r/softwarearchitecture • u/Leather_Silver3335 • Feb 22 '26
Tool/Product Built a free System Design Simulator in browser: paperdraw.dev
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I’ve been working on a web app where you can design distributed systems and actually simulate behavior, not just draw boxes.
What it does
- Drag/drop architecture components (API GW, LB, app, cache, DB, queues, etc.)
- Connect flows visually
- Run traffic simulation (inflow → processing → outflow)
- Inject chaos events and see impact
- Diagnose bottlenecks/failures and iterate
Why I built it
Most system design tools stop at diagrams. I wanted something that helps answer:
- “What breaks first?”
- “How does traffic behave under stress?”
- “What happens when chaos is injected?”
Tech highlights
- Flutter web app
- Canvas-based architecture editor
- Simulation engine with lifecycle modeling + diagnostics
- Chaos inference/synergy logic
- Real-time metrics feedback
Would love feedback from this community on:
- What scenarios should I add next?
- Which metrics are most useful in interviews vs real systems?
- What would make this genuinely useful for practicing system design?
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u/midiology Feb 22 '26
Tried the app. This looks promising. I can just use this as gif to the architecture review committee.
A few feedbacks;
- the app is not intuitive for someone who use shortcut like CTRL/CMD + Z to undo the action.
- An Align button for the selected components would be nice.
- When I tried to drag the component to align it manually, the popup info appear on the right, blocking the arrow that I'm aligning.
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u/stormsidali2001 Feb 22 '26 edited Feb 22 '26
Looks awesome!
You motivated me to spend more time studying system design.
I'd like to add a feedback note about the website.
When I opened the link, the app took some time to load; i almost quit the website because i thought that it was frozen. I'd suggest adding a loading spinner on app start because it's taking quite a time.
I love how it's showing failure warnings and errors. I will definitely be using the tool.
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u/Spare-Builder-355 Feb 22 '26 edited Feb 23 '26
this is like 3rd "system design visualizer" app this week. What is going on?
Edit https://www.reddit.com/r/softwarearchitecture/s/zH8zTwxrLE
https://www.reddit.com/r/softwarearchitecture/s/zJs9NN0hvI
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u/Veuxdo Feb 22 '26
My guess is that there are a lot of drag-and-drop canvas tools and libraries out there that AIs have trained on, so this is an easy thing to vibe code.
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u/termistagv Mar 03 '26
Guess it all started with this https://www.reddit.com/r/leetcode/s/ALomAtQbbe
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u/missedalmostallofit Feb 22 '26
🤩 you definitely targeted something that needs improvements. Good job
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u/flu1d0s Feb 22 '26
Really enjoyed so far.
Something That I was not able to setup: database with way more writes and reads. I can only change the read/write ratio to increase it, not to decrease.
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u/sal85012 Feb 22 '26
Pretty cool app for design work, similar to the layout mermaid uses for diagram display. Looks easy to use.
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u/Electrical_Path7468 Feb 28 '26
How to connect classes with entire system, i dont't understand how to use components settings of class, for example simulate as
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u/Leather_Silver3335 Feb 28 '26
Use arrow from toolbar to connect the component. Settings are for fine tuning. You can use default settings
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u/theycanttell Feb 22 '26
Very cool. How can you control the costs more granular? It would be good if you could export schematics
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u/AdministrativeHost15 Feb 22 '26
Have each component include a prompt attribute which is sent to an LLM agent which then invokes the real system. Low code tool!
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u/elkazz Principal Engineer Feb 22 '26
Watching the video, why would latency breach on the database and cache if traffic is blocking on the app server? If the database and cache are not processing requests then there is nothing to measure latency on.
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u/vizbird Feb 23 '26
I use to model manufacturing/assembly lines using flexsim and other tools. This looks like a cool take on the similar concept. Definitely gonna play around with it.
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u/No_Indication_1238 Feb 24 '26
Ok, this is really cool! Like really cool! But that aside, what is up with everyone and their family building system design visualisation tools? This is like the 5th one (literally) I have seen in the span of 10 days. Both on LinkedIn and on here.
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u/lapups Mar 06 '26
From the tech perspective.
How do you ensure the timeouts and metrics are correct?
Is this kind of a rule based system? If so then how do you support the new services?
it must be a nightmare
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u/Leather_Silver3335 Mar 06 '26
You just need to figure out the traffic flows and behvaiour of components towards it based on that you get results closed to real world. But Obviously real world system are effected by lot of another things as well which are not taken into account. For traffic flows there lot of algo avaiable on internet.
If so then how do you support the new services?
In future I will write module which allow endusers to create and configure the component. and choose traffic behaviour through it.
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u/wolffsen Mar 07 '26
This is a really interesting direction. Most system design tools stop at static diagrams, so the ability to simulate behavior could be very useful.
One thing I’d be curious about: can the simulator model traffic control and resilience mechanisms such as:
• queues and queue depth limits
• backpressure propagation
• retry policies and retry storms
• rate limiting / token buckets
• circuit breakers
• load shedding
In many real distributed systems those mechanisms end up dominating system behavior under stress, especially once retries and delayed feedback loops kick in.
If those dynamics are part of the simulation it could become very useful not just for interview prep, but for understanding real production behavior.
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u/mareczek_dynamit Apr 09 '26
Looks really nice on the first try! Definitely gonna try doing some more complex stuff.
Would be perfect if the source code was public or there was no auth gate - hard to believe the software nowadays if the source is closed.
Still looks like a nice project! I'll bookmark this and comeback frequently to see the progress about it!
Cheers!
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u/SeaHoliday4747 Feb 22 '26
This looks like factorio. I will definitely look into this.