settings

Documentation

Overview

Package settings implements storage for infrequently changing global settings.

Settings are represented as (key, value) pairs, where value is JSON serializable struct. Settings are cached internally in the process memory to avoid hitting the storage all the time.

DEPRECATED. Available only on GAEv1. Use command line flags instead.

Index

• Variables
• func Get(c context.Context, key string, value any) error
• func GetUncached(c context.Context, key string, value any) error
• func Set(c context.Context, key string, value any) error
• func SetIfChanged(c context.Context, key string, value any) error
• func Use(c context.Context, s *Settings) context.Context
• type Bundle
• type EventualConsistentStorage
• type ExternalStorage
  • func (s *ExternalStorage) FetchAllSettings(context.Context) (*Bundle, time.Duration, error)
  • func (s *ExternalStorage) Load(c context.Context, r io.Reader) error
• type MemoryStorage
  • func (m *MemoryStorage) FetchAllSettings(c context.Context) (*Bundle, time.Duration, error)
  • func (m *MemoryStorage) UpdateSetting(c context.Context, key string, value json.RawMessage) error
• type MutableStorage
• type Settings
  • func GetSettings(c context.Context) *Settings
  • func New(storage Storage) *Settings
  • func (s *Settings) Get(c context.Context, key string, value any) error
  • func (s *Settings) GetStorage() Storage
  • func (s *Settings) GetUncached(c context.Context, key string, value any) error
  • func (s *Settings) IsMutable() bool
  • func (s *Settings) Set(c context.Context, key string, value any) error
  • func (s *Settings) SetIfChanged(c context.Context, key string, value any) error
• type Storage

Variables

var (
	// ErrNoSettings can be returned by Get and Set on fatal errors.
	ErrNoSettings = errors.New("settings: settings are not available")

	// ErrBadType is returned if Get(...) receives unexpected type.
	ErrBadType = errors.New("settings: bad type")

	// ErrReadOnly is returned by Set(...) if the storage is read only.
	ErrReadOnly = errors.New("settings: the storage is read only")
)

Functions

func Get

func Get(c context.Context, key string, value any) error

Get returns setting value (possibly cached) for the given key.

It will be deserialized into the supplied value. Caller is responsible to pass correct type here. If the setting is not set or the context doesn't have settings implementation in it, returns ErrNoSettings.

func GetUncached

func GetUncached(c context.Context, key string, value any) error

GetUncached is like Get, by always fetches settings from the storage.

Do not use GetUncached in performance critical parts, it is much heavier than Get.

func Set

func Set(c context.Context, key string, value any) error

Set overwrites a setting value for the given key.

New settings will apply only when existing in-memory cache expires. In particular, Get() right after Set() may still return old value.

Returns ErrNoSettings if context doesn't have Settings implementation.

func SetIfChanged

func SetIfChanged(c context.Context, key string, value any) error

SetIfChanged is like Set, but fetches an existing value and compares it to a new one before changing it.

Avoids generating new revisions of settings if no changes are actually made. Also logs who is making the change.

Returns ErrNoSettings if context doesn't have Settings implementation.

func Use

func Use(c context.Context, s *Settings) context.Context

Use injects Settings into the context to be used by Get and Set.

Types

type Bundle

type Bundle struct {
	Values map[string]*json.RawMessage // immutable
	// contains filtered or unexported fields
}

Bundle contains all latest settings.

type EventualConsistentStorage

type EventualConsistentStorage interface {
	MutableStorage

	// GetConsistencyTime returns "last modification time" + "expiration period".
	//
	// It indicates moment in time when last setting change is fully propagated to
	// all instances.
	//
	// Returns zero time if there are no settings stored.
	GetConsistencyTime(c context.Context) (time.Time, error)
}

EventualConsistentStorage is MutableStorage where settings changes take effect not immediately but by some predefined moment in the future.

type ExternalStorage

type ExternalStorage struct {
	// contains filtered or unexported fields
}

ExternalStorage implements Storage interface using an externally supplied io.Reader with JSON.

This is read-only storage (it doesn't implement MutableStorage interface), meaning settings it stores can't be change via UpdateSetting (or admin UI).

