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So in this session we are going to learn about Redis Transactions



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and basically Redis Transaction is supported by these set of 5 commands.



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OK.



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which  we are going to use. Now to give you an example of Inconsistent Transaction. So let's say we have,



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you know, User 1 which is going to see the total number of available courses and then it is going to



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enroll for the course.



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So let's have an user 1 queries Redis,  to see the total number of courses available along with the price.



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Redis replies that there are total 3 courses available and the price is 500$ and you



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know, when user 1 tries to enroll for the course. Just before that there is another user User 2. It enrolls for



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all the three courses



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Okay.



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Successfully and now when User 1 tries to enroll.



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Redis



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And if Redis  allows it to enroll. It is going to be inconsistent state, because we have only 3



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courses available to enroll.



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And basically we have already allowed 3, User2 for three enrollments and if we allow User



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1 also for the enrollment, then basically we are going to have you know, more number of enrollments



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than the available seats.



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So if you are, you know, by Atomic Transaction, what it means is that basically when User 1 is issuing



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this transaction. User 1 is going to issue this transaction as a Transaction.



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Okay



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So if, you know, if the value of the Ticket is changing or let's say the availability of the Ticket is



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changing. That basically the transaction is going to get failed.



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Okay.



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So in transaction basically all the commands execute successfully or  the transaction fails.



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Okay.



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That is the property of a  transaction. So by that what I mean is, Transaction basically is going to guarantee



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you that all operation which, you are performing is going to be executed sequentially and also either



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all of them are going to be successful or none of them are going to be successful.



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Okay.



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So it is going to make sure that you don't run into an Inconsistent State.



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Okay.



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Now here let's say we have a Key, which is called Ticket Available and now let's say we just want to see



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how many tickets available and we want to decrement the value of ticket and that's it.



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Okay.



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And  both of these basically should be running as a Transaction. It should not be running as Individual



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Command.



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Okay.



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So by Individual Command, what I mean is you know,  running Get Ticket Available.



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Okay.



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This is running as a Single Command and then you have the option to Increment Ticket Available.



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Okay.



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and then let's say i wrote a  wrong Command.



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It doesn't impact previous commands.



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Okay.



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So these, all of these three commands which we ran, it is executed one by one.



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Now we want to do, is we want to run set of commands as a Transaction. Now to start a transaction



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you type multi, okay.



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That is how you indicate Redis that basically now you are going to execute a transaction.



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You don't want to execute commands line by line.



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Okay.



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So now  if I type ticket, Get Ticket Available you see the response which I have get is Queued, which means



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you know this command is not executed.  It has been Queued.



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And if I do



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Decrement Ticket Available and then we want to see the value again



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Okay now we have issued 3 commands but none of them has been executed by Redis.



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So it is only going to be executed when I type



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exec.



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Okay.



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So basically that is a way of telling that Execute this Transaction.



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So once I hit enter. Now you see all the three commands has been executed together.



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OK.



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And this this



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No transaction basically has been executed.



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And we got the Consistent Reply.



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Now let's say what happens, if there is an Error in one of the commands



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so once you write Execute. The previous transaction is already ended here.



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And if we want to start a New Transaction. We need to write Multi again.



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So I write Multi.  Hit enter.



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and now I'm going to write Get  Ticket Available and then we want to decrement the value.



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However, this time, let's say,you know, I don't write Decrement Ticket Available.



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I probably wrote let's say, wrong command and once I hit enter. It is going to give me an error.



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OK.



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But since we are running it as a Transaction. If I run the next command.  OK let's say if I run



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Decrement Ticket Available again



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and then I run  Get Ticket Available.



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OK.



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So what do you think is going to happen, when I run Execute in this



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OK.



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So if you're thinking that this is going to get successful. It is wrong, because you see one of our command



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in this transaction is failed.



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So basically this whole transaction is going to be treated as a Failed Transaction.



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Okay.



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So I'm going to hit enter and you see that Execution is Aborted. Transaction is Discarded because of



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previous errors. So when you have error in transaction. The whole transaction is aborted.



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It is not that one line of transaction is going to run and another line of transaction is not going to run.



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and the same has been explained here in the example as well.



