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‫Hey, guys, what's up and this video I want to talk about, how can you build an online booking system

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‫and not just the actual act of building an online booking system, but the most important feature,

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‫which is prevent double booking or prevent race condition when two people try to book the same seat

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‫at the same exact millisecond?

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‫OK, I've seen a lot of systems that actually don't have this problem solved.

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‫Right.

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‫So I'm going to talk about it.

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‫I'm going to talk about how to prevent this condition, can talk about exclusive locks, ride.

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‫And I'm not going to show you how I built this application because it's very straightforward, not really

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‫magic.

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‫I might I might make another video to actually show you how the how I built the application from scratch.

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‫It's it's it's really straightforward.

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‫I'm using Express, Norges and Postgres as the back.

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‫I'm going to go through the application right now.

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‫The idea here as we are building an online system that books, I don't know, seats.

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‫Right.

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‫You're you're buying tickets at a concert.

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‫And this is one user and this is just another instance, maybe another user.

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‫Right.

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‫And trying to open that.

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‫And these are seats.

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‫Seats are green.

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‫I mean, they are available.

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‫Red are booked.

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‫OK, how about we actually try to implement this thing?

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‫So if I click on this seat number three and I say, hey, my name is Ali, I know in a book this seat,

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‫I'm going to get back a message, say, hey, this was booked successfully.

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‫Right.

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‫And we're going to go through the logic.

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‫And now what what exactly am I doing here?

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‫And if I do this and I'm going to fresh here, I can see that this now is booked by Ali.

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‫You can see there is a tooltip here.

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‫I didn't I really it's a bad front.

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‫And I know it's just does the trick.

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‫And if I go to the other using a refresh, you can see that this has taken effect or so of that user

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‫like, say, book jack, book number nine.

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‫And you can see that I am now actually debugging this and I'm going to show you exactly what happened.

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‫So how about we go through the code and I made a put request here.

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‫And what we're doing here is we get the ID, which is the seat ID and the name from the prompt.

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‫And I am connecting through a Posterous pool.

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‫So I get to pick up one client from the pool.

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‫And we talked about Norges Postgrads and how we actually make transactions.

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‫I'm going to reference the videos here.

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‫If you want details this, I'm going to go quickly through them.

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‫I'm going to begin a transaction because obviously this is a transaction.

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‫Right?

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‫And the first thing we're going to do in the transaction is this.

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‫I'm going to select Star from Seeds.

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‫I need to actually query this seat and see if it's actually booked or not.

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‫That makes sense.

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‫Right.

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‫A lot of booking system actually does that.

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‫And so where ID is a good one and is booked equals zero.

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‫That means it's not it's not booked.

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‫And that's perfect.

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‫When I do this query, I'm going to get the result back.

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‫And the real count is one that means, hey, you actually found this seat and it's available for you.

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‫All right.

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‫So far, so good.

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‫But if that seat is not available, you're going to get an error.

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‫We're going to send back in our seats already booked.

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‫This is to avoid this double booking.

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‫But we're going to show that this is not enough in a minute.

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‫Right.

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‫A lot of people actually do this.

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‫This party, Aquarian, check that thing.

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‫But this is not enough.

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‫So I'm going to go and once we actually pass this, that we are safe to actually execute an updated

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‫statement and said to the go away, please, is his book the equal one and the name equal, then where

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‫it equal to one?

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‫We just physically set that statement.

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‫So let's go ahead and just run this thing and we commit the transaction.

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‫We release the client back to the pool.

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‫Right.

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‫And then we send the result till we get back.

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‫We get back also here booked successfully.

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‫Right.

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‫And then if for a year.

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‫Right.

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‫And I'm not going to refresh actually, if I actually click this nine now and I'm going to book it for

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‫which is already book, we know that it's book.

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‫Right.

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‫If I say I don't know, saying that, OK, we get an error failed.

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‫Couldn't book because it's already booked and the reason is because we have queried and that Kuwait

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‫actually failed.

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‫Returned.

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‫There's no record on gonorrhea.

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‫All right, Hosain, what are you doing that all of the stuff, y'know, here is the floor of this design.

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‫The problem with this design is.

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‫Double booking can easily happen, not on my single machine, obviously, but if you have a flux of

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‫requests coming at the same time, you will get into a situation where both statements actually at the

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‫same time you can imagine them executing impera.

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‫They query the seat is available right.

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‫The first transaction query and says, oh, the seat is available and it reaches here.

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‫And the second query also queries and also finds out that the seat, the same seat is also available.

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‫Right.

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‫So both of them will succeed on this if statement.

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‫They will pass it and both of them will execute the update.

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‫And that's the problem in this case last to an end.

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‫But incorrectly, that client will get the message that actually your booking has been succeed.

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‫And we saw a lot of back back maybe 10 years ago when online cinema became a thing.

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‫Right.

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‫We always get double booking because of this problem, because it's a popular system.

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‫Right.

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‫A lot of people quit at the same time.

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‫So we're going to show how to fix that.

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‫First of all, let's reproduce this.

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‫Let's show you actually this is going to happen, but I'm going to do this with the power of debugging

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‫is what I'm going to do.

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‫I am going to book I'm going to book seat number 15 now, and I am going to sign him to wreck.

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‫All right.

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‫And this will essentially go to the debugging statement, passes all of that stuff.

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‫And I'm going to post that here.

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‫I'm going to intentionally not resume the code to go to continue.

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‫But guess what?

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‫It actually passed the select statement that actually checks whether whether the it exists or not.

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‫So the seat is available.

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‫Here's what I'm going to do.

