How I would design a reservation system 

you are changing your source of truth to something which doesn't guarantee strong consistency and is in memory volatile

thanks for pointing this out. I made this video with the assumption of ACID compliant databases, but I probably should have put more thought in to talking about it. Doing some type of conditional update to obtain the lock and have it throw an exception if the lock already exists and isn't expired is probably the best approach assuming you have consistent writes in your database.

If you populate expiry & reservation within a transaction why would the order matter if you roll back the transaction if any write fails?

@@CanRau I should have been more careful about my choice of the word “transaction”. I meant transaction as in a single round trip from the user perspective. When using a formal transaction mechanism like the ones provided by many relational databases, you are 100% correct that the order does not matter. However, distributed systems, and certain NoSQL databases do not support formal transaction mechanisms and in those cases we need to manage things ourselves and the order can help us simplify things (i.e. avoid having to write a manual rollback or compensating transaction).

Aaah I see that makes sense! thanks for clarifying 🙌

But this is more than ok solution for small apps where you have anything less than dozen bookings per hour. The chances of this happening are so low that it is much cheaper to manually fix these issues when they occur then to implement something more advanced. Obviously, on a large scale when you have millions of bookings per day we would have to do way way wayyy better than this.

In that case it sounds like you’d need another flag or status on the seat to mark it as “pending” which would also act as a reservation to prevent people from buying. I guess I’m wondering what you do if the webhook takes hours to come in, or never comes in? After a certain amount of time you’d need to void the persons payment and open the seat probably? That might be a rare edge case

@@WebDevCody It is indeed very rare edge case. So rare than we basically ignored it, we assumed all the webhooks will be received after 10 minutes. We agreed that when it happens we will fix it manually by support. It never happened. But now that we are speaking about it I am curious to investigate and try to solve this problem just for fun.

But again, this decision to ignore the edge case is justified when you are operating on a small scale. "Once in a million" edge case will happen once a year on small scale which is fine most of the time. But it will happen dozens of times per day on large scale and no support can fix that.

This is perfect example how back end starts being relatively easy on a small scale but it then becomes super super suuuper hard on larger scale.

yeah that sounds like a potential edge case you'd need to think about. I think you could just try to obtain the lock right before a user submits the payment info. It's either that, or you need to keep refreshing the lock periodically if the user is actively doing something on your buy seat wizard.

What were you looking for?

I'm still waiting in the ticket queue for months for my taylor swift tickets 😝

😂 why you do late to the party?!

If two users ask at the same time, you’d want an ACID compliant database and use a transaction or a conditional write to claim the reservation. That would prevent multiple users from claiming the lock

Websockets or push server events

@hijazi479 So can you give me insights On how to solve this case

Also, great videos! I love the way you break these topics down. 😄

I don’t think disc latency is important. Having an acid compliant data store with transactions would be necessary to prevent double booking.

Sure, but what if user2 loads the seat map and just waits for a minute. By that time other seats could have been bought. So either you have to add polling or websockets to make the Ui refresh, or just refresh the seats if the user clicks one that was already reserved

Date times should probably just be stored as ISO dates in UTC timezone, Zulu time

Yeah for sure, the ui should show open / closed cabins. Ones you can’t buy should be red or grayed out. You could just join the cabins with non expired reservations and send that list back from the api to show the open rooms

@@WebDevCody Makes sense. Returning everything but having its status displayed for the users.

@@onakoyakorede but yes a more sophisticated system would be use events or websockets to update the view for users, but that might add lot of complexity and costs for a larger scale system. Sometimes just refreshing when the user clicks an already reserved room can save a ton of money and complexity

Hey I'd like to hear your thoughts on this topic about a scenario what I think is quite common on some "super-sales" kind of events. Say a *ticket* to a highly wanted *gig* is at a -70% sale or something. And the sales event starts at 8am. Multiple persons refresh into the page at the exact same time (with a date "error" percentage due to internet speeds). How to "keep the truth" in these kinds of scenarios for users? The UI for the end user cannot be what was initially loaded right. The application cannot send requests every second to check availability=true or not right, because that would make the app heavy. I'd really like to hear a very simplified explanation how these kinds of situations are handled or a keyword or two what i should search and study. I'm asking as a noob fe-oriented fullstack dev who's more interested in system architecture. I've recently built a web store with our team where the first user to click the *product* into his *cart* is the one the *product* is reserved to. But I think there lays a problem where if another person who has loaded the same products list at the same time as the other... you see where this is going. This app we've created has a very low traffic/orders placed per day amount, but i am very interested in how to scale applications. Thanks alot for possible responses.

@@satansauce666 if you have an event sale starting at a certain time, you may want to just have logic in the UI to show new content at 8am.

Lmaoooooooo hey! that’s my line!

You better back off, my wife won’t like it