Merge branch 'store-build-tags'

After the recent optimization work, I wanted to experiment with using
build tags to gate which store backends are included in a randomizer
binary. This _might_ be useful for my Cloud Run instance if I build a
Firestore-only version of the standard container image.

Author: featherbread

SERVERMORE.md

@@ -35,23 +35,34 @@ token (the newer signing secret configuration isn't supported):
 
 ## Storage Backends
 
+By default, the `randomizer-server` build supports all of the following storage
+backends, and permits you to select between them using environment variables.
+If you must optimize your build time or binary size, you can run `go build`
+with `-tags` listing the comma-separated build tags of the backends you wish to
+support (e.g. `go build -tags=randomizer.bbolt,randomizer.dynamodb …`).
+
 ### bbolt
 
+`-tags=randomizer.bbolt`
+
 [bbolt][bbolt] is the local key-value database engine behind systems like etcd,
 Consul, InfluxDB 2.x, and more.
 
 The bbolt backend's only prerequisite is persistent disk storage. Since a
 single running server locks the database file, this backend won't support high
 availability or zero-downtime deployment.
 
-The bbolt backend is active by default if no other backends have environment
-variables set. You can set `DB_PATH` to the desired location of the database on
-disk, otherwise it defaults to "randomizer.db" in the current directory.
+To activate the bbolt backend, set `DB_PATH` to the desired location of the
+database on disk. If you haven't used build tags to customize your server's
+supported backends, and no other store is configured, the server activates the
+bbolt backend by default as if `DB_PATH=randomizer.db` had been set.
 
 [bbolt]: https://go.etcd.io/bbolt
 
 ### DynamoDB
 
+`-tags=randomizer.dynamodb`
+
 DynamoDB is Amazon's fully managed NoSQL key-value store.
 
 The DynamoDB backend requires a pre-existing table with the randomizer schema.
@@ -67,6 +78,8 @@ in the code are unstable, and are subject to removal or behavior changes.)
 
 ### Google Cloud Firestore
 
+`-tags=randomizer.firestore`
+
 Firestore is Google's fully managed document database. This mode is especially
 useful to run `randomizer-server` on [Cloud Run][Cloud Run], with a level of
 operational ease comparable to the AWS Lambda solution (though the randomizer

internal/store/bbolt/factory.go

@@ -1,16 +1,22 @@
 package bbolt
 
 import (
+	"context"
 	"os"
 
 	bolt "go.etcd.io/bbolt"
 
 	"github.com/ahamlinman/randomizer/internal/randomizer"
+	"github.com/ahamlinman/randomizer/internal/store/registry"
 )
 
+func init() {
+	registry.Provide("bbolt", FactoryFromEnv, "DB_PATH")
+}
+
 // FactoryFromEnv returns a store.Factory whose stores are backed by a local
 // Bolt database (using the CoreOS "bbolt" fork).
-func FactoryFromEnv() (func(string) randomizer.Store, error) {
+func FactoryFromEnv(_ context.Context) (func(string) randomizer.Store, error) {
 	path := pathFromEnv()
 
 	db, err := bolt.Open(path, os.ModePerm&0644, nil)
@@ -31,6 +37,5 @@ func pathFromEnv() string {
 	if path := os.Getenv("DB_PATH"); path != "" {
 		return path
 	}
-
 	return "randomizer.db"
 }

internal/store/dynamodb/factory.go

@@ -9,8 +9,14 @@ import (
 
 	"github.com/ahamlinman/randomizer/internal/awsconfig"
 	"github.com/ahamlinman/randomizer/internal/randomizer"
+	"github.com/ahamlinman/randomizer/internal/store/registry"
 )
 
+func init() {
+	registry.Provide("dynamodb", FactoryFromEnv,
+		"DYNAMODB", "DYNAMODB_TABLE", "DYNAMODB_ENDPOINT")
+}
+
 // FactoryFromEnv returns a store.Factory whose stores are backed by Amazon
 // DynamoDB.
 //

internal/store/firestore/factory.go

@@ -8,11 +8,17 @@ import (
 	"cloud.google.com/go/firestore"
 
 	"github.com/ahamlinman/randomizer/internal/randomizer"
+	"github.com/ahamlinman/randomizer/internal/store/registry"
 )
 
+func init() {
+	registry.Provide("firestore", FactoryFromEnv,
+		"FIRESTORE_PROJECT_ID", "FIRESTORE_DATABASE_ID")
+}
+
 // FactoryFromEnv returns a store.Factory whose stores are backed by a Google
 // Cloud Firestore database.
-func FactoryFromEnv() (func(string) randomizer.Store, error) {
+func FactoryFromEnv(_ context.Context) (func(string) randomizer.Store, error) {
 	projectID, ok := os.LookupEnv("FIRESTORE_PROJECT_ID")
 	if !ok {
 		return nil, errors.New("missing FIRESTORE_PROJECT_ID in environment")

