The specification

Everything the engine does, written out.

The sales page has the short version. This one has every screen in the dashboard, every message the engine will and will not send, how money is handled, who can see what, and what it is built on. Nothing here is a plan — it is a description of the build you download.

Where it appears

The four surfaces, all views of one database.

A customer’s phone, a dashboard, a screen on the wall and an app in a pocket. A change on one is on the others at once.

The timetable and a member's page

Members, and somebody new

The studio's own page opens on the week, and nobody signs in to book anything.

The week as the studio runs it: every class with its instructor, its room and how many mats are left.
Four ways to pay: one class at A$30, a fortnight of every class at A$55, ten classes at A$260, or the membership at A$189 a month.
The membership books three days before everybody else, pauses one to eight weeks, and ends at the close of what is paid.
A member has no password. "Send me my link" takes an email and says the same thing either way, so nobody learns who is a member.
Their own page lists what is coming, what they wait for, their pass or membership, late cancellations and any fee owed, and gives their classes as a calendar to subscribe to.
A browser that opened the page remembers them, and "Forget this browser" undoes it. The link is kept out of the addresses of the timetable and of a class.
Gift vouchers for an amount or one class, good for three years, arriving now, on a day chosen, or as a page to print.

A full class, and its line

Anybody who was too late

A full class takes a name for its line and says the place they would have.

Joining the line costs nothing, and nothing is taken until the mat is theirs.
When a mat comes free the first in line is written to and has a set time to take it — thirty minutes, and fewer near the start.
If they let it pass it goes to the next in line.
Nothing is offered between nine at night and six in the morning.
In the last hour before a class everybody still waiting is told at once, and the first to answer has the mat.
From the desk the line can be reordered, a name taken out, and a free mat given to the first at once.

The dashboard

Instructors, the desk and the owner

Today opens on the classes, the three cards that ask for something, and the day's private sessions.

Payments that are late on a membership kept at the desk, and until what day what they booked still stands.
"A word to have": the members who cancelled late three times in thirty days, or who owe fees.
The week, each class with how full it is, and a mark on one somebody else is taking or that was called off.
One class's sheet: the roster with a tick each, opening twelve hours before, a mark on somebody coming for the first time, and what they told the studio.
Somebody can be added at the door by name, and taken off. "Close the list" ends the class: whoever was not ticked did not come.
"This date only" hands one date to another instructor or moves it to the other room — nobody rebooks, and everyone on the roster and in the line is told.
"Call this date off" asks for a reason and cancels every booking as if in time: a pass has its class back, a voucher its amount, and money goes back the way it came.

The staff phone app

Instructors, at the door

One codebase for iPhone and Android, carrying the studio's own name and icon.

The day's classes, each with how many are booked.
One class's roster, ticked as people arrive.
A mark on somebody coming for the first time.
Somebody added at the door, by name.
A free mat given to the first in line.

The ten rules

What the engine refuses to break.

These are the decisions a booking engine has to make and almost never writes down. Each one is enforced in the database, not in a screen.

How long it takes

The chosen services added together, with the largest prep-before and the largest clean-up-after of them wrapped around the whole booking.

Who can do what

A staff member must do every service in the booking. Pick two and only the people who do both are offered.

The working day

Each person's own weekly hours. Two stretches on one day make a split shift, and the gap between them is not bookable.

Time already taken

Blocked, with its buffers — unless the booking was released by a cancellation, a no-show or an expired hold. A new booking's buffers stack on its neighbours'.

Closures and extra openings

A holiday, a week off, a late Thursday. Set for one person or the whole location, and they add time as readily as they remove it.

Notice

Nothing sooner than the lead time. Anywhere from no notice at all to a week.

How far ahead

Nothing later than the booking window — one day to a full year.

The slot grid

Start times step by the slot interval, measured from the beginning of each stretch of hours rather than from midnight.

Anyone available

Everybody's free times merged, each start shown once. On booking, whoever has the fewest bookings that day gets it first.

