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With Docker, you can create so-called container networks,

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also called just networks.

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So, what are networks then?

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Well, the idea is that you might have multiple containers

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and you want to allow communication

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between these containers.

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So, essentially the scenario we have

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with the node web API container and the MongoDB container.

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Now, with Docker, you can put all these containers

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into one and the same network

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by adding the --network option on the Docker run command.

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This then creates a network in which all containers

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are able to talk to Java,

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and Docker is then automatically doing this

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IP look up and resolving stuff,

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which we did manually a couple of minutes ago.

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And that's a really useful feature

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for having multiple, isolated containers

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with their own duties and tasks,

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which still are able to talk to Java,

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which is exactly what we want here in our example.

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So, let's see how we could utilize networks here.

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And for that, I'm going to stop the favorites container

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and I'm then also going to stop the MongoDB container,

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because I wanna restart both, but in a different way.

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So, let's stop favorites and let's stop MongoDB,

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and let's then explore how we can use networks here.

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Let's start with MongoDB.

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We started the MongoDB container with this command here,

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Docker run -d --name mongodb and then using the Mongo image.

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Now, we can add another option here

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and that's the --network option,

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and here we can then pick any name of our choice.

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Let's say, favorites net.

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So, favorite-net is my network name.

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Now, if I hit enter, this fails to run

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because I just stopped the MongoDB container,

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I did not remove it,

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so, therefore I will actually run Docker container prune

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to remove all stopped containers,

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so that this MongoDB container is also removed

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and thereafter,

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we would be able to run a container named MongoDB again,

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but still, if I try to run this

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with this --network option, if I hit enter,

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I get an error that the network favorites net is not found.

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Unlike with volumes, for networks,

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Docker will not automatically create them for you

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if you want to run a container using a network,

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instead you have to create them on your own.

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So, first of all,

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I'm going to remove this newly started MongoDB container,

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which then was stopped because we failed to

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find that network,

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but once that is done, we can create a network

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with the Docker network command.

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With --help you again see all options,

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and the most important option here at the moment

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is to create option.

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With Docker network create, you create a new Docker network,

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and then you don't need any sophisticated configuration,

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you just need to pick a name, for example, favorites-net.

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That's your custom network name,

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and you can simply hit enter thereafter,

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and this network gets created and set up by Docker.

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It's a Docker internal network,

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which you can then use on Docker containers

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to let them talk to Java.

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All the hard work and heavy lifting

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is taken care of by Docker here.

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Now, with Docker network LS,

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you can inspect all existing networks

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and you see your own network here,

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as well as a couple of built in default networks,

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which you can ignore here,

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and with that created, with that network existing,

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you can try this Docker run MongoDB command again

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with this --network flag here

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using that network, favorites net, which we just set up,

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and if you now hit enter, this container starts

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and it stays up and running,

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but it's now part of this favorites net network.

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And now any other container

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can also be part of that network,

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and if two containers or possibly more containers also

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are part of the same network, they can talk to Java.

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The only remaining question

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is how they can talk to Java.

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How should this note JS code look like

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to ensure that this request works?

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This IP address here

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might be the IP address of the other MongoDB container,

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but I mentioned that we don't want to hard code it in here.

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MongoDB should automatically resolve it instead,

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and the cool thing is

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if two containers are part of the same network,

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you can just put the other containers name in here.

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So, in my case, MongoDB,

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because my other container is named MongoDB here.

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I picked this name when I started the MongoDB container,

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so, you can then use the name here.

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If you picked a different name, use that different name,

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if you didn't pick a name, use the auto-generated name

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and plug it in here as a domain.

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And just as with host.docker internal,

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which was translated to your local host IP address

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as seen from inside this Docker container,

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the other Docker container's name

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will also be translated by Docker

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to the IP address of that container,

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and this automatic transformation will work

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if both containers are part of the same network.

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So, save this updated code

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and then build the image again because the code was updated

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and thereafter run the note application again

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with Docker run --name

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favorites -d --rm,

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and then exposing the port

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and now very important, when our a flag,

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a flag to put this container

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into the same network as our MongoDB container is part of.

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So, at --network here as well

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and then it was the favorites net.

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So, this same network we used for the MongoDB container,

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both containers have to be part of the same network

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in order to talk to Java with this simplification

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of just using the other container's name.

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If you now hit enter this, it starts

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and it stays up and running,

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which is always a good sign, proves that it works.

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If you inspect the logs

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of the favorites

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container,

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you should see something like this,

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which is actually a warning,

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which actually proves that it works

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because it's just this warning and no connection error.

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So, indeed the node application was able to connect

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to the MongoDB database running in another container,

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and we still see both containers up and running here.

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Now with that, if you again tried it with Postman,

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I can again, get my favorites

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and we see an empty response, an empty aray of favorites,

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but a success response, no error and that is what matters,

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and we can send a post request to slash favorites

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and the data does get stored

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until after we can also get it here.

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And this now works and proves that both containers

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again, are able to communicate,

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this time, however, with this simplification

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of using the built in network feature,

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and that's important.

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Two containers normally are not able to talk to Java,

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unless you create such a container network

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or you manually look up the container IP as you saw,

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but with such a container network,

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as you see here, it works

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and this is therefore how you typically ensure

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that multiple containers can talk to Java

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because this is a common requirement,

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it's not a niche scenario, indeed it is very common

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that two or more containers need to talk to Java

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and with networks

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set up the way you saw it here over the last lectures,

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this is a breeze,

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and by using the container names here in your code,

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it's then especially simple to do as you saw.

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And I also wanna point out something else.

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When we launch a container like the MongoDB container,

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which another container connects to,

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in our case, the favorites container

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will connect to the MongoDB container,

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then as you saw, you don't need to publish any port

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when running that to be connected to container.

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When I run the MongoDB container,

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I don't have the -p option here.

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The reason for that is that the -p option is only required

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if we plan on connecting to something in that container

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from our local host machine

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or from outside the container network.

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Now, in this case, we don't do that.

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The only thing which connects to this MongoDB container

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will be our favorites container,

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which will be part of the same Docker network,

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and in such cases,

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when you have a container to container connection,

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then you don't need to publish the port

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because internally in that container network,

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all the containers can freely communicate with each other

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and you don't need to expose any ports.

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That's really only needed if you wanna connect

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to a container from your local host machine

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or from outside that container in general.