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In the last section we saw that making a change to our index such as file and then rebuilding the image

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caused the copy step to be completely invalidated.

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We had to rerun copy in every step after it which means we had to sit through and wait for all of our

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dependencies to be installed through run npm install.

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So in this section we're going to try to figure out a way to avoid having to completely reinstall all

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dependencies just because we made a little change to one the source code files inside of our project.

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So here's what we'll do.

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We're gonna take our current build process inside of the docker file and we're going to split the corporate

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Copy operation out into two different steps.

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The only thing that npm install right here requires in order to run successfully is the package not

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JS on file.

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That's all we care about.

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We don't care about the index such as file or any other file inside of here.

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So during this initial copy step I'm going to say the only thing that I want to copy is the package

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not just on file so I just change the instruction right here.

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It now says look in the current working directory or look in the directory specified by the build context

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argument of Docker run or something Docker built and find the package stop Jace on file and then copy

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that into the current working directory of the container will then rerun or Samuel will then run npm

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install and then after we install all those dependencies we'll copy over everything else and so we'll

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do a copy dot slash dot slash like so so.

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Think about what is going to happen now as you start to work on your project.

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You can make as many changes as you want to the index not just file and it will not invalidate the cache

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for either of these steps right here.

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The only time that NPM is npm install is going to be executed.

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Again if we make a change to that step or any step above it.

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And so in other words the really only effect this is going to have is if we make a change to the package

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not just on file.

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That's really the only situation in which npm install will normally be executed again during the build

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process.

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All right.

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So let's try building this again and then starting the container up and just making sure that everything

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still works.

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All right.

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I'm going to flip back over I'll do a docker build dash T.

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Steven Greider again your Docker I.D. not mine.

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Simple web and I will specify that Bill context with that simple dot.

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So I'm gonna rerun this.

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Now we have made a change to the docker files so we are gonna see some steps rerun and then we eventually

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get our successfully tagged and built image.

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Now before trying to run it I want you to run the same command a second time so same thing.

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No changes whatsoever so that time it went extremely quickly because we were able to use the cached

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version of every single step.

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So now let's imagine that we are doing some normal changes to our application and a normal change would

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probably be a change the index not just file.

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So maybe we decide that we don't want to say bye there anymore.

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Maybe instead we want to send back.

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How are you doing.

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So we've now made a change the index not J.S. file and we definitely need to rebuild our image because

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we do not have any support for kind of hot reloading of the index Dot.

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J ust file a project file into the container so we have to rebuild the image.

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So I'll go back over.

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I'm going to rerun the same command for a third time and we saw that it executed very quickly because

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the only change that we made to any step along the way was step number five.

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We copied over the entire contents of that directory Docker detected that we made a change to one of

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those files and so it reran that step and every step after it.

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But we did not have to rerun the NPM install step and reinstall those modules which would have taken

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you know in our application not very long but in a real application it could take several minutes to

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run.

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All right.

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So the real lesson to learn here is that yes it does make a difference.

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The order in which you put the L.D. instructions into your Docker file and wherever possible it is kind

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of nice to segment out the copy operations to make sure that you are only copying the bare minimum for

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each successive step.

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All right.

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So that's pretty much it for this application now.

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I hope that it was not terribly annoying for me to show you kind of like some the wrong way of doing

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things and incorrect them along the way.

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I hope that it was kind of nice to see the error messages appear and see the changes that we made.

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Of course we could have put the right Docker file together from the get go but I don't know.

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You probably may have not learned quite as much as seeing the errors occur and then fixing them in as

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they occurred.

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No I think this application has turned out pretty well but it's definitely a very very basic operation

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so let's take a quick break right now.

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We're going to come back the next section we'll start working on a much more advanced Docker project

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so I'll see you in just a minute.