A person's own length and price

A senior stylist's cut can be longer and dearer than the shop's. Their number replaces the service's, for them only.

And beyond the ten

The settings an owner can move, and how far

Only people who still take bookings are offered.
Busy times read in from a staff member's own calendar block the diary.
A service that needs a room or a piece of equipment needs a free unit of it.
A service marked in-house only is never offered to a customer, though staff can book it.

Notice before a booking: 0 minutes to 7 days
How far ahead: 1 to 365 days
Slot step: 5, 10, 15, 20, 30 or 60 minutes
Cancellation window: 0 to 168 hours
Reminders: Up to five, each 1 to 168 hours before
Deposit: None, a fixed amount, a percentage, or per service
Hold while a deposit is paid: 5 to 30 minutes
Prep and clean-up per service: 0 to 240 minutes each
Currency: Any ISO 4217 code, until the first booking
Automatic data clean-up: Off, or 1 to 120 months

Not a generic booking tool

The parts that are the shape of a fitness and yoga.

Its own words

Staff member: Instructor
Service: Class
Customer: Member
Location: Room
Appointment: Booking

Its starting rules

A reminder: Three hours before — the evening before for an early class
A mat offered from the line: Thirty minutes to take it, fewer near the start
Nothing offered: Between nine at night and six in the morning
In the last hour: Everybody still waiting is told at once
A late cancellation on the membership: A$15
Not coming, on the membership: A$20, written the next morning
The first late cancellation in thirty days: Costs nothing
A membership pauses for: One to eight weeks, twelve in a year
A price change is told: Thirty days ahead

The studio

Lantern Yoga, 12 Sample Lane, Fitzroy North VIC 3068 — a house with two rooms.

The week

28 classes, 284 of 440 mats booked, with full classes and people waiting on them.

The rooms

The Sun room at 20 mats and the Garden room at 10, each refusing a second class at the same hour.

The team

Six instructors — Imogen Hart, Sophie Lindqvist, Dev Raman, Kiri Tane, Marco Bellini and Hana Okafor.

The prices

A$30 one class, A$55 a fortnight, A$260 for ten, A$189 a month — Australian dollars, GST in every price.

The fees

A$15 late, A$20 for not coming, on the membership only, and the first late cancellation in thirty days free.

The vouchers

An amount or one class, good for three years, arriving now, on a chosen day, or as a page to print.

The time

Melbourne's, written the Australian way — and the small print marks where Victorian law wants a lawyer.

Past the engine

What the pack adds that this trade needs.

The waitlist

A line that moves by itself, and never takes money early

A full class is the normal state of a good studio. The pack treats the line as a first-class thing rather than a mailing list.

A place in the line is a number the member can see, and leave.
An offer has a clock on it: thirty minutes, fewer as the class gets close, then it passes to the next.
Quiet hours are kept — nothing is offered between nine at night and six in the morning.
In the last hour the studio stops being fair and starts being useful: everybody waiting is told at once.

Passes and memberships

Four ways to pay, and all of them drawn down honestly

One class, two kinds of pass and a membership billed every month, each with its own rules for what a late cancellation costs.

A pass keeps its class back when somebody cancels late or does not come; one class paid at once does not.
The membership charges A$15 late and A$20 for not coming, and the first late cancellation in thirty days costs nothing.
A fee is owed, never taken: the member pays it from their own page, on the provider's page, or at the desk.
A membership pauses one to eight weeks, twelve in a year, and ends at the close of what is already paid.

No password anywhere

A member's page that opens from an email

Nobody signs in to book. The page that holds a member's bookings, passes and fees opens from a link, and the studio never learns who asked for one.

"Send me my link" says the same thing whether or not the address is known.
A browser that opened the page remembers the member, and can be made to forget.
The link is kept out of the addresses of the timetable and of a class, so sending a class to a friend does not send the member's page with it.
Whoever holds the link can book, cancel and change — and the email says so.

Rooms and mats

A class is an hour, a room, an instructor and a number

A weekly class is refused where its instructor already teaches at that hour, or its room already holds a class.

