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 booking site

Patients, on a phone mostly

Four steps with no account and no password: a treatment, a therapist, a time, and a name with an email address.

Ten treatments in four kinds, each with how long it takes and what it costs, and a row of buttons to show one kind at a time.
Only the people who give everything chosen are offered — the chiropractor is not offered for a physiotherapy assessment.
A time is offered only when the therapist and a room are both free; the two gym treatments wait on the gym.
More than one treatment can be ticked: a follow-up and acupuncture are then one session, with one length and one price.
A first assessment asks the practice's own questions on the same step, so the forty-five minutes go on the patient and not on a form.
One of those questions is there for safety: somebody who answers yes to it is told to call their doctor or 111 today.
Nothing is paid online. A session is paid at the practice, and a late cancellation is charged at the desk, not by the app.

The patient's own page

A patient with a plan, by their own link

What the plan is for, the goal in their words, how far along it is, and what an insurer agreed to.

A stroke for each session — filled for one done, orange for the next, outlined for the ones still to come.
Every session with its day and where it stands: done, missed, next — and each one still ahead is theirs to move or cancel.
Where somebody else pays: how many sessions were agreed, how many are used, how many are left, and the day the agreement is good until. The patient sees the count the desk sees.
A plan with nothing booked ahead says so and asks them to get in touch.
Never a note, nor the number a session measured. Never who sent them, and never an insurer's reference.
The page says so itself, and says they can ask for a copy of everything held about them.
No account and no password: the link is the key, and the desk copies it from the plan's sheet.

Today, the diary and a plan

The front desk, the therapists and the owner

Where the dashboard opens, and everything recorded against one plan and one session.

Today leads with the two lists a practice that works in courses needs: the plans somebody has to decide about, and the authorisations nearly spent.
A plan to close is one where every planned session was held and nobody has said how it ended; a lapsed plan is one with nothing booked for longer than the practice's weeks.
The diary by day is a column for each therapist; a session needs a room as well, and the day can be read by room.
A session repeated every week is booked as one block — all of it or none of it — and with Make it a plan ticked the form asks the goal, who referred the patient and what was authorised.
Move the rest asks where the next session goes, and the ones after it follow by as many days; the sessions already held stay where they were.
What is used of what an insurer covered is counted from the plan's own sessions. Nobody keeps a tally by hand.
A plan with sessions still booked is not closed over them: the form offers to call them off with it, and refuses otherwise.

The staff phone app

Therapists, between two patients

One codebase for Android and iPhone, carrying the practice's own name and mark, reading the same server as the dashboard.

The day, with its counts and each session's room; the week, with seven days and their counts.
A session: check in, complete, did not come — the last two ask first, and both can be taken back the same day.
Which session of which plan it is, and what the note before planned and measured, so the therapist starts where the last session stopped.
The note is taken there, in the room: what they said, what was found, what was done, what comes next, and a measure from 0 to 10.
A note is written once the session has begun, and not before.
Its words are shown only to the people patients are booked with, and the owner.
A quick booking: a patient, a treatment, a therapist and a free time.

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 physio and chiropractic.

Its own words

Staff member: Therapist
Service: Treatment
Customer: Patient
Location: Room
Appointment: Session

Its starting rules

Notice before an online booking: 2 hours
How far ahead: 90 days
Slot step: 15 minutes
Free change until: 24 hours before
Deposit: None — a session is paid at the practice
A plan lapses after: 4 weeks with nothing booked
A missed session counts against what is covered: Yes — insurers differ, the practice says
What a note measures: Pain, 0 to 10, unless renamed
Reminders: 48 hours and 2 hours before
A note's words: The people patients are booked with, and the owner
Classes, memberships, walk-in line, recalls: Off — a practice works in courses

The practice

Kestrel Physiotherapy — 9 Sample Row, Leeds, open Monday to Friday with two late evenings, and Saturday morning.

The team

Five — Maya Okafor the owner, Tom Reiner for runners and acupuncture, Priya Nair, Elias Brandt the chiropractor, and Jo Mbeki on massage and the gym.

The rooms

Three treatment rooms and a gym; the gym holds one patient and their therapist, and shuts for two hours in the sample for the treadmill's service.

The day

A Saturday with 7 sessions and £387 taken; the week around it with 108.

The treatments

From a £38 half-hour sports massage to an £85 running assessment, with shockwave therapy kept off the booking page for therapists to book.

The plans

Seven, each stopped somewhere different: Robin's 3 of 6, Ada's six all held and waiting to be closed, Cal's lapsed at five weeks, Eve's eight planned where six were authorised.

The one running out

Finn's is a solicitor's case whose authorisation is good for days — so it is on Today, with how many sessions are booked past what it covers.

The money

Pounds sterling, and British dates — “Thu 12 Nov”, “17:30”, on London's clock.

Past the engine

What the pack adds that this trade needs.

Treatment plans

A course of treatment is one thing, not six bookings

A practice does not sell appointments; it sells a course with a goal and an end. The pack makes that the object everything else hangs off.

A plan carries a title, a goal in the patient's own words, who treats and how many sessions are planned, from 1 to 52.
Its sessions are booked as one block, a week apart, all of it or none of it, and told to the patient in one email with every date.
Move the rest moves the next session and the ones after it follow by as many days; the sessions already held stay where they were.
Where a plan stands — under way, nothing booked, to close, lapsed, closed — is worked out from its own sessions each time the page is opened.

A note after each session

Clinical words, kept away from everything that leaves the building

A session's note is clinical information. The pack decides who may read it once, and then holds that line in every place the words could escape.

Shown with its words on a session's sheet, a plan's sheet and in the phone app — to the people patients are booked with, and the owner.
Counted without its words for the front desk: that a note was written, and on Today as notes still to write.
Never in an email, a text or a log, never in a reminder, and never on the patient's own page.
In the copy of their data a patient asks for, and gone when their details are erased.

Who pays

An authorisation counted from the sessions, not from a tally

Where an insurer, a solicitor or an employer pays, somebody has to know how much is left before the next session is booked — and usually that somebody is a spreadsheet.

A plan carries who referred the patient, who pays, and an authorisation: its reference, how many sessions it covers and until when.
What is used and what is left are counted from the plan's own sessions; whether a missed session counts is the practice's setting.
Where more sessions are booked than were agreed, the sheet says how many still to come are not covered.
Today lists the agreements nearly spent — one session left, or a fortnight to the day. The app tells the desk; what to do is the practice's.

Rooms

A time is free only when a therapist and a room both are

Most booking tools check one calendar. A practice with three treatment rooms and a gym has two scarce things, and the gym holds exactly one patient.

Every treatment names the room it needs; a treatment room turns around in five minutes.
The gym is one room, used by one patient and their therapist at a time, and its two treatments keep five minutes after them.
A room can be shut for a few hours — the sample shuts the gym for the treadmill's service, and nothing in it is offered then.
The report says how much of the time each room and the gym had somebody in it.

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 physio and chiropractic pack

See the pack The licences