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So if you just want to take a note, you can take a note from here.



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I'm going to give the link of this Blog in the Resource Section.



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Now we have something called Discarding a Transaction. Now let's say



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let me clear the screen first and let's say, in the middle of the transaction



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we realize that we don't want to continue with the transaction.



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We just want to discard this transaction.



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Then you can use Discard



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Command.



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So let's say rather than, you know, decrementing the value.



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By mistake we incremented the value of ticket available.



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And now I do not want this transaction to run.



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So if I type Execute, basically it is going to execute this transaction and the value of ticket available



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is going to be increased. Whereas actually we wanted it to be decreased.



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So in this case I can issue Discard Command and hit enter.



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And now we are out of this transaction.



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Okay.



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So this Multi is now closed. This Transaction is now closed.



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And if I type Get Ticket Available.  Now we're still going to see 4 okay.



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We don't see the Increased Value, which is it should have been 5 in this case



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now since we are running on a Single Client.



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And we are able to perform operations sequentially. Now let's assume



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what happens if there are Multiple Clients  or Multiple Applications are trying to, you know, get



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the Values or Perform Operations on the same keys.



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Okay.



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What is going to happen and how you avoid or how you ensure that transaction is run, you know with



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in, in a Consistent Mode.



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Okay, so in that case basically what we need is, something which is called a Watch Statement.



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Okay.



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In Redis.



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So you see, right now we have ticket available value of 4 and I am going to start a new transaction.



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But before I start a new transaction, I'm going to make sure I run Watch Ticket Available.



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So basically what it is going to do is,  it is going to ensure that, if the value of the good available



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is modified by any other client, this command is



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Basically this transaction is going to return us Null.



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OK.



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Which is Redis way of saying that someone else or some other client also modified the value of Ticket Available.



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So it cannot modify the value of Ticket Available.



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Okay now let's say, I wanted to decrease the value of Ticket Available, by issuing this Command.



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However let's say we had another Client. Okay which already issued a different Command on this



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Ticket Available keys. So to simulate that scenario.  I'm going to open a New Session and then we'll



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be logged into this Redis and then we are going to issue some commands or let's say probably I am



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going to increase the value of ticket available or let's say decrease the value of the ticket available.



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Okay.



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And I'm going to run this 3 time.



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Now if you see, we have value of ticket available, modified by some other client.



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And in this case, this client 1 is unaware that, you know, value of ticket available has been changed



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and it still goes on and, you know, enrolls.



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And which results in reducing the Ticket Available Value.



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okay.



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Oh I guess we made a mistake here.



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So after Watch we did not start the transaction with Multi



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Command.



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Okay.



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So that's why this is executing individually. It is not executing as a transaction.



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Okay.



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So yeah.



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So in this case, let me first run a Set



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Command and Set the value of ticket available to Let's say 4. Okay



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And then I'm going to run  Watch Ticket Available and



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so after the Watch Command, you just need to make sure you are running it as a transaction.



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Okay.



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So  the way to do that is, you use Multi



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and then I'm going to run these commands. So get ticket available and



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decreased or Decrement Ticket Available.



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But before we do that again, we are going to simulate that there is another client, which is changing



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the value and now we are going to come back to client 1, which is unaware that someone else has also



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changed the value.



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However since we were running it with the watch Command on ticket available. So if any other client has



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changed the value of  ticket available. This whole transaction is going to get failed.



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Okay.



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So now, here if I write Execute Statement and if I hit enter. It is going to reply us with a Nil.



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Okay.



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So what it means is, that someone else has changed the value of this key and that is Redis way of saying



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that, that's why this command has failed.



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Now let's say



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if no other client has changed the value of ticket available. In that case what would have happened.



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So let's see that.



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So I set the  Ticket  Value to 4, which is the initial value.



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And then we are going to write



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okay again we want to introduce Watch on this key



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and then I'm going to run a transaction by hitting Multi.



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And then we are going to see the value of  ticket available and then we are going to decrement the



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value of ticket available



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and then I'm going to execute this. Hit enter.



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And now you see the transaction has been successfully executed because there was no interference in



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this transaction by any other client. Okay



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So these are the ways, how you handle transaction in Redis.