The Sun room holds 20 mats, the Garden room 10.
Changing a weekly class changes the dates nobody has booked; a date somebody booked keeps what it was given.
"Take off the timetable" removes the dates ahead that nobody booked, and leaves the booked ones to be called off one by one.
A class whose instructor has been switched off stays on the timetable, marked as needing somebody.

The owner’s dashboard

Menu by menu, and what it does.

Seven screens. The menu reads in the trade’s own nouns, so an owner is never translating from software words into their own.

Today

The morning screen.

Bookings made today, what is still to come, takings so far, and a list of anything waiting on a person — a deposit to refund, a late change to approve.

Appointments

Day or week, one column per person.

Open a booking to check a customer in, mark it done, or record a no-show. Drag it to another staff member. Add a walk-in without leaving the screen.

Services

What can be booked.

Grouped into categories, each with a length, a price, and padding before or after for clean-up.

Staff

People and their hours.

Which services each person does, their weekly hours, time off, extra openings, and a login of their own.

Clients

Everyone who has booked.

Visit history, total spend, private notes, and the privacy actions — download their data, stop emails, erase them.

Reports

What actually happened.

Bookings, value, utilisation, no-shows and cancellations for any period, by person and by service, with where the booking came from. CSV download.

Settings

The rules of the business.

Booking link, profile, locations, time zone, notice period, how far ahead people can book, cancellation window, time step, currency, deposits, reminder times and automatic data clean-up.

Plus the platform admin console, on the Full and All-packs licences.

Every account and every business on the deployment: roles, sessions, bans, and signing in as somebody to help them, with a banner that says you are.

Who can do what

Routes ask for a permission, not a role.

Which is why a client who needs a third role — a receptionist who may not change prices — defines it in one file and changes no route at all.

FIG. 07Who can do whatRoutes ask for a permission, never for a role — which is why a third role is one file and no route changes.

Action	Staff	Owner
See, make, change and cancel bookings	Yes	Yes
Check in, complete, record a no-show	Yes	Yes
See, add and edit customers	Yes	Yes
Download a customer's data, or erase them	No	Yes
Services, categories, rooms and equipment	See	Yes
Staff, their hours, time off and closures	See	Yes
Booking settings	See	Yes
Text messages and their wording	No	Yes
Reports	No	Yes
Refund a deposit	No	Yes
Invite members and change roles	No	Yes
The business name, logo, and deleting it	No	Yes

Four kinds of person

Customer

No account needed

Books in a few taps. Every email carries a private link to manage that booking — no password to forget, no sign-up to abandon.

Staff

Works the calendar

Check-ins, walk-ins, moves and no-shows. Sees their own clients. Cannot change settings, see reports, or touch money.

Owner

Runs the business

The full dashboard, every report, all settings, staff logins and the money.

Platform admin

Runs the platform

Every account and business, impersonation to help someone, role changes, bans. Full and All-packs licences only.

Messages

Every message that goes out — and what never is.

The second list matters more than the first. Booking software that quietly markets to a client’s customers is a problem the client discovers after launch, in public.

FIG. 04What a customer actually receivesAnd the longer list of what never arrives.

Booked

the moment it is made

Confirmed

mail goes out

2 h before

reminder

The visit

and then silence

to the customer

Confirmation

Calendar file, private link, stamp card

to the staff

New booking

To the staff member, unless they made it

to the customer

Reminder

At each time the owner set

And what never leaves the building

A follow-up after the visit
A review request
Any marketing email, ever
Anything about a no-show
Anything while a deposit is pending
A reminder whose visit has already begun

Booking software that quietly markets to a client’s customers is a problem they find out about in public. This one has no sender for it.

