ccache

package module
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 Go to latest Published: Nov 21, 2014 License: MIT Imports: 5 Imported by: 0

README

CCache

CCache is an LRU Cache, written in Go, focused on supporting high concurrency.

Lock contention on the list is reduced by:

  • Introducing a window which limits the frequency that an item can get promoted
  • Using a buffered channel to queue promotions for a single worker
  • Garbage collecting within the same thread as the worker

Setup

First, download the project:

go get github.com/karlseguin/ccache

Configuration

Next, import and create a Cache instance:

import (
  "github.com/karlseguin/ccache"
)

var cache = ccache.New(ccache.Configure())

Configure exposes a chainable API:

var cache = ccache.New(ccache.Configure().MaxSize(1000).itemsToPrune(100))

The most likely configuration options to tweak are:

  • MaxSize(int) - the maximum number size to store in the cache (default: 5000)
  • GetsPerPromote(int) - the number of times an item is fetched before we promote it. For large caches with long TTLs, it normally isn't necessary to promote an item after every fetch (default: 3)
  • ItemsToPrune(int) - the number of items to prune when we hit MaxSize. Freeing up more than 1 slot at a time improved performance (default: 500)

Configurations that change the internals of the cache, which aren't as likely to need tweaking:

  • Buckets - ccache shards its internal map to provide a greater amount of concurrency. Must be a power of 2 (default: 16).
  • PromoteBuffer(int) - the size of the buffer to use to queue promotions (default: 1024)
  • DeleteBuffer(int) the size of the buffer to use to queue deletions (default: 1024)

Usage

Once the cache is setup, you can Get, Set and Delete items from it. A Get returns an *Item:

Get
item := cache.Get("user:4")
if item == nil {
  //handle
} else {
  user := item.Value().(*User)
}

The returned *Item exposes a number of methods:

  • Value() interface{} - the value cached
  • Expired() bool - whether the item is expired or not
  • TTL() time.Duration - the duration before the item expires (will be a negative value for expired items)
  • Expires() time.Time - the time the item will expire

By returning expired items, CCache lets you decide if you want to serve stale content or not. For example, you might decide to serve up slightly stale content (< 30 seconds old) while re-fetching newer data in the background. You might also decide to serve up infinitely stale content if you're unable to get new data from your source.

Set

Set expects the key, value and ttl:

cache.Set("user:4", user, time.Minute * 10)
Fetch

There's also a Fetch which mixes a Get and a Set:

item, err := cache.Fetch("user:4", time.Minute * 10, func() (interface{}, error) {
  //code to fetch the data incase of a miss
  //should return the data to cache and the error, if any
})
Delete

Delete expects the key to delete. It's ok to call Delete on a non-existant key:

cache.Delete("user:4")
Extend

The life of an item can be changed via the Extend method. This will change the expiry of the item by the specified duration relative to the current time.

Replace

The value of an item can be updated to a new value without renewing the item's TTL or it's position in the LRU:

cache.Replace("user:4", user)

Replace returns true if the item existed (and thus was replaced). In the case where the key was not in the cache, the value is not inserted and false is returned.

Tracking

CCache supports a special tracking mode which is meant to be used in conjunction with other pieces of your code that maintains a long-lived reference to data.

When you configure your cache with Track():

cache = ccache.New(ccache.Configure().Track())

The items retrieved via TrackingGet will not be eligible for purge until Release is called on them:

item := cache.TrackingGet("user:4")
user := item.Value()   //will be nil if "user:4" didn't exist in the cache
item.Release()  //can be called even if item.Value() returned nil

In practive, Release wouldn't be called until later, at some other place in your code.

There's a couple reason to use the tracking mode if other parts of your code also hold references to objects. First, if you're already going to hold a reference to these objects, there's really no reason not to have them in the cache - the memory is used up anyways.

More important, it helps ensure that you're code returns consistent data. With tracking, "user:4" might be purged, and a subsequent Fetch would reload the data. This can result in different versions of "user:4" being returned by different parts of your system.

LayeredCache

CCache's LayeredCache stores and retrieves values by both a primary and secondary key. Deletion can happen against either the primary and secondary key, or the primary key only (removing all values that share the same primary key).

LayeredCache is useful for HTTP caching, when you want to purge all variations of a request.

LayeredCache takes the same configuration object as the main cache, exposes the same optional tracking capabilities, but exposes a slightly different API:

cache := ccache.Layered(ccache.Configure())

cache.Set("/users/goku", "type:json", "{value_to_cache}", time.Minute * 5)
cache.Set("/users/goku", "type:xml", "<value_to_cache>", time.Minute * 5)

json := cache.Get("/users/goku", "type:json")
xml := cache.Get("/users/goku", "type:xml")

cache.Delete("/users/goku", "type:json")
cache.Delete("/users/goku", "type:xml")
// OR
cache.DeleteAll("/users/goku")

Size

By default, items added to a cache have a size of 1. This means that if you configure MaxSize(10000), you'll be able to store 10000 items in the cache.

However, if the values you set into the cache have a method Size() int64, this size will be used. Note that ccache has an overhead of ~350 bytes per entry, which isn't taken into account. In other words, given a filled up cache, with MaxSize(4096000) and items that return a Size() int64 of 2048, we can expect to find 2000 items (4096000/2048) taking a total space of 4796000 bytes.

Documentation

Overview

An LRU cached aimed at high concurrency

An LRU cached aimed at high concurrency

Index

Constants

This section is empty.

Variables

View Source 
var NilTracked = new(nilItem)

Functions

This section is empty.

