GCP Secret Manager, Live-Reloaded in Go with mamori

Introduction

Google Cloud Secret Manager has a clean, opinionated model: a secret is a container, and inside it live immutable, numbered versions. You never edit a version — you add a new one (1, 2, 3, …) and, when it's ready, you move traffic to it, usually by resolving the latest alias (or by disabling the old version).

It's a genuinely good design. But it collides with how most Go services consume secrets: you call AccessSecretVersion("latest") exactly once, at startup, and cache the result. From that moment your process is pinned to whatever latest meant at boot. Add version 4, disable version 3, and your service happily keeps using 3 until someone restarts it — which, at best, is a scheduled redeploy and, at worst, an incident when the disabled version stops resolving.

mamori is a Go library I built to make config and secrets watchable. Point it at a GCP secret and it notices when latest moves, pulls the new version, re-validates your whole config, and swaps it in atomically — while your service keeps running.

This is the GCP-focused companion to the launch article, Introducing mamori: Typed, Watchable Config & Secrets for Go. That one covers the full model; here we focus on Secret Manager's version semantics.

The gcp-sm scheme

The providers/gcp module registers the gcp-sm scheme. The grammar is gcp-sm://<project>/<secret>[#json-key][?version=<v>]:

gcp-sm://my-project/db-password        # resolves the "latest" enabled version
gcp-sm://my-project/api-key?version=3  # pin to version 3, forever
gcp-sm://my-project/creds#password     # one key out of a JSON secret payload

That ?version= option is the important knob, and it maps directly onto GCP's version model:

• No ?version → mamori tracks latest. When you add a new version and it becomes latest, mamori reconciles it in. This is what you want for rotating credentials.
• ?version=N → mamori pins to an immutable version N. It will never move. This is what you want for config you want to promote deliberately and reproducibly (think: "this deploy runs config version 7, full stop").

Setup

go get github.com/xavidop/mamori
go get github.com/xavidop/mamori/providers/gcp

Register with a blank import; it uses Application Default Credentials, so on GKE it picks up Workload Identity automatically, and locally it uses your gcloud auth application-default login:

import _ "github.com/xavidop/mamori/providers/gcp"

The service account you bind needs roles/secretmanager.secretAccessor on the secrets it reads (and the ability to see version metadata to detect a new latest). Scope it to the specific secrets, not the whole project.

Load, then watch latest move

package main

import (
    "context"
    "log"
    "time"

    "github.com/xavidop/mamori"
    "github.com/xavidop/mamori/secret"

    _ "github.com/xavidop/mamori/providers/gcp"
)

type Config struct {
    // Tracks "latest": rotates in when you add a new enabled version.
    DBPassword secret.String `source:"gcp-sm://my-project/db-password"`

    // Pinned to an immutable version: deliberate, reproducible.
    StripeKey secret.String `source:"gcp-sm://my-project/stripe-key?version=7"`

    // One field out of a JSON secret.
    APIToken secret.String `source:"gcp-sm://my-project/creds#api_token"`
}

func main() {
    ctx := context.Background()

    w, err := mamori.Watch[Config](ctx,
        mamori.WithPollInterval(60*time.Second),
        mamori.OnChange(func(ev mamori.Change[Config]) {
            if ev.Changed("DBPassword") {
                pool.Rotate(ev.New.DBPassword.Reveal()) // live, no restart
                log.Println("db-password: new version reconciled")
            }
        }),
        mamori.OnError(func(err error) { metrics.Inc("config_error") }),
    )
    if err != nil {
        log.Fatal(err)
    }
    defer w.Close()

    cfg := w.Get() // last VALID config, lock-free
    _ = cfg
}

Every secret field here is a secret.String, which redacts itself in fmt, slog, and JSON — so nothing leaks into logs — and only reveals plaintext through an explicit .Reveal(). mamori's go vet analyzer will even fail your build if you store a gcp-sm:// value in a plain string.

What reconciliation actually does here

When you rotate — add version 4 to db-password and let it become latest — mamori's poll cycle notices that the resolved version changed, fetches it, and then:

1. Re-validates the entire struct. If the new secret somehow fails a validate rule, the update is rejected: Get() keeps returning the last good value and OnError gets a *ValidationError. A bad version can't take your service down.
2. Swaps atomically. w.Get() transitions from old to new in one lock-free store — no reader ever sees a partially-updated config.
3. Fires a diff-aware callback. OnChange tells you which fields moved (ev.Fields) with old/new versions, so you only rebuild what actually changed.

Because Secret Manager has no push channel for "latest moved", mamori polls — and it polls politely: the interval is jittered by ±20% so a large fleet doesn't stampede the API on the same tick, and change detection compares version metadata before pulling the payload, so an unchanged secret is cheap.

Structured secrets with flatten

Teams often store a whole connection blob as one JSON secret. Decode it straight into a nested struct:

type Config struct {
    DB DBConfig `source:"gcp-sm://my-project/db-conn" flatten:"json"`
}

type DBConfig struct {
    Host     string        `mapstructure:"host"`
    Port     int           `mapstructure:"port"`
    Password secret.String `mapstructure:"password"`
}

One AccessSecretVersion call, one JSON document, cleanly typed — with the password still redacting itself.

Version pinning as a deploy strategy

The ?version= toggle enables a genuinely nice workflow. Keep operational secrets on latest so rotations flow in automatically, but pin config that changes behavior to explicit versions so a rollout is deterministic and rollback is trivial:

type Config struct {
    DBPassword   secret.String `source:"gcp-sm://my-project/db-password"`             // latest: auto-rotate
    FeatureFlags []byte        `source:"gcp-sm://my-project/flags?version=12"`         // pinned: promote on purpose
}

Bumping ?version=12 to ?version=13 becomes a reviewable, revertible change in your code — while credentials keep rotating themselves in the background.

Wrapping up

GCP Secret Manager's immutable-versions model is great, but only if your app actually follows latest at runtime. mamori makes it do that: it watches the secret, reconciles a new version into your running Go process, validated and redacted, and lets you pin the fields you want frozen.

go get github.com/xavidop/mamori/providers/gcp

• Full introduction: Introducing mamori
• Docs: mamorigo.dev/docs
• Source: github.com/xavidop/mamori

mamori is MIT-licensed and at v0.1.0 — early and moving fast, so issues and feedback are very welcome. Happy coding!