How can I fix my docker build failing while my go build succeeds? Dockerfile includes go mod download

Question

I am using "github.com/confluentinc/confluent-kafka-go/kafka" in a very simple consumer. It's pretty much what confluent has as a kafka/go tutorial.

"go build ." and "go run ." succeed, "docker build ." does not.

The error is:

 > [builder 7/7] RUN go build -o /app/bin/main .:
#12 9.826 # gitlab.com/.....
#12 9.826 ./main.go:24:24: undefined: kafka.ConfigMap
#12 9.826 ./main.go:30:18: undefined: kafka.NewConsumer

Here is my Dockerfile:

FROM golang:1.20.2-alpine3.16 AS builder
RUN apk --update add git
WORKDIR /app
COPY go.mod go.sum /app/
RUN go mod download
COPY . .
RUN go build -o /app/bin/main .

FROM scratch
WORKDIR /app
COPY --from=builder /app/bin/main /app/bin/main
ENTRYPOINT ["/app/bin/main"]

I am at a loss as to why this happens, especially since it claims that some functions in the package are undefined. I have not redeclared "kafka" anywhere in my code.

Answer 1

As correctly pointed out by Zeke in the comments, Kafka binaries are included in the Kafka Go client, so check out the Update section where I demonstrate such scenario.

That's because you need the native (C/C++) librdkafka. dependencies in order to build it. That will imply in most cases having gcc available as well. That's the underlying Kakfa lib used by your code.

You also need to make go build aware of that. I've added and modified these two lines on your example and was able to build it.

# Add this
RUN apk add alpine-sdk 

# Change this
RUN GOOS=linux GOARCH=amd64 go build -tags musl -o /app/bin/main .

Functional example:

FROM golang:1.20.2-alpine3.16 AS builder
RUN apk add alpine-sdk 
RUN apk --update add git
WORKDIR /app
COPY . .
RUN go mod download

RUN GOOS=linux GOARCH=amd64 go build -tags musl -o /app/bin/main .

FROM scratch
WORKDIR /app
COPY --from=builder /app/bin/main /app/bin/main
ENTRYPOINT ["/app/bin/main"]

I've used this quickstart code and the build process was OK.

Update

As Zeke pointed out, Kafka already has the prebuilt binaries.

Breaking down the Dockerfile below:

• gcc and musl-dev are required for compilation using the selected image.
• go build -tags musl option for Alpine, as per the Kafka SDK docs.
• -ldflags '-extldflags "-static"' will statically compile everything in a single binary, including the dependencies. This will include for example C std lib, and will keep the scratch image clean. If dynamic is chosen, then one has to add the dependencies to the scratch image section directly.

The Dockerfile which generated the image with ~10MB size:

FROM golang:1.20.4-alpine3.17 AS builder
RUN apk add --no-progress --no-cache gcc musl-dev
WORKDIR /build
COPY . .
RUN go mod download

RUN go build -tags musl -ldflags '-extldflags "-static"' -o /build/main

FROM scratch
WORKDIR /app
COPY --from=builder /build/main .
ENTRYPOINT ["/app/main"]

Integration with Kafka using test code was successful when running in my demo.