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So how can we now add such a volume

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to our container?

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One of the easiest ways of adding a volume

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is that we add a special instruction

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to the Docker file that creates the image

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which we'll later use to create a container.

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There we can add the volume instruction.

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And this takes an array full of texts,

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full of strings and quotes,

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where we can specify the different paths

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inside of the container file system.

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So, which are going to get used

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inside of our application code, which we wanna persist.

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So in our example, for example

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I am saving my permanent files to the feedback folder.

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I'm also saving files to the temp folder

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but as the name implies, this folder is only temporary.

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It does not need to be saved.

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But everything in the feedback folder should be saved.

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So in my application code,

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I'm writing to the feedback folder

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which of course in the end is inside of the app folder

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since that's our general working directory

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where we copied the entire application into.

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And therefore the volume

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which I wanna save is /app/feedback.

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That is the path inside of my container

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and that's important, inside of my container,

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which should be mapped to some folder

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outside of the container

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and where data should therefore survive.

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Now you might be wondering

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how can we control to which folder

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on our hosting machine does this map?

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I will come back to this later.

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At the moment we don't control this.

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We let docker control this

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and I'll come back to why this makes sense.

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So if I saved this ,

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this Docker file with this instruction added

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we can rebuild this image with docker build.

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Now that we added this volume instruction, docker build.

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and of course, maybe give it again, a tag

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and I will name this feedback-node

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and give it a tag of volumes.

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We already have feedback-node latest.

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Now I give it a special volumes edition, you could say,

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which is the same image as before,

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but with this extra volume feature.

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Hit enter and now this image is created and tagged.

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And now we can run a container based on this image.

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So docker run feedback-node:volumes

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So using that tag, which I just assigned

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and I'll run it in detached mode,

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I'll publish the internal port 80

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to my host port 3000.

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And we can also add the --rm flect to remove that container

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if we stop it, because thanks to volumes,

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this should now not be a problem anymore.

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We can also sign a name and I'll name it, feedback-app.

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If I enter, I get an error because I already have this app.

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I already have this container.

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It's actually even a running container.

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So I will stop this running container here.

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And then also remove it thereafter

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since that was not a container created with --rm.

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So I'll run docker rm feedback-app.

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And now with that removed

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now we can restart this container based on this new image.

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If I now hit enter, this is up and running.

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Therefore, of course, we can go back to local host 3000

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add this feedback one more time, save this

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and now we have a problem.

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Now this starts spinning and it crashes.

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So clearly something went wrong.

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And in order to see what went wrong,

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I'll run docker logs feedback-app

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to look into the output of this container.

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And we see that we got an error here,

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unhandled rejection warning.

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And the problem here seems to be

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that cross-device link is not permitted.

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So when we try to move the awesome.txt file

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from the temp folder to the feedback folder that crashes.

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And that must have something to do

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with our recently added volume

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because it worked before.

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Now this error, which we're facing

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is stemming from this specific code from the rename method

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which I'm using here in node to be precise.

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Now this is no node course

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so I won't bore you with the details.

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But in the end, the rename method doesn't work

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if the file gets moved across multiple devices.

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And thanks to the volume which we specified here

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this is kind of what's happening under the hood.

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Docker indeed will not just move the file

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to some other folder inside of the container file system

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but it moves it kind of out of the container.

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And it's this specific rename method

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which doesn't like things like this.

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Now the work around is simple.

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You can replace the rename method with the copyFile method

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and there after simply add a new line

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where you call fs.unlink and point at the tempFilePath

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and add await in front of that.

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That's all.

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This essentially will copy the file

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and manually delete it thereafter.

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If you change this, you should be good.

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Now of course, we need to rebuild our image

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because we changed the source code

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and that's still not being picked up automatically

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at this point.

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So to do that, I'll first of all remove the image

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with the docker rmi command

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and then there it's the feedback-node:volumes tag

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which I wanna remove like this.

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And then we just need to rerun this docker build command

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to rebuild the image.

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I'll use the same tag as before, since I just removed it.

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And once this was rebuilt, we can run this container again

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still using this image which we just rebuilt

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and removing the container if it shuts down.

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And if you now back to local hosts 3000/

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and you try this again, now this works.

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And now if you access feedback/

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whatever title you picked .txt, you see your file here.

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And now hopefully the file is still there

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if we stop and remove this container and restart it.

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So let's go back and let's stop this feedback app container.

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And once it is stopped, let's rerun it

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by running the docker run command again.

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And this will of course create a brand new container based

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on the same image as before.

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And if we do that, then go back and reload

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well, no, this file is still not there.

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So what's up with this volumes thing.

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Why am I teaching you about volumes

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if we just add an instruction that doesn't do anything

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except for break our code?

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What's the idea behind that?

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Why is it not working?