In the last post, we created some unit tests that will allow us to automatically test our application. In this post, we will cover the process of Dockerizing our application in preparation for deploying with AWS Elastic Beanstalk.
What is Docker?
Explaining fully the features and advantages / disadvantages of Docker would take an entire blog post in itself, so if you aren't familiar with the concept of containers then watch the following video:
The easiest way to use Docker is with Docker Desktop (mac / windows). This will install both the
For Linux users, you have to first install Docker Engine, and then install Docker Compose separately.
Creating the Docker File
First, create an empty file in the root of your repo called
Dockerfile (no file extension). This file will contain a list of repeatable instructions that tells Docker how to build our container.
# select a starting image to build off FROM rust as build # set our working directory in the container as /repo WORKDIR /repo # copy all our files across from our local repo to the /repo directory in the container COPY Cargo.lock . COPY Cargo.toml . COPY src src # build the release RUN cargo install --path . # allow requests to port 8000 EXPOSE 8000 # this command is run when we actually start the container CMD ["aws-rust-api"]
Building and running the container
We can build the Docker image with the following command (make sure to run it from the root of the repo). This can take a while - we'll work on optimizing build times later.
docker build -t url-shortener .
-t url-shortener part tags our image so that we can run it in the next step, whilst the
. indicates the Dockerfile is in the current directory.
Note: Ensure that the Docker daemon is running, or the Docker commands may fail.
Once the container has been built, we can start it with the following command:
docker run -it --rm --name my-running-app -p 8000:8000 url-shortener
-p 8000:8000means that we are binding port 8000 of our local machine to port 8000 of the Docker container.
The application should appear to start as normal - however, you will notice that you cannot connect. This is because our program is listening on
127.0.0.1, but we need it to listen on all available network interfaces (
We can fix this using a
Rocket.toml configuration file to configure the address and port for Release mode.
[release] address = "0.0.0.0" port = 80
The file is read by the Rocket instance on program start up, as long as it is in the directory where the program is run. We can put it in there by adding the following line to the Dockerfile, just before the
COPY Rocket.toml .
You should also change the
EXPOSE line to expose port 80 (rather than 8000).
We can build the image and run it again, and you should be able to connect to it by going to http://127.0.0.1. Remember to bind to port
80 instead of
8000 in the
docker run command this time!
To simplify the process of creating and running the Docker images, we will create a
docker-compose.yml file in the root of the repo.
version: "3.9" services: url-shortener: build: . ports: - "80:80"
Starting our container is now as simple as running
docker-compose up, but make sure to append
--build if you want to force an image rebuild.
If you try and change a small part of your code, you may notice that a tiny change requires downloading and installing every dependency again when building the image. This is because of how Docker's cache works - if a copied file changes at all, then every step after that copy must be recalculated.
We can fix this by splitting our Docker file into two builds - the first build to install dependencies without copying our code, and the second to build the code without reinstalling dependencies.
# ... # copy all our files across from our local repo to the /repo directory in the container COPY Cargo.lock . COPY Cargo.toml . # cache dependencies by creating an empty # lib.rs file and building the project RUN mkdir src RUN echo "// empty file" > src/lib.rs RUN cargo build --release # now copy the code over COPY src src # build the release RUN cargo install --offline --path . # ...
Note: remember the
--offlineflag for the second build stage, or dependencies may be fetched again!
This means that we should only have to reinstall dependencies when either of our
Cargo.lock files change.
This step isn't as noticeable as the previous step, but it can help reduce the bloat of our image. We can do this by using two separate images - one purely for building, and the other for running our application.
# ... # build the release RUN cargo install --offline --path . # use a slim image for actually running the container. FROM rust:slim WORKDIR /app # allow requests to port 80 EXPOSE 80 # install the program onto the current image COPY --from=build /usr/local/cargo/bin/aws-rust-api /usr/local/bin/aws-rust-api # copy config file COPY Rocket.toml . # this command is run when we actually start the container CMD ["aws-rust-api"]
Congratulations! Your program is almost ready to be deployed on AWS Elastic Beanstalk. If you have any issues, you compare your code with the part-4 tag of my repo.
In the next post, we will create a statically-served frontend for our application with Svelte and Bulma. Make sure to click the "Follow" button if you want to be alerted when the next part is available!
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Disclaimer - I'm a (mostly) self-taught programmer, and I use my blog to share things that I've learnt on my journey to becoming a better developer. Because of this, I apologize in advance for any inaccuracies I might have made - criticism and corrections are welcome!
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