4. Where tokens live

The service never receives a token. It receives a reference, scheme:name, and the scheme says which store to ask. This is why one running service can post for several accounts, and why you can move from one cloud to another without changing what the callers send.

Reference	Meaning	Read	Update
env:NAME	an environment variable of the service	yes	no
envfile:/path/.env#KEY	one KEY=value line of a file	yes	yes
gcp:projects/P/secrets/S	Google Secret Manager, latest version	yes	adds a version

env: and envfile: are always there. gcp: is there in the released binaries and in go install of cmd/instagram-publisher.

Updating matters for token refresh: the refreshed token is written back to the same reference, so a read-only store (env:) can post but cannot be refreshed by the service.

Why only some stores are in your binary

A Go program contains only what it imports. Each cloud store is its own package (secret/gcp), and the program that starts the service lists the stores it wants. A build without the Google package does not contain it.

This is cmd/instagram-publisher/main.go, in full where it matters:

func secrets() *secret.Resolver {
	r := secret.NewResolver() // env:, envfile:
	r.Register("gcp", gcp.New(nil))
	return r
}

Adding your own store

A store is two methods:

type Store interface {
	Get(ctx context.Context, rest string) (string, error)
	Set(ctx context.Context, rest, value string) error
}

rest is everything after the first colon. Return secret.ErrReadOnly from Set if the store cannot be updated. Then write your own small main:

package main

import (
	"net/http"

	"github.com/javadebadi/instagram-publisher/publisher"
	"github.com/javadebadi/instagram-publisher/secret"
)

func main() {
	r := secret.NewResolver()
	r.Register("aws", awsStore{}) // your type: Get and Set against AWS Secrets Manager
	http.ListenAndServe(":8080", publisher.New(publisher.Config{Secrets: r}))
}

publisher and secret are ordinary public packages, so this file can live in your own repository, and callers then send "token_ref": "aws:my/secret/name". Values are never put into error messages; keep that property in your store.