We keep what the game needs, and nothing to sell.

The short version

System Design Duel has no ads, no analytics and no third-party trackers. Videos play in YouTube's own player, and YouTube's privacy terms apply to them. You can take your daily practice without signing in to anything. We keep your answers, because they are how questions and players are rated, and if you play duels we keep your matches and your rank. Other players can see your username, your robot avatar and how you stand on the ladder, and nothing else. You can delete your account, and everything we hold with it, from inside the app.

Who we are

System Design Duel is made by BrayLabs LLC, an Oregon limited liability company, and we are the ones responsible for the data described here. You can reach us at support@systemdesignduel.com, or by post at BrayLabs LLC, 5441 S Macadam Ave, Ste R, Portland, OR 97239.

This website sets no cookies and runs no analytics. Cloudflare serves it and keeps the usual short-lived request logs.

Accounts

The first time you open the app it creates an anonymous account for you: a random identifier, a generated username and a robot avatar. There is no sign-up, no password and nothing to remember. That is enough for daily practice, the whiteboard and your streak.

Signing in with Apple is optional, and it is what duels and the leaderboard require. It ties the account on your phone to that sign-in, so your progress can be restored on another device. When you do it, Apple passes us an identifier for you and an email address. Apple lets you hide your real address, and the app works the same if you do. We use the address to recognise your account. We do not send marketing to it and we do not share it.

We never ask for your real name, your phone number, your contacts or your location.

What other players can see

Your username, your robot avatar, your rank and rating, your record of wins and losses, and the topics you are strongest in. These appear on the leaderboard, on your player page, and to anyone you duel. Pick a username you are happy to be known by. You can change it, and your avatar, on the You tab.

Your email address, your individual practice answers and your streak are never shown to other players. In a duel, your opponent sees which of that match's questions you got right and how the rounds went.

What we keep

• Your answers. For each question you answer for the first time: which question, whether you were right, how long you took, and when. This is how the app learns how hard a question is and what you are ready for next. It is sent whether or not you have signed in.
• Your progress. Your streak, your level, your review schedule and your settings live on your phone. Once you have signed in with Apple, a copy is kept on our server so it can be restored on a new device.
• Your duels. Each match, its questions, your answers and their timings, and how your rating moved. We also look at answer timings to spot automated play.
• What you send us. A thumbs up or down on a question and any note you add, a report that a question is wrong, and any topic you ask the app to teach. These are read by a person, so please keep personal details out of them.
• Your subscription. Whether your account has Pro, which plan, and when it renews or ends. We never see your card or your Apple ID.
• Your device's notification address. If you allow notifications and play duels, the token that lets us tell this phone it is your turn.

Lessons are fetched from our server the first time you open them. To stop the question bank being copied in bulk, the server counts how many new lessons are fetched each day, for each account and for each internet address. The address is never stored: we keep only a one-way hash of it, made with a secret key, beside the day and the count. That record is not tied to your account and is deleted after two days.

The app's database and sign-in are built on Supabase. Everything above is kept there, readable only by you and by us, apart from the public details listed earlier.

Notifications

The daily reminder is scheduled on your phone by the app itself and involves no server. Duel notifications, such as a turn waiting for you or a match that has finished, are sent from our server through Expo's push service and Apple's. Both are off until you allow them, and you can turn either off again in the app or in iOS Settings.

Videos

Some explanations come with a segment of a video from YouTube. The app uses YouTube API Services to show them. A video's card shows its thumbnail, which your phone loads from YouTube. The video itself plays only when you open it, in YouTube's embedded player, which is run by Google. Loading a thumbnail or playing a video sends Google your internet address and details of your device, and the player tells Google what you watch and may store cookies, as it does on any website. We do not send Google your account, your username or your answers, and we receive nothing back about what you watch.

What Google does with this is covered by the Google Privacy Policy, and use of the player by YouTube's Terms of Service.

Purchases

Pro is bought through the App Store, and Apple handles the payment. We use RevenueCat to confirm that a purchase is real and to keep track of your subscription. RevenueCat receives your anonymous account identifier and the details of the purchase from Apple, and tells us whether Pro is active. Neither of us receives your payment details.

Who else is involved

• Apple, for sign-in, purchases and notifications.
• RevenueCat, for subscription status.
• Expo, to deliver duel notifications to your phone.
• Google, through YouTube, when a video's thumbnail loads or a video plays, as described under Videos.

That is the whole list. We do not sell your data, share it with advertisers or data brokers, or use it to build a profile of you for anyone else. The app contains no analytics, advertising or crash-reporting kits.

Every question links to the source it was checked against. Those links open the source's own website, which has its own privacy policy and is not ours.

Deleting your account

On the You tab, open your account and choose Delete account. This erases your username, rank, matches, answers, backup, subscription record and notification address from our server, and your progress from the phone. It happens straight away and cannot be undone. Matches you played disappear from your opponents' lists too.

Deleting your account does not cancel a subscription, because the subscription is held by Apple. Cancel it in your Apple account settings, or you will still be charged.

If you cannot get into the app, email support@systemdesignduel.com from the address your account uses and we will delete it for you.

How long we keep things

For as long as you have an account. Delete the account and everything tied to it goes with it. Backups of the database age out on their own shortly afterwards.

Your choices

You can ask us for a copy of what we hold about you, ask us to correct it, or ask us to delete it, by emailing support@systemdesignduel.com. We answer every request, and we do not charge for it. If you live somewhere with a data protection authority, you also have the right to complain to it.

Children

System Design Duel is written for working and student engineers. It is not meant for anyone under 16, and we do not knowingly keep data about them. If you believe a child has an account, tell us and we will delete it.

Changes

If this policy changes, the date at the top changes with it, and a change that matters is announced in the app before it takes effect.