go mod — Optimization¶

Speed up dependency download/verification and keep the graph lean. Numbers are illustrative.

Exercise 1: Cache the module cache in CI¶

Before — every CI job re-downloads all modules from the network.

After:

- uses: actions/cache@v4
  with:
    path: ~/go/pkg/mod        # GOMODCACHE
    key: gomod-${{ hashFiles('**/go.sum') }}

Keyed on go.sum, the cache invalidates only when dependencies change.

Exercise 2: Layer the Dockerfile¶

Before — COPY . . before downloading modules, so any source change re-downloads everything.

After:

COPY go.mod go.sum ./
RUN go mod download           # cached unless go.mod/go.sum change
COPY . .
RUN go build ./...

Exercise 3: Use a private proxy¶

Before — GOPROXY=direct fetches each module from its VCS origin (slow, fragile, rate-limited).

After:

export GOPROXY=https://proxy.internal.example.com,direct

A proxy serves prebuilt module zips and survives upstream outages.

Exercise 4: Keep the graph pruned and lean¶

Before — an old go 1.16 directive loads the full (unpruned) module graph; go mod operations are slower on large dependency trees.

After — bump the go directive to enable pruning:

go mod tidy -go=1.21

Exercise 5: Drop unnecessary dependencies¶

Before — a heavy dependency pulled in for a tiny utility bloats the graph and build.

After — find and remove it:

go mod why -m github.com/heavy/dep    # find what pulls it in
# replace with stdlib or a lighter alternative, then:
go mod tidy

Exercise 6: readonly to avoid surprise graph work¶

Before — builds occasionally mutate go.mod/go.sum, causing re-resolution and inconsistent CI.

After:

export GOFLAGS=-mod=readonly

Dependency changes become deliberate, so routine builds never pay resolution cost.

Measurement checklist¶

• Cache GOMODCACHE in CI keyed on go.sum.
• Layer Dockerfiles so go mod download caches independently of source.
• Use a private GOPROXY for speed and resilience.
• Keep the go directive modern so the graph stays pruned.
• Remove unjustified heavy dependencies (go mod why).
• Use -mod=readonly so routine builds do no resolution work.