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‫The seat is available.

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‫But that guy did an updated.

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‫Yet Rick did not actually reserve the seat because he didn't reach the statement.

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‫Right.

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‫Now, if I go to the other client and I also try to book this guy and I say, Edmund, want to book

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‫this guy this, I don't have a debugger.

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‫This guy will completely succeed because there's no debugging.

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‫Right.

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‫We'll find out this is available and it will actually commit the transaction or this is book.

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‫Now, here's what I do now.

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‫I'm going to just release the code, go ahead and continue.

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‫And you can see that both of them actually booked the same seat.

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‫And that's the problem.

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‫This is what it's called, race condition in online system or double booking.

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‫And here is how we can fix this problem, guys, using the concept of exclusive locks, how about we

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‫jump into it and fix it?

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‫The problem here, guys, is.

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‫Both of these guys are attempting to edit the same row technically, right?

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‫So the solution, one solution at least as.

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‫The first person or process to get to the role should obtain an exclusive lock on that row so that the

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‫other process, when it tries to execute it, it will have to wait until the other transaction actually

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‫completed.

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‫And how do we do this in postgrads?

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‫It is very simple.

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‫The same thing you select that you're selecting this role, right.

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‫The only thing you need to do is add two things at the end.

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‫So for update, that's it.

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‫When you do select star from Select where ID and you query that role.

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‫You also say for update and that is a call, the role level luck and few databases actually support

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‫that.

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‫Not all databases.

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‫Right.

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‫So this is a very important feature in some databases, I suppose.

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‫I think MySQL also supported.

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‫Oracle definitely supported.

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‫And you do it for update.

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‫And the syntax 692 is almost identical in all of them.

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‫So you do select staff for updates.

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‫Wow.

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‫All right.

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‫The moment you do that, the database will have an exclusive lock on that role.

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‫That means if someone tries to update that rule or selected for update as well, they will have to wait.

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‫And this is what I don't like about progress in Oracle, because I use Oracle for 15 years and an oracle.

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‫That's why Oracle is expensive.

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‫One of the reasons, as in Oracle, you can actually specify a time out here, says, hey, wait four

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‫or five seconds right here is actually an indefinite wait.

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‫I'm going to show that now in a minute.

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‫All right.

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‫So let's go ahead and try to do now that we have fixed that, let's refresh this thing and let's also

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‫rerun our my second application right on board, eight, eight, eight, eight.

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‫And let's check this up.

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‫Right.

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‫And for fun, let's just clear all that box.

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‫Right.

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‫I'm going to update Sytek will push back.

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‫What a double this.

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‫No, no one is looking at anything just like that.

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‫All right.

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‫Everything is now unbooked.

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‫Sweet guys.

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‫All right, here's what I'm going to do.

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‫I'm going to do the same thing.

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‫Going to book seat fifteen.

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‫I'm going to one, for example, booking seats fifteen.

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‫We go there, hopefully the debugger anytime soon now.

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‫OK, we selected the same stadium, but now it's for update.

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‫There is a little bit of difference in the database.

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‫It's freaking out now, right?

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‫We got it now.

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‫Here's the thing.

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‫This guy will always be the winner no matter what, I'm going to find out how.

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‫So the other process now will try to book 15.

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‫Was it 15?

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‫I think it's 15.

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‫Let's just make sure I think it's 15.

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‫Yep, 15 or so.

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‫15.

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‫If I click 15 and I say, Melissa, I want to book 15 is what we're going to do.

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‫Look at that, we didn't receive anything is just waiting.

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‫Now, the process is just stock ride and this is what I don't like, how our policy there is no option.

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‫Am I wrong?

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‫Correct me if I'm wrong.

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‫There is no option to actually give up and time out after after a certain amount of waiting.

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‫Right.

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‫So this guy is waiting and this is a bad user experience.

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‫That's the limitation of this now.

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‫And this is where your database actually shines.

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‫Now, if I go and say, release that cracken here, go back this guy.

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‫This is what happened.

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‫Something happened in your right book successfully is the guy who actually the person who actually made

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‫that transaction book successfully, but this guy was waiting technically right here.

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‫They were stuck.

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‫And the moment this process actually released the transaction, what happened is that Locke got unblocked

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‫and now we selected the brand new committed change, which was what it was actually committed, which

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‫was the rate is already booked.

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‫I mean, this result is zero.

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‫That means this either already booked.

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‫All right.

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‫And that's what happened, essentially, guys.

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‫So this is now a bulletproof online booking system that is essentially race safe or or you cannot get

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‫double booking with them, obviously comes with a price.

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‫But that price, remember, nobody in their right mind will put a debugger, obviously, and this will

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‫take a finite amount of time.

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‫Milliseconds is very fast.

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‫So the wait will almost be unnoticeable.

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‫Right.

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‫So that's only a good encoding book already booked, right.

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‫All right, guys, that was a short video talking about this.

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‫Let me know, guys, if you actually want me to build this application from Scott showing you how to

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‫highlight what all the species we explained all that stuff before everything.

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‫Right.

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‫That gate requires.

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‫This is a port also for fun.

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‫I added a delete message to move it, to delete that booking, to unblock that thing.

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‫And obviously, this is a very simple I'm going to reference the source code below for you guys if you're

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‫interested.

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‫And the database also, it's just a Bosco's on Docker, very straightforward.

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‫And all the tutorials, all this post Christodoulos, we have done them before.

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‫Let me know if you have any questions.

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‫Let me know if I missed anything.

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‫I'm going to see you on the next one.

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‫You guys stay awesome.