How they leave the building

Every change to a booking writes a history row, and in the same transaction the messages that change calls for. One function decides who is told what.
A reminder is simply an outbox row with a later time. Moving the booking drops and re-queues them; cancelling drops them.
Each message is rendered from the booking as it stands when it is sent, so a confirmation delayed by a provider outage still shows the right time.
A failure that might pass is retried after 1 minute, 5, 15, an hour, then every few hours — eight attempts over about a day. A refusal of the address is given up at once.
Text messages go through the business's own provider account: Twilio over SMS or WhatsApp, Meta's WhatsApp Cloud API, or any gateway that accepts a JSON POST.
Nothing is texted without consent, STOP and START replies are honoured for every booking that number has, and a monthly ceiling stops a runaway bill.

Every message, and exactly when it goes

Confirmation: Once the booking is confirmed. Carries a calendar file, the private link to manage it, and the loyalty card where the pack has one.
Moved: When the time changes. Not when only the staff member changes at the same time.
Cancelled: With a refund note where one applies and a link to book again. Its calendar file removes the event.
Reminder: At each reminder time the owner set. Dropped if the booking moved, was cancelled, or the visit has already begun.
New, moved, cancelled: On the same changes, unless they made the change themselves.
Account mail: Confirm your email, reset your password, a magic link, a six-digit code, a change of email, an invitation to a business.

Money

Stripe built in, any gateway in one file.

Payments go through one interface. A provider says whether it is configured, starts a charge, says how to pay it, calls it off, and reads its own webhook. Nothing downstream knows which provider took the money — which is what makes a country Stripe does not serve a day of work instead of a rewrite.

FIG. 05The deposit holdWhere booking software usually loses money.

0:00Booking written
The row exists before the charge, so a slot someone else takes never strands a payment
0:00Time held
Pending. Nobody else can book it
15:00Hold expires
Unless the customer asked for more time — three holds at most
—Webhook decides
Confirmed only on the provider's signed message. The page never takes the browser's word

Refunds and disputes

A deposit for a time that is no longer theirs, or a cancellation inside the window, puts the booking in “needs your attention”. The owner presses refund, and pressing twice is still one refund.
The engine never refunds by itself. An owner can also refund at the provider and mark it handled.
A chargeback is flagged above everything else, because the business has a deadline for its evidence, and the refund button is refused so nobody pays twice.

Any country

Stripe is built in, with its own card form on the page and 3-D Secure handled there. Card numbers never reach the server, which keeps the business in PCI's shortest self-assessment.
Where Stripe does not serve a country, a local gateway is one file against one interface. Nothing downstream knows which provider took the money.
A stand-in gateway in the test suite lets that file be tested without any keys at all.
Money is stored as whole units of the smallest denomination, one currency per business, and a booking keeps its price as booked — so a price rise never moves an old booking.

A booking’s life

Seven states,and the rules between.

FIG. 06The booking state machineA released time goes straight back on sale.

Pending
Holding its time while a deposit is paid
Confirmed
Booked. The customer has the email and the link
Arrived
Checked in. Opens 12 hours before
Completed
Done, and a loyalty stamp earned

Any state above can leave the flow

Cancelled

Time released, reminders dropped, refund flagged if one is owed

No-show

Time released, counted on the customer. No fee, no message, undoable the same day

Expired

The hold ran out. Time goes back on sale and the charge is called off

Completed and no-show are only reachable after the visit has started. A released time goes straight back on sale, and the database — not the code — is what refuses the overlap if someone takes it first.

Calendars

Read both ways, never obeyed.

A feed to subscribe to

Every staff member has a private address, 30 days back and 400 days ahead, that Apple Calendar, Google Calendar or Outlook can subscribe to. Renewing it cuts off every old copy at once. It needs no keys at all.

Their own calendar, read in

Paste the secret address of a personal calendar and the engine keeps only the busy times — start and end, never the title and never who is attending. Those times block the booking page.

Google and Microsoft, both ways

Busy times come in; bookings go out, 180 days ahead, written once and not rewritten when nothing changed. A failing connection backs off for up to a day, then says on the staff page that it needs looking at.

Clashes are flagged, not obeyed

An outside event landing on a booking already made never moves or cancels it. The booking gets a clash flag, and the flag clears itself if the outside event goes away.

