it to other minds. It should enable others to parse it and ask useful questions.

If your diagram is ugly, you're probably mixing levels of abstraction without acknowledging it. It's a forcing function on articulating what you know and what is outstanding. Something that is black boxed should be referenced as a black box.

I use a lot of data viz because it's a high bandwidth way to show relationships, dynamics, order of complexity and its location, information problems, scope, and de-noise data. So much can be explained by having AI make you a uml sequence diagram of a concept. it is unreasonbly effective. If you are making a "chart for management" and using powerpoint or native excel charts, you're probably creating garbage though.

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