Updating container apps



Scope

As a small feature request, we’ve been asked by the product team to change the “empty text” when we don’t have any todo list items. They would like to change it to the following: 

You have no todo items yet! Add one above!

Pretty simple, right? Let’s make the change.

Update the source code

In the src/static/js/app.js file, update line 56 to use the new empty text.

-                <p className="text-center">No items yet! Add one above!</p>
+                <p className="text-center">You have no todo items yet! Add one above!</p>

Let’s build our updated version of the image, using the same command we used before.

 $ sudo docker build -t getting-started .

Let’s start a new container using the updated code.

$ sudo docker run -dp 3000:3000 getting-started

Uh oh! You probably saw an error like this (the IDs will be different):

docker: Error response from daemon: driver failed programming external connectivity on endpoint laughing_burnell 
(bb242b2ca4d67eba76e79474fb36bb5125708ebdabd7f45c8eaf16caaabde9dd): Bind for 0.0.0.0:3000 failed: port is already allocated.

So, what happened? We aren’t able to start the new container because our old container is still running. It is because the container is using the host’s port 3000 and only one process on the machine (containers included) can listen to a specific port. To fix this, we need to remove the old container.

Replace the old container

To remove a container, it first needs to be stopped. Once it has stopped, it can be removed. We have two ways that we can remove the old container. Feel free to choose the path that you’re most comfortable with.

Remove a container using the CLI

1. Get the ID of the container by using the docker ps command.

$ sudo docker ps

2. Use the docker stop command to stop the container.

 # Swap out <the-container-id> with the ID from docker ps
 $ sudo docker stop <the-container-id>

3. Once the container has stopped, you can remove it by using the docker rm command.

$ sudo docker rm <the-container-id>

You can stop and remove a container in a single command by adding the “force” flag to the docker rm command. For example: docker rm -f <the-container-id>.

 

Star the updated container

Now start the updated container and check your browser (http://localhost:3000).

$ sudo docker run -dp 3000:3000 getting-started

Recap

While we were able to build an update, there were two things you might have noticed:

  • All of the existing items in our todo list are gone! That’s not a very good app! We’ll talk about that shortly.

  • There were a lot of steps involved for such a small change. In an upcoming section, we’ll talk about how to see code updates without needing to rebuild and start a new container every time we make a change.

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