How can I connect a Go application in a Docker container to a local MongoDB instance?

Dockerizing Go App with Local Mongo Connection

When working on Golang projects with Docker, integrating a local MongoDB database poses a connectivity challenge. If a Go project using MongoDB is run outside Docker, the database connection is successful. However, when the same project is run within a Docker container, the connection fails.

Understanding the Issue

Docker creates a virtual environment for containerized applications. This means that the container runs in a separate network space, distinct from the host system. Consequently, connecting to external resources, such as a locally running MongoDB instance, becomes problematic.

Solution: Utilizing host.docker.internal

Docker assigns a unique IP address to each container and provides a special hostname, "host.docker.internal." This hostname acts as a gateway to connect to the host system from within the container. Since MongoDB is running locally on the host system, using "host.docker.internal" as its hostname resolves the connectivity issue.

Modified Dockerfile

To incorporate this solution, modify the Dockerfile as follows:

FROM golang
WORKDIR $GOPATH/src/myapp
ADD . /go/src/myapp
RUN go get ./...
RUN go install myapp
ENV MONGO_HOST=host.docker.internal
CMD /go/bin/myapp --mongodbhost=$MONGO_HOST
EXPOSE 8080
EXPOSE 27017

Connection String

Within the Go application, update the MongoDB connection string to use "host.docker.internal" as the hostname. For example:

connectionString := fmt.Sprintf("mongodb://host.docker.internal:21017/database")
client, err := mongo.Connect(ctx, options.Client().ApplyURI(connectionString))

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