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;;; data.cache - Implementation of various cache strategies
;;;
;;; Copyright (c) 2015-2016 Shiro Kawai <shiro@acm.org>
;;;
;;; Redistribution and use in source and binary forms, with or without
;;; modification, are permitted provided that the following conditions
;;; are met:
;;;
;;; 1. Redistributions of source code must retain the above copyright
;;; notice, this list of conditions and the following disclaimer.
;;;
;;; 2. Redistributions in binary form must reproduce the above copyright
;;; notice, this list of conditions and the following disclaimer in the
;;; documentation and/or other materials provided with the distribution.
;;;
;;; 3. Neither the name of the authors nor the names of its contributors
;;; may be used to endorse or promote products derived from this
;;; software without specific prior written permission.
;;;
;;; THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
;;; "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
;;; LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
;;; A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
;;; OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
;;; SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED
;;; TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
;;; PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
;;; LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
;;; NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
;;; SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
;;;
;; This class takes care of various cache algorithms.
;; The cache works like a dictionary that you can associate value
;; to a key. The entry may disappear silently, when it is expired
;; according to the cache policy. This module is inspired by Clojure's
;; core.cache, but note that the API spec is different from Clojure's.
;;
;; User-side API
;;
;; (make-***-cache [args ...] :key storage comparator ...) procedure
;; Arguments varies depending on the alrogithm.
;;
;; (cache-lookup! cache key [default]) => value procedure
;; (cache-through! cache key value-fn) => value procedure
;; (cache-evict! cache key) => void method
;; (cache-clear! cache) => void method
;; (cache-write! cache key value) => void method
;;
;; (cache-storage cache) => dictionary procedure
;; (cache-compartor cache) => comparator procedure
;;
;; Implementor-side API
;;
;; An implementation of cache algorithm must provide at least the
;; following two methods:
;;
;; (cache-check! cache key) => Maybe (key . value)
;; Check if key has a value in cache. If not, returns #f.
;; If it does, returns a cons of the key and the value.
;;
;; (cache-register! cache key value) => (key . value)
;; Called when cache doesn't have an entry for the key, in order
;; to insert the value with the key.
;; This inserts the key-value pair into the cache, and returns
;; cons of them.
;;
;; The initialize method must take care of set up the internal state.
;; It should also handle the case when the existing dictionary is given.
;;
;; The default method of cache-evict! and cache-clear! only takes
;; care of the storage. If the algorithm needs to modify other internal
;; states, it should override those methods.
;;
;; The default method of cache-write! is evict! + register!. In most
;; cases there exists a better way.
(define-module data.cache
(use gauche.collection)
(use gauche.dictionary)
(use data.queue)
(use data.heap)
(use srfi-114)
(export <cache>
;; Protocol
cache-storage cache-comparator
cache-lookup! cache-through!
cache-check! cache-register! cache-write!
cache-evict! cache-clear!
;; Some auxiliary procedures for implementors
cache-populate-queue! cache-compact-queue!
cache-renumber-entries!
;; Concrete implementations
make-fifo-cache
make-ttl-cache
make-ttlr-cache
make-lru-cache
make-counting-cache cache-stats))
(select-module data.cache)
;; storage and comparator
(define-class <cache> ()
(;; private. must be treated read-only.
[storage :init-keyword :storage :init-value #f]
[comparator :init-keyword :comparator :init-value #f]
))
(define-inline (cache-storage cache) (slot-ref cache 'storage))
(define-inline (cache-comparator cache) (slot-ref cache 'comparator))
(define-method initialize ((c <cache>) initargs)
(next-method)
(if (cache-storage c)
(unless (cache-comparator c)
(slot-set! c 'comparator (dict-comparator (cache-storage c))))
(begin
(unless (cache-comparator c)
(slot-set! c 'comparator default-comparator))
(slot-set! c 'storage (make-hash-table (cache-comparator c))))))
;;;
;;; Cache external API
;;;
(define (cache-lookup! cache key . maybe-default)
(if-let1 kv (cache-check! cache key)
(cdr kv)
(if (pair? maybe-default)
(car maybe-default)
(errorf "cache ~s doesn't have an entry for key ~s" cache key))))
(define (cache-through! cache key value-fn)
(cdr (or (cache-check! cache key)
(cache-register! cache key (value-fn key)))))
(define-method cache-evict! ((cache <cache>) key)
(dict-delete! (cache-storage cache) key))
(define-method cache-clear! ((cache <cache>))
(dict-clear! (cache-storage cache)))
(define-method cache-write! ((cache <cache>) key value)
(cache-evict! cache key)
(cache-register! cache key value))
;; Some common routines
;; Most of cache implementations uses a queue as an auxiliary struct.
;; They keep <key> -> (<n> . <value>) in the storage, and
;; pushd (<key> . <n>) into the queue, where <n> is a number and
;; monotonically increasing within the queue. There may be multiple
;; entries with the same <key> in the queue.
;; Although the meaning of <n> depends on the algorithm, some
;; operations can be common.
