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HLT Reservation App

  • hlt_rsvt_next

    Including all front-end codes, and part of data access layer which I plan to replace with REST API client. Built with Next.js.

  • hlt_rsvt_server

    Including GraphQL and RESTful APIs. Built with Nest.js

data flow diagram

Due to the time limitation, there are still lots of thing I didn't implemented: unit tests, Playwright for e2e testing, docker files, etc. Hope this project could show the rough ideas of my design.

Some implicit bussiness rules

  1. Customers have to login before making reservations (while the login process should be as easy as possible) - DONE
  2. Employees have to login before manipulating reservations - DONE
  3. A reservation should have an "expiration", when the reserved time passed and employee didn't mark it as "Done", the reservation should be treated as "Expired". - DONE
  4. The seats are not infinite. When a reservation is made, the available seats should be decreased by one. - TODO

    • Unlike tickets or goods, we could not simply use a number to represent the available seats. We have a totalCount of seats, but the availableCount is calculated by the reservation's time.

    • To make it simpler, we can assume that every reservation will reserve the seat for 2 hours, for example. Before a reservation is created, a getAvailableSeats(reservationTime, tableSize) function should be called, to calculate how many seats have been reserved in the time range.

      Something like:

      async function getAvailableSeats(reservationTime: number, tableSize: number): Promise<number> {
    const timeFrom = reservationTime - 7200 * 1000;
    const timeUntil = reservationTime + 7200 * 1000;
    // any reservations have time fall in this range are treated as possibly conflict
    const sql = `
        SELECT COUNT(1) FROM reservations WHERE (status = 'pending' OR status = 'preorder') AND tableSize = $1 AND time > $2 AND time < $3
    `;
    const args = [tableSize, timeFrom, timeUntil];
    const reservedCount = await SOME_QUERY(sql, args);
    const availableCount = TOTAL_SEATS[tableSize] - reservedCount;

    return availableCount;
}
  5. To avoid some potential concurrent conflict during the available seats calculation, especially when multiple reservations are created at the same time. We may also make the "reservation create" a two-step process. First we create a reservation order with an unique id after the availableCount checks passed. The status of the newly created reservation should be preorder. Then we use a message queue to handle the preorder one by one, do the checks again before mark it as pending.

How to start locally

Before starting the project, make sure a couchbase instance is ready, you can use a docker image to start it:

docker run -d --name couch -p 8091-8097:8091-8097 -p 9123:9123 -p 11207:11207 -p 11210:11210 -p 11280:11280 -p 18091-18097:18091-18097 couchbase

Visit localhost:8091 to finish the initial configuration.

Then modify the env file, one in hlt_rsvt_server and one in hlt_rsvt_next, to match your couchbase instance.

# hlt_rsvt_server/.env
HLT_RSVT_COUCH_CONNSTR=couchbase://localhost
HLT_RSVT_COUCH_USERNAME=admin
HLT_RSVT_COUCH_PASSWORD=123456
HLT_RSVT_JWT_SECRET=A_TEST_JWT_SECRET

# hlt_rsvt_next/.env.local
HLT_RSVT_COUCH_CONNSTR=couchbase://localhost
HLT_RSVT_COUCH_USERNAME=admin
HLT_RSVT_COUCH_PASSWORD=123456
HLT_RSVT_GQL_SERVER_URL=http://127.0.0.1:3001/graphql
HLT_RSVT_REST_SERVER_URL=http://127.0.0.1:3001

Or just cp hlt_rsvt_server/.env.example hlt_rsvt_server/.env and cp hlt_rsvt_next/.env.example hlt_rsvt_next/.env.local as templates.

Then install the deps, start the server and the client:

cd hlt_rsvt_server
npm i && npm start
cd hlt_rsvt_next
npm i && npm run build && npm start

Visit localhost:3000 to see the app.

At the customer login page, input any phone number, the verification code is always 000000.

At the employee login page, use admin as username, and anything for password.

Some thoughts during the development

I spent the first two days trying the loopback.js framework. It's an awesome framework for team working, provides a powerful cli tool to generate most boilerplate codes, which helps aligning every developer's codes. It's also an opinionated framework imo. And that's why I gave it up at last. For a small demo project it's a little bit heavy, and for beginner like me it's hard to show my own ideas through it.

Then I put my eyes on Next. I've heard about it a lot but haven't got chances to try it. It seems like very suitable for small project. So I spent the next two days following the tutorial. It was a pleasant experience. The idea of mixing server side rendering and client side rendering is very interesting. But I'm not sure if it's a good choice for a larger project. Accessing database from front-end (although it's placed in a "server-side" part) seems too It may lead to a "big ball of mud" code base very easily. That's why I built a pure back-end server with Nest. Once I found the Next part take too much responsibility, I can easily replace most business logic with RESTful APIs calling.