Accounts and security

The part nobody asks about until after launch.

Six ways to sign in, limits that are on in every environment, and a double-booking guard that lives in the database rather than in code somebody might refactor.

Six ways to sign in

Email and password: Web and phone
Magic link by email: Web and phone
Six-digit code by email: Web and phone
Passkey: Web
Two-factor with an authenticator app, with backup codes: Web and phone
Apple, Google and GitHub: Web and phone, with each provider's keys

Rate limits, per caller

Signing in with a password: 5 a minute
Trying a six-digit code: 3 a minute
Asking for a link, a code or a password reset: 3 in 5 minutes
Reading the booking page's free times: 120 a minute
Making bookings: 10 in 10 minutes, 5 an hour per email address, 200 in 10 minutes per business
Cancelling or moving from a booking link: 20 in 10 minutes

A refusal answers with a retry time, in the visitor’s own language. There is no CAPTCHA anywhere in the product.

Signed links, not guessable ones

A booking's own page carries an HMAC signature. Nothing about the link is stored, a booking always has the same link, and it stops working a week after the visit. An altered link gets one flat answer that never says which part was wrong.

Every row belongs to one business

The database itself enforces it: a booking cannot name another business's staff member, customer, location or service. An integration test proves one business cannot read or write another's rows.

The double-booking guard is in the database

Not in the code. A Postgres exclusion constraint refuses two overlapping bookings for the same person, buffers included, and the API turns that refusal into a message the page can translate.

Secrets are sealed

A staff calendar token, a business's text credentials — encrypted with AES-256-GCM under a key that lives in the host's settings, never in the repository, and never sent back to a browser.

Cross-site writes are refused

Forms by a CSRF check, JSON by an origin check. Machine routes carry no cookie and prove themselves with a secret or a signature instead.

Headers that are actually set

An enforced content policy, framing denied outright, two years of HSTS on production builds, no sniffing, a strict referrer policy, and camera, microphone and location switched off.

Privacy

A compliance kit, not a promise to add it.

Export, erase and retention are built, and six documents ship with the code — including the answers to Apple’s and Google’s privacy forms, which is the step that delays a launch by a fortnight.

Export

Everything the business holds about one customer as JSON — who they are, every booking with its services and prices, payments, what happened to each booking, the messages sent, and their places in any queue. The whole customer list downloads as CSV. Owners only, and typed text is written so a spreadsheet will not run it as a formula.

Erase

Name, email, phone and notes gone for good, along with what was typed on their bookings. The bookings themselves stay without a name, so the figures still add up. Refused while they have a booking ahead. It cannot be undone.

Retention

Erase customers who have been away a number of months you choose, 1 to 120, off until an owner turns it on. A customer with a booking ahead, or money still waiting on a person, is left alone until it is settled.

Stops, per customer

“Don't email this customer” stops every email from that business to them. Texts have their own switch on the record, the customer's own switch on their booking page, and STOP replies. A bounce or a complaint protects the person from every business on the deployment.

Cookies

Six, every one of them essential or functional, each listed with its purpose and how long it lasts. Booking needs no cookie at all. No advertising cookie, no tracking cookie, and no analytics unless the owner switches one on.

The compliance kit

Six documents: who is controller and who processor, what to choose at each provider to keep data in the EU, the card-payment note for Stripe's shortest self-assessment, every cookie and storage entry, the data inventory that answers Apple's and Google's privacy forms, and a one-page handout for the owner.

One command prints the outside providers your own settings actually select, by name and purpose, and the privacy page shows the same list. The engine itself sends nothing home: no telemetry, no licence check, no call to its maker.

The kit is a set of templates, not legal advice, and the demo business’s legal facts are sample facts. Your client’s lawyer fills in theirs.

How it is built

One deployment, no worker to babysit.

No queue, no cache server, no second service. The API is mounted inside the web app by one catch-all route, and one HTTP call a minute runs every background job — which is the whole reason it fits on a $5 host.