;; Queue must be an empty queue. Fills queue according to the contents of
;; storage. This is called from initialize, in order to set up a cache
;; with existing storage. Returns maximum <n>.
(define (cache-populate-queue! queue storage)
(let1 entries (dict->alist storage) ; entries :: [(key n . val)]
(fold (^[entry maxn]
(enqueue! queue (cons (car entry) (cadr entry))) ; (key . n)
(if (> (cadr entry) maxn)
(cadr entry)
maxn))
0
(sort entries < cadr))))
;; When the algorithm increments <n> monotonically, it can eventually fell
;; to bignum, but we don't want that. This routine renumbers entries,
;; while removing duplicate keys in the queue.
(define (cache-renumber-entries! queue storage)
(let1 seen (make-hash-table (dict-comparator storage))
(define cnt (- (size-of storage) 1))
(dolist [kn (sort (dequeue-all! queue) > cdr)]
(unless (hash-table-exists? seen (car kn))
(hash-table-put! seen (car kn) #t)
(queue-push! queue (cons (car kn) cnt))
(dict-update! storage (car kn) (^[nv] (cons cnt (cdr nv))))
(dec! cnt)))))
;; We allow duplicate keys in the queue, but we don't want the queue to
;; get too long (e.g. Repeatedly hitting the same key in LRU cache would
;; quickly pile up entries in the queue). So occasionally we want to
;; compact the queue, by removing the duplicate entries.
;; This routine is quite similar to cache-renumber-entries! but we don't
;; change <n>'s, so we don't need to mutate the storage.
(define (cache-compact-queue! queue storage)
(let1 seen (make-hash-table (dict-comparator storage))
(dolist [kn (sort (dequeue-all! queue) > cdr)]
(unless (hash-table-exists? seen (car kn))
(hash-table-put! seen (car kn) #t)
(queue-push! queue kn)))))
;;;
;;; Cache implementations
;;;
;; FIFO Cache
;; - To recover the order from dict, we keep (<n> . <value>) in the storage.
;; <n> being increasing order of nonnegative integers.
;; - The queue holds (<key> . <n>)
;; - We don't want <n> to become bignums in long-running process, so
;; when we see <n> gets too big, we renumber entries.
(define-class <fifo-cache> (<cache>)
([capacity :init-keyword :capacity]
;; private
[queue :init-form (make-queue)]
[counter :init-value 0]))
(define (make-fifo-cache capacity :key (storage #f) (comparator #f))
(make <fifo-cache> :storage storage :comparator comparator
:capacity capacity))
(define-method initialize ((c <fifo-cache>) initargs)
(next-method)
(set! (~ c'counter)
(+ 1 (cache-populate-queue! (~ c'queue) (cache-storage c)))))
(define-method cache-check! ((cache <fifo-cache>) key)
(and-let1 nv (dict-get (cache-storage cache) key #f)
(cons key (cdr nv))))
(define (%fifo-add! cache key value)
(let ([dict (cache-storage cache)]
[queue (~ cache'queue)])
(when (>= (size-of dict) (~ cache'capacity))
;; NB: The queue may have multiple entries for the key. We only
;; concern the newest entry, so we loop if the queue head is old.
;; The queue should never be empty during this loop, because size of
;; queue >= size of dict.
(let loop ([kn (dequeue! queue)])
(let1 nv (dict-get dict (car kn) #f)
(if (and (pair? nv) (= (cdr kn) (car nv)))
(dict-delete! dict (car kn))
(loop (dequeue! queue))))))
(dict-put! dict key (cons (~ cache'counter) value))
(%fifo-touch! cache queue key)))
(define (%fifo-touch! cache queue key)
(let ([n (~ cache'counter)]
[storage (cache-storage cache)])
(enqueue! queue (cons key n))
(cond [(= n (greatest-fixnum))
(cache-renumber-entries! queue storage)
(set! (~ cache'counter) (size-of storage))]
[(> (queue-length queue) (* 3 (size-of storage)))
(cache-compact-queue! queue storage)
(set! (~ cache'counter) (+ n 1))]
[else
(set! (~ cache'counter) (+ n 1))])))
(define-method cache-register! ((cache <fifo-cache>) key value)
(%fifo-add! cache key value)
(cons key value))
(define-method cache-write! ((cache <fifo-cache>) key value)
(%fifo-add! cache key value))
;; We don't provide cache-evict!. Leaving stale entry in the queue
;; doesn't do harm.
(define-method cache-clear! ((cache <fifo-cache>))
(dict-clear! (cache-storage cache))
(dequeue-all! (~ cache'queue))
(undefined))
;; LRU Cache
;; - LRU cache is actually a variation of FIFO cache, except that
;; we touch the entry on read operaion as well.
(define-class <lru-cache> (<fifo-cache>) ())
(define (make-lru-cache capacity :key (storage #f) (comparator #f))
(make <lru-cache> :storage storage :comparator comparator
:capacity capacity))
(define-method cache-check! ((cache <lru-cache>) key)
(and-let* ([nv (dict-get (cache-storage cache) key #f)]
[nv2 (cons (~ cache'counter) (cdr nv))])
(dict-put! (cache-storage cache) key nv2)
(%fifo-touch! cache (~ cache'queue) key)
(cons key (cdr nv))))
;; TTL Cache
;; - Timestamps is a heap with (<timestamp> . <key>). There can
;; be multiple entries with the same <key>.
