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So now that we had a look at two kinds of communication,

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there is one last kind of communication,

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and that's from container to container.

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For that, we first of all need a second container,

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because up to this point,

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we always had only 1 container working on its own.

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Now I want to have 1 container for the note app

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and one container for the MongoDB database,

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Because as I mentioned earlier, already,

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it is a good practice to have every container focus

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on just one thing.

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So one container for the node Web API,

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and one container for the database

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and therefore, I will first of all,

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stop my running node application container here

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with Docker stop favorites,

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and once this is stopped,

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I want to put my MongoDB database inside of a container.

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So I will no longer use the locally installed one,

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I could uninstall it

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and if you never installed it in the first place,

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which you probably didn't,

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because I told you that you won't need it,

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then you can now follow along because now

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is the point of time,

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where we're going to set up MongoDB together,

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but inside of a container

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and that means that we're going

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to use two containers instead of just one now.

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For that, let's start by creating a MongoDB container

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and you could think that we're going

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to write another Docker file for that,

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but we don't even need to do this,

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because if you search for Docker MongoDB,

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you will find an official Mongo image

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on Docker Hub.

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So just as before, with the official node

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and Python images, this is an official MongoDB image.

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Now you'll find detailed instructions on how

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to use it on this Docker Hub page.

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That's true for all kinds of images by the way.

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But we're going to set it up step by step together anyways,

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so you don't need to go through these instructions for now.

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Instead, what we're going to do here

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is we're going to run a container

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with Docker run based on this Mongo image.

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Now that is an image which already exists here

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on Docker Hub.

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So I'm using this official image named Mongo.

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If I run Docker run Mongo,

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this would create a new container based on

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this MongoDB image

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and this image will spin up a MongoDB database.

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So if I hit Enter here,

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you should see some output like this

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and this is now the running MongoDB database container.

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Now, this is blocking the terminal here,

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because I did not run it in detached mode.

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Hence, I'll actually quit this

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with Ctrl + C,

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and then rerun it.

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But now with Dash + D to run it in detached mode,

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and also with Dash + Dash + name to give it a name

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and I'll name it MongoDB.

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So with that this is now running,

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we can see this running MongoDB container.

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So that's now container with

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a MongoDB database inside of it.

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Now, I want to alter my node js application code such

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that we can connect to this database running in

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this upper container

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and that's now the tricky part,

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host.docker internal will no longer work,

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because that refers to my local host machine IP address,

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not to the IP address of some other container

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but instead we can do something interesting,

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we can run a number of command

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and that's the Docker container inspect command to

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inspect this MongoDB container we just started.

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Here we get a lot of information about this container,

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but the interesting bit of

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information can be found if you scroll up a bit,

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this IP address here,

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which you find under network settings in

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this object here, there this IP address.

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This is the IP address of your container

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and this IP address can be used to contact this container.

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So if I copy this IP address, which could be different

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on your system,

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so make sure you copy your IP address which you see

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in your in your terminal when you inspect this container.

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If we use this IP address here,

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instead of host.docker internal,

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then this node application should be able to connect to

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this upper container and to the MongoDB database in here.

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This port is the default port so

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this shouldn't require any changes

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and therefore we should now be able to talk to

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this our container.

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Hence we can build

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this image again the favorites node image

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because we updated the source code

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and we can then try running

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this favorites node application container based on

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this updated image.

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So if I hit Enter on this Docker run command,

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let's inspect the running containers

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and I see we got containers up and running

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the MongoDB container and my node application container.

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With that back here in postman,

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if I send a request to localhost 3000 favorites,

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talking to this dockerized node API,

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I get back a response with an empty favorites array.

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The fact that it's empty makes a lot of sense,

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because this no longer is my MongoDB database

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on my host machine

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but instead, this is a brand new MongoDB database

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in our MongoDB container

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and therefore it of course does not have access

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to the data stored in my local host MongoDB database.

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It's a totally separate, isolated database.

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After all, separation and isolation are core concepts

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and core reasons for using Docker, right.

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So we got no data back because there initially is no data

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but and that's super important.

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We can communicate here, we don't get an error

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and that shows that our approach works,

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and that the two containers are able to talk to each other.

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I can also try storing a favorite again and that works

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and if I thereafter send a GET request to favorites,

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that data is there.

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So this cross container communication works.

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But of course, it's not very convenient.

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We had to look up the IP address

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of the our container in order to then use it here.

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Definitely not a lot of work,

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but still quite cumbersome.

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It also means that we always have

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to build a new image

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whenever the MongoDB container IP address changed,

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because we hard code that IP address here ,in the node app

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and this of course, is not ideal.

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So thankfully, there is an easier way

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of having multiple Docker containers talk

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to each other.