They still can be dynamically reloaded via Load() though.

func (*ExternalStorage) FetchAllSettings

func (s *ExternalStorage) FetchAllSettings(context.Context) (*Bundle, time.Duration, error)

FetchAllSettings is part of Storage interface, it returns a bundle with all latest settings and its expiration time.

The bundle has expiration time of 10 sec, so that Settings{...} implementation doesn't go through slower cache-miss code path all the time a setting is read (which happens multiple times per request). In practice it means changes loaded via Load() apply at most 10 sec later.

func (*ExternalStorage) Load

func (s *ExternalStorage) Load(c context.Context, r io.Reader) error

Load loads (or reloads) settings from a given reader.

The reader should supply a JSON document with a dict. Keys are strings matching various settings pages, and values are corresponding settings (usually also dicts). After settings are loaded (and the context is properly configured), calling settings.Get(c, <key>, &output) deserializes the value supplied for the corresponding key into 'output'.

On success logs what has changed (if anything) and eventually starts serving new settings (see comments for FetchAllSettings regarding the caching).

On failure returns an error without any logging or without changing what is currently being served.

type MemoryStorage

type MemoryStorage struct {
	Expiration time.Duration // default expiration time of in-memory cache
	// contains filtered or unexported fields
}

MemoryStorage implements Storage interface, using memory as a backend. Useful in unit tests.

func (*MemoryStorage) FetchAllSettings

func (m *MemoryStorage) FetchAllSettings(c context.Context) (*Bundle, time.Duration, error)

FetchAllSettings fetches all latest settings at once.

func (*MemoryStorage) UpdateSetting

func (m *MemoryStorage) UpdateSetting(c context.Context, key string, value json.RawMessage) error

UpdateSetting updates a setting at the given key.

type MutableStorage

type MutableStorage interface {
	Storage

	// UpdateSetting updates a setting at the given key.
	UpdateSetting(c context.Context, key string, value json.RawMessage) error
}

MutableStorage knows how to fetch settings from permanent storage and mutate them there.

type Settings

type Settings struct {
	// contains filtered or unexported fields
}

Settings represent process global cache of all settings. Exact same instance of Settings should be injected into the context used by request handlers.

func GetSettings

func GetSettings(c context.Context) *Settings

GetSettings grabs Settings from the context if it's there.

func New

func New(storage Storage) *Settings

New creates new Settings object that uses given Storage to fetch and save settings.

func (*Settings) Get

func (s *Settings) Get(c context.Context, key string, value any) error

Get returns setting value (possibly cached) for the given key.

It will be deserialized into the supplied value. Caller is responsible to pass correct type and pass same type to all calls. If the setting is not set returns ErrNoSettings.

func (*Settings) GetStorage

func (s *Settings) GetStorage() Storage

GetStorage returns underlying Storage instance.

func (*Settings) GetUncached

func (s *Settings) GetUncached(c context.Context, key string, value any) error

GetUncached is like Get, by always fetches settings from the storage.

Do not use GetUncached in performance critical parts, it is much heavier than Get.

func (*Settings) IsMutable

func (s *Settings) IsMutable() bool

IsMutable returns true if the storage supports MutableStorage interface.

func (*Settings) Set

func (s *Settings) Set(c context.Context, key string, value any) error

Set changes a setting value for the given key.

New settings will apply only when existing in-memory cache expires. In particular, Get() right after Set() may still return old value.

func (*Settings) SetIfChanged

func (s *Settings) SetIfChanged(c context.Context, key string, value any) error

SetIfChanged is like Set, but fetches an existing value and compares it to a new one before changing it.

Avoids generating new revisions of settings if no changes are actually made. Also logs who is making the change.

type Storage

type Storage interface {
	// FetchAllSettings fetches all latest settings at once.
	//
	// Returns the settings and the duration they should be cached for by default.
	FetchAllSettings(c context.Context) (*Bundle, time.Duration, error)
}

Storage knows how to fetch settings.

May also optionally implement MutableStorage if it supports mutating settings. Otherwise the settings are assumed to be updated via some external mechanism.

Methods of Storage can be called concurrently.