internal/store/registry/registry.go

@@ -0,0 +1,43 @@
+// Package registry facilitates environment-based store setup without requiring
+// every store to be linked into a randomizer binary.
+package registry
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/ahamlinman/randomizer/internal/randomizer"
+)
+
+// Registry provides environment keys and factory constructors for the store
+// backends linked into this binary.
+var Registry = map[string]Entry{}
+
+// Entry represents a single store backend.
+type Entry struct {
+	// EnvironmentKeys is the list of environment variables that this store's
+	// factory checks for configuration. If any one of these keys is set in the
+	// environment (and no conflicting keys are set), this store will be selected
+	// as the backend for this randomizer instance.
+	EnvironmentKeys []string
+
+	// FactoryFromEnv creates a factory for this backend based on its environment
+	// variables.
+	FactoryFromEnv func(context.Context) (func(partition string) randomizer.Store, error)
+}
+
+// Provide registers a new store backend, or panics if a backend is already
+// registered under this name.
+func Provide(
+	name string,
+	factoryFromEnv func(context.Context) (func(partition string) randomizer.Store, error),
+	environmentKeys ...string,
+) {
+	if _, ok := Registry[name]; ok {
+		panic(fmt.Errorf("%s already registered as a store backend", name))
+	}
+	Registry[name] = Entry{
+		EnvironmentKeys: environmentKeys,
+		FactoryFromEnv:  factoryFromEnv,
+	}
+}

internal/store/store.go

@@ -4,34 +4,75 @@ package store
 
 import (
 	"context"
+	"errors"
+	"fmt"
+	"maps"
 	"os"
+	"slices"
 
 	"github.com/ahamlinman/randomizer/internal/randomizer"
-	"github.com/ahamlinman/randomizer/internal/store/bbolt"
-	"github.com/ahamlinman/randomizer/internal/store/dynamodb"
-	"github.com/ahamlinman/randomizer/internal/store/firestore"
+	"github.com/ahamlinman/randomizer/internal/store/registry"
 )
 
+// haveAllStoreBackends indicates whether we can safely use the bbolt fallback
+// explained in the [FactoryFromEnv] comment. If we fall back to bbolt even
+// though we don't know the full set of possible environment keys (because we
+// used build tags to exclude some backends), we might activate it for a user
+// who meant to configure one of those missing stores instead.
+var haveAllStoreBackends bool
+
 // Factory represents a type for functions that produce a store for the
 // randomizer to use for a given "partition" (e.g. Slack channel). Factories
 // may panic if a non-empty partition is required and not given.
 //
 // Factory is provided for documentation purposes. Do not import the store
-// package just to use this alias; this will link support for all possible
+// package just to use this alias; this may link support for all possible
 // store backends into the final program, even if this was not intended.
 type Factory = func(partition string) randomizer.Store
 
-// FactoryFromEnv constructs and returns a [Factory] based on available
-// environment variables. If a known DynamoDB environment variable is set, it
-// will return a DynamoDB store. Otherwise, it will return a bbolt store.
-func FactoryFromEnv(ctx context.Context) (func(string) randomizer.Store, error) {
-	if envHasAny("DYNAMODB", "DYNAMODB_TABLE", "DYNAMODB_ENDPOINT") {
-		return dynamodb.FactoryFromEnv(ctx)
+// FactoryFromEnv constructs and returns a [Factory] based on runtime
+// environment variables, using one of the store backends included in the
+// binary based on Go build tags.
+//
+// Each store backend defines a set of environment variables for configuration.
+// On startup, the randomizer selects one store backend based on the presence
+// of its environment variables. It may fail if the environment has conflicting
+// or missing store configurations, or use a default bbolt configuration if no
+// build tags have been used to restrict the backends available in this binary.
+func FactoryFromEnv(ctx context.Context) (Factory, error) {
+	if len(registry.Registry) == 0 {
+		return nil, errors.New("no store backends available in this build")
+	}
+
+	candidates := make(map[string]struct{})
+	for name, entry := range registry.Registry {
+		if envHasAny(entry.EnvironmentKeys...) {
+			candidates[name] = struct{}{}
+		}
+	}
+
+	var chosen string
+	if len(candidates) == 0 && haveAllStoreBackends {
+		chosen = "bbolt"
+	}
+	if len(candidates) == 1 {
+		for name := range candidates {
+			chosen = name
+		}
+	}
+
+	if chosen == "" && len(candidates) == 0 {
+		available := slices.Sorted(maps.Keys(registry.Registry))
+		return nil, fmt.Errorf(
+			"can't find environment settings to select between store backends: %v", available)
 	}
-	if envHasAny("FIRESTORE_PROJECT_ID", "FIRESTORE_DATABASE_ID") {
-		return firestore.FactoryFromEnv()
+	if chosen == "" {
+		options := slices.Sorted(maps.Keys(candidates))
+		return nil, fmt.Errorf(
+			"environment settings match multiple store backends: %v", options)
 	}
-	return bbolt.FactoryFromEnv()
+
+	return registry.Registry[chosen].FactoryFromEnv(ctx)
 }
 
 func envHasAny(names ...string) bool {

internal/store/store_all.go

@@ -0,0 +1,13 @@
+//go:build !randomizer.bbolt && !randomizer.dynamodb && !randomizer.firestore
+
+package store
+
+import (
+	_ "github.com/ahamlinman/randomizer/internal/store/bbolt"
+	_ "github.com/ahamlinman/randomizer/internal/store/dynamodb"
+	_ "github.com/ahamlinman/randomizer/internal/store/firestore"
+)
+
+func init() {
+	haveAllStoreBackends = true
+}

internal/store/store_bbolt.go

@@ -0,0 +1,5 @@
+//go:build randomizer.bbolt
+
+package store
+
+import _ "github.com/ahamlinman/randomizer/internal/store/bbolt"

internal/store/store_dynamodb.go

@@ -0,0 +1,5 @@
+//go:build randomizer.dynamodb
+
+package store
+
+import _ "github.com/ahamlinman/randomizer/internal/store/dynamodb"

internal/store/store_firestore.go

@@ -0,0 +1,5 @@
+//go:build randomizer.firestore
+
+package store
+
+import _ "github.com/ahamlinman/randomizer/internal/store/firestore"