Types

type Cache 

type Cache struct {
	*Configuration
	// contains filtered or unexported fields
}

func New 

func New(config *Configuration) *Cache

Create a new cache with the specified configuration See ccache.Configure() for creating a configuration

func (*Cache) Clear 

func (c *Cache) Clear()

this isn't thread safe. It's meant to be called from non-concurrent tests

func (*Cache) Delete 

func (c *Cache) Delete(key string) bool

Remove the item from the cache, return true if the item was present, false otherwise.

func (*Cache) Fetch 

func (c *Cache) Fetch(key string, duration time.Duration, fetch func() (interface{}, error)) (interface{}, error)

Attempts to get the value from the cache and calles fetch on a miss. If fetch returns an error, no value is cached and the error is returned back to the caller.

func (*Cache) Get 

func (c *Cache) Get(key string) *Item

Get an item from the cache. Returns nil if the item wasn't found. This can return an expired item. Use item.Expired() to see if the item is expired and item.TTL() to see how long until the item expires (which will be negative for an already expired item).

func (*Cache) Replace 

func (c *Cache) Replace(key string, value interface{}) bool

Replace the value if it exists, does not set if it doesn't. Returns true if the item existed an was replaced, false otherwise. Replace does not reset item's TTL nor does it alter its position in the LRU

func (*Cache) Set 

func (c *Cache) Set(key string, value interface{}, duration time.Duration)

Set the value in the cache for the specified duration

func (*Cache) TrackingGet 

func (c *Cache) TrackingGet(key string) TrackedItem

Used when the cache was created with the Track() configuration option. Avoid otherwise

type Configuration 

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

func Configure 

func Configure() *Configuration

Creates a configuration object with sensible defaults Use this as the start of the fluent configuration: e.g.: ccache.New(ccache.Configure().MaxSize(10000))

func (*Configuration) Buckets 

func (c *Configuration) Buckets(count uint32) *Configuration

Keys are hashed into % bucket count to provide greater concurrency (every set requires a write lock on the bucket). Must be a power of 2 (1, 2, 4, 8, 16, ...) [16]

func (*Configuration) DeleteBuffer 

func (c *Configuration) DeleteBuffer(size uint32) *Configuration

The size of the queue for items which should be deleted. If the queue fills up, calls to Delete() will block

func (*Configuration) GetsPerPromote 

func (c *Configuration) GetsPerPromote(count int32) *Configuration

Give a large cache with a high read / write ratio, it's usually unecessary to promote an item on every Get. GetsPerPromote specifies the number of Gets a key must have before being promoted [3]

func (*Configuration) ItemsToPrune 

func (c *Configuration) ItemsToPrune(count uint32) *Configuration

The number of items to prune when memory is low [500]

func (*Configuration) MaxSize 

func (c *Configuration) MaxSize(max int64) *Configuration

The max size for the cache [5000]

func (*Configuration) PromoteBuffer 

func (c *Configuration) PromoteBuffer(size uint32) *Configuration

The size of the queue for items which should be promoted. If the queue fills up, promotions are skipped [1024]

func (*Configuration) Track 

func (c *Configuration) Track() *Configuration

By turning tracking on and using the cache's TrackingGet, the cache won't evict items which you haven't called Release() on. It's a simple reference counter.

type Item 

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

func (*Item) Expired 

func (i *Item) Expired() bool

func (*Item) Expires 

func (i *Item) Expires() time.Time

func (*Item) Extend 

func (i *Item) Extend(duration time.Duration)

func (*Item) Release 

func (i *Item) Release()

func (*Item) TTL 

func (i *Item) TTL() time.Duration

func (*Item) Value 

func (i *Item) Value() interface{}

type LayeredCache 

type LayeredCache struct {
	*Configuration
	// contains filtered or unexported fields
}

func Layered 

func Layered(config *Configuration) *LayeredCache

See ccache.Configure() for creating a configuration

func (*LayeredCache) Clear 

func (c *LayeredCache) Clear()

this isn't thread safe. It's meant to be called from non-concurrent tests

func (*LayeredCache) Delete 

func (c *LayeredCache) Delete(primary, secondary string) bool

Remove the item from the cache, return true if the item was present, false otherwise.

func (*LayeredCache) DeleteAll 

func (c *LayeredCache) DeleteAll(primary string) bool

Deletes all items that share the same primary key

func (*LayeredCache) Fetch 

func (c *LayeredCache) Fetch(primary, secondary string, duration time.Duration, fetch func() (interface{}, error)) (interface{}, error)

Attempts to get the value from the cache and calles fetch on a miss. If fetch returns an error, no value is cached and the error is returned back to the caller.

func (*LayeredCache) Get 

func (c *LayeredCache) Get(primary, secondary string) *Item

Get an item from the cache. Returns nil if the item wasn't found. This can return an expired item. Use item.Expired() to see if the item is expired and item.TTL() to see how long until the item expires (which will be negative for an already expired item).

func (*LayeredCache) Replace 

func (c *LayeredCache) Replace(primary, secondary string, value interface{}) bool

Replace the value if it exists, does not set if it doesn't. Returns true if the item existed an was replaced, false otherwise. Replace does not reset item's TTL nor does it alter its position in the LRU

func (*LayeredCache) Set 

func (c *LayeredCache) Set(primary, secondary string, value interface{}, duration time.Duration)

Set the value in the cache for the specified duration

func (*LayeredCache) TrackingGet 

func (c *LayeredCache) TrackingGet(primary, secondary string) TrackedItem

Used when the cache was created with the Track() configuration option. Avoid otherwise

type Sized 

type Sized interface {
	Size() int64
}

type TrackedItem 

type TrackedItem interface {
	Value() interface{}
	Release()
	Expired() bool
	TTL() time.Duration
	Expires() time.Time
	Extend(duration time.Duration)
}

Source Files

View all Source files

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