;; - The storage holds (<timestamp> . <value>).
(define-class <ttl-cache> (<cache>)
([ttl :init-keyword :ttl] ; time to live, in seconds
[timestamper :init-keyword :timestamper] ; a thunk to return a timestamp
[timestamps :init-form (make-queue)])) ; (<key> . <timestamp>)
(define (make-ttl-cache ttl :key (storage #f) (comparator #f)
(timestamper sys-time))
(make <ttl-cache> :storage storage :comparator comparator
:ttl ttl :timestamper timestamper))
(define-method initialize ((c <ttl-cache>) initargs)
(next-method)
(cache-populate-queue! (~ c'timestamps) (cache-storage c)))
(define (%ttl-sweep! c)
(let ([cutoff (- ((~ c'timestamper)) (~ c'ttl))]
[ts (~ c'timestamps)]
[tab (cache-storage c)])
(let loop ()
(and-let* ([kt (queue-front ts #f)]
[ (< (cdr kt) cutoff) ])
(dequeue! ts)
;; The dict-get below may return #f, if there's more than one
;; entry of the key in the heap with the same timestamp.
(and-let* ([tv (dict-get tab (car kt) #f)]
[ (<= (car tv) (cdr kt)) ])
(dict-delete! tab (car kt)))
(loop)))))
(define (%ttl-timestamp c) ((~ c'timestamper)))
(define-method cache-check! ((c <ttl-cache>) key)
(%ttl-sweep! c)
(and-let1 tv (dict-get (cache-storage c) key #f)
(cons key (cdr tv))))
(define-method cache-register! ((c <ttl-cache>) key val)
(let1 t (%ttl-timestamp c)
(dict-put! (cache-storage c) key (cons t val))
(enqueue! (~ c'timestamps) (cons key t)))
(cons key val))
(define-method cache-clear! ((c <ttl-cache>))
(dict-clear! (cache-storage c))
(dequeue-all! (~ c'timestamps)))
(define-method cache-write! ((c <ttl-cache>) key val)
(if-let1 tv (dict-get (cache-storage c) key #f)
(let1 t (%ttl-timestamp c)
(set! (car tv) t)
(set! (cdr tv) val)
(enqueue! (~ c'timestamps) (cons key t)))
(cache-register! c key val))
(undefined))
;; TTLR Cache
;; - TTL with refreshing. The timestamp of the entry is updated every
;; time it is read. It's kind of combination of LRU + TTL.
;; - Most methods are inherited from TTL cache. The only difference
;; is cache-check!, in which we 'touch' the entry.
(define-class <ttlr-cache> (<ttl-cache>)
())
(define (make-ttlr-cache ttl :key (storage #f) (comparator #f)
(timestamper sys-time))
(make <ttlr-cache> :storage storage :comparator comparator
:ttl ttl :timestamper timestamper))
(define-method cache-check! ((c <ttlr-cache>) key)
(%ttl-sweep! c)
(and-let* ([tv (dict-get (cache-storage c) key #f)]
[t (%ttl-timestamp c)])
(set! (car tv) t)
(enqueue! (~ c'timestamps) (cons key t))
(when (> (queue-length (~ c'timestamps)) (* 3 (size-of (cache-storage c))))
(cache-compact-queue! (~ c'timestamps) (cache-storage c)))
(cons key (cdr tv))))
;; Counting cache
;; - This is a wrapper cache to count cache misses/hits
;; NB: counting cache's storage directly points to inner-cache's storage.
;; We assume that any modification on the storage is done via inner-cache's
;; method, so that it won't cause inconsistency.
(define-class <counting-cache> (<cache>)
([inner-cache :init-keyword :inner-cache]
[hits :init-value 0]
[misses :init-value 0]))
(define (make-counting-cache inner-cache)
(make <counting-cache>
:inner-cache inner-cache
:storage (cache-storage inner-cache)
:comparator (cache-comparator inner-cache)))
(define-method cache-check! ((cache <counting-cache>) key)
(rlet1 r (cache-check! (~ cache'inner-cache) key)
(if r
(inc! (~ cache'hits))
(inc! (~ cache'misses)))))
(define-method cache-register! ((cache <counting-cache>) key value)
(cache-register! (~ cache'inner-cache) key value))
(define-method cache-write! ((cache <counting-cache>) key value)
(cache-write! (~ cache'inner-cache) key value))
(define-method cache-evict! ((cache <counting-cache>) key)
(cache-evict! (~ cache'inner-cache) key))
(define-method cache-clear! ((cache <counting-cache>))
(cache-clear! (~ cache'inner-cache)))
(define-method cache-stats ((cache <counting-cache>))
`(:hits ,(~ cache'hits) :misses ,(~ cache'misses)))
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