FIG. 02The whole deploymentFour callers, one app, one database. Nothing else to keep alive.

Customers

booking page, queue, screen

Owners & staff

dashboard, admin console

Staff phone app

iPhone and Android

Scheduler

one call a minute

One deployment · one address

Pages & dashboard

Next.js 16, React 19, rendered on the server

API at /api

Hono 4, mounted in the same process

↓ they share one set of packages ↓

booking rulesniche packauthemaili18nlinksstoragejobsrate limitsui

One Postgres

Every row belongs to one business, enforced by the database

Outside providers

Mail, payments, texts, calendars, picture bucket

No queue. No cache server. No worker to keep alive. That is the whole reason it runs on a $5 host, and the reason a handover is one repository and one database rather than a diagram like this one with six boxes missing.

And what each piece is

Web app: Next.js 16 and React 19. The marketing pages, the booking flow, the queue, the screen view, a booking's own page, the dashboard, the admin console and the legal pages.
API: Hono 4 under /api, mounted inside the same app by one catch-all route. Same program, same address: there is no second server to deploy.
Staff phone app: Expo SDK 57 and React Native 0.86, one codebase for iPhone and Android, signing in against the same server and calling the same routes.
Database: One Postgres, 17 or 18. The schema is written with Drizzle and changed only by numbered SQL migrations.
Sign-in: Better Auth 1.6 with its plugins for businesses, the platform admin, two-factor, passkeys, magic links, email codes and the phone app.

What is actually tested

100%

of the rules engine

Every line and every branch of the booking rules, held at full coverage by the build. The booking tests run a second time under another time zone to prove nothing reads the server's own.

booking the same second

Twenty simultaneous bookings for one slot, against Postgres 17 and 18. Exactly one wins. The rest are offered another time.

< 300ms

to find free times

A benchmark on the availability query that the build refuses to go past.

browser set-ups

End-to-end tests across five, with an accessibility gate that fails on a single axe violation on the dashboard or the booking page.

Hosting

Three routes, one of them recommended

Cloudflare Workers

The recommended host, and the one the build is tuned for. The whole app and its API run as one Worker, Postgres through Hyperdrive, pictures in R2, and a cron trigger calling the tick every minute. From $5 a month.

Vercel

Project root is the web app, and the cron entry that calls the tick every minute is already written. A bucket is required, because pictures do not survive a deploy.

Any Node server

A production build behind one reverse proxy, with the jobs worker kept alive beside it. Every setting is spelled out in full rather than assumed.

Running it

It refuses to half-work

Start-up checks refuse to serve customers on a half-configured deployment: a missing link secret, an unknown payment provider, a bucket with no public address, a console mail sender. Every route answers 500 and the log names each problem on its own line.
A doctor command runs the same checks from your machine against the live settings, and also checks the database, the extension the booking guard needs, your sending domain's DNS records, and the phone app's identity.
A health route answers ok, degraded or down, and tells a monitor how long ago the tick last finished — the one alarm worth setting.
Migrate first, then deploy. A status command shows what is pending without changing anything, and the release notes say what each one does.
One verify command is the gate: the repository's own checks, formatting, lint, types, migrations, the doctor and every test, in order, stopping at the first failure.

White label

A check fails the build if our name is anywhere a client could see it

Nothing a customer or a staff member sees names the engine. The product name, the logo, the installed-app icon, the link-preview picture and the cookie prefix are all the client's.
A strict check fails the build if a page, a language file, an email, store text or a public file still carries the engine's name or the template's.
Colours, corner radius and the font pair reach every screen as variables, so a change lands everywhere without editing a component. The phone app takes the same four values.
The client folder is the only place a project writes: the brand, the wording overlay, the legal facts, its own routes, its own event subscribers and its own tables.
Any engine file a project does change is recorded in one list, and a check fails if that list is out of date — so the next engine update merges cleanly.

That is the whole of it

Back to the fitness and yoga pack

See the pack The licences