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// Copyright 2016 The Upspin Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Package storage implements a low-level interface for storing blobs in
// stable storage such as a database.
package storage // import ""
import (
// Storage is a low-level storage interface for services to store their data
// permanently. Storage implementations must be safe for concurrent use.
type Storage interface {
// LinkBase returns the base URL from which any ref may be downloaded.
// If the backend does not offer direct links it returns
// upspin.ErrNotSupported.
LinkBase() (base string, err error)
// Download retrieves the bytes associated with a ref.
Download(ref string) ([]byte, error)
// Put stores the contents given as ref on the storage backend.
Put(ref string, contents []byte) error
// Delete permanently removes all storage space associated
// with a ref.
Delete(ref string) error
// StorageConstructor is a function that initializes and returns a Storage
// implementation with the given options.
type StorageConstructor func(*Opts) (Storage, error)
var registration = make(map[string]StorageConstructor)
// Opts holds configuration options for the storage backend.
// It is meant to be used by implementations of Storage.
type Opts struct {
Opts map[string]string // key-value pair
// DialOpts is a daisy-chaining mechanism for setting options to a backend during Dial.
type DialOpts func(*Opts) error
// Register registers a new Storage under a name. It is typically used in init functions.
func Register(name string, fn StorageConstructor) error {
const op errors.Op = "cloud/storage.Register"
if _, exists := registration[name]; exists {
return errors.E(op, errors.Exist)
registration[name] = fn
return nil
// WithOptions parses a string in the format "key1=value1,key2=value2,..." where keys and values
// are specific to each storage backend. Neither key nor value may contain the characters "," or "=".
// Use WithKeyValue repeatedly if these characters need to be used.
func WithOptions(options string) DialOpts {
const op errors.Op = "cloud/storage.WithOptions"
return func(o *Opts) error {
pairs := strings.Split(options, ",")
for _, p := range pairs {
kv := strings.SplitN(p, "=", 2)
if len(kv) != 2 {
return errors.E(op, errors.Invalid, errors.Errorf("error parsing option %s", kv))
o.Opts[kv[0]] = kv[1]
return nil
// WithKeyValue sets a key-value pair as option. If called multiple times with the same key, the last one wins.
func WithKeyValue(key, value string) DialOpts {
return func(o *Opts) error {
o.Opts[key] = value
return nil
// Dial dials the named storage backend using the dial options opts.
func Dial(name string, opts ...DialOpts) (Storage, error) {
const op errors.Op = "cloud/storage.Dial"
fn, found := registration[name]
if !found {
return nil, errors.E(op, errors.Invalid, errors.Errorf("unknown storage backend type %q", name))
dOpts := &Opts{Opts: make(map[string]string)}
var err error
for _, o := range opts {
if o != nil {
err = o(dOpts)
if err != nil {
return nil, err
return fn(dOpts)