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StampedeProtector
in package
implements KeyValueStore

Cache is usually used to reduce performing a complex operation. In case of a cache miss, that operation is executed & the result is stored.

A cache stampede happens when there are a lot of requests for data that is not currently in cache. Examples could be:

  • cache expires for something that is often under very heavy load
  • sudden unexpected high load on something that is likely to not be in cache In those cases, this huge amount of requests for data that is not at that time in cache, causes that expensive operation to be executed a lot of times, all at once.

This class is designed to counteract that: if a value can't be found in cache we'll write something else to cache for a short period of time, to indicate that another process has already requested this same key (and is probably already performing that complex operation that will result in the key being filled)

All of the follow-up requests (that find that the "stampede indicator" has already been set) will just wait (usleep): instead of crippling the servers by all having to execute the same operation, these processes will just idle to give the first process the chance to fill in the cache. Periodically, these processes will poll the cache to see if the value has already been stored in the meantime.

The stampede protection will only be temporary, for $sla milliseconds. We need to limit it because the first process (tasked with filling the cache after executing the expensive operation) may fail/crash/... If the expensive operation fails to conclude in < $sla milliseconds. This class guarantees that the stampede will hold off for $sla amount of time but after that, all follow-up requests will go through without cached values and cause a stampede after all, if the initial process fails to complete within that time.

Tags
author

Matthias Mullie [email protected]

copyright

Copyright (c) 2014, Matthias Mullie. All rights reserved

license

LICENSE MIT

Table of Contents

Interfaces

KeyValueStore
Interface for key-value storage engines.

Methods

__construct()  : mixed
add()  : bool
Adds an item under new key.
cas()  : bool
Replaces an item in 1 atomic operation, to ensure it didn't change since it was originally read, when the CAS token was issued.
decrement()  : int|bool
Decrements a counter value, or sets an initial value if it does not yet exist.
delete()  : bool
Deletes an item from the cache.
deleteMulti()  : array<string|int, bool>
Deletes multiple items at once (reduced network traffic compared to individual operations).
flush()  : bool
Clears the entire cache (or the everything for the given collection).
get()  : mixed|bool
Retrieves an item from the cache.
getCollection()  : KeyValueStore
Returns an isolated subset (collection) in which to store or fetch data from.
getMulti()  : array<string|int, mixed>
Retrieves multiple items at once.
increment()  : int|bool
Increments a counter value, or sets an initial value if it does not yet exist.
replace()  : bool
Replaces an item.
set()  : bool
Stores a value, regardless of whether or not the key already exists (in which case it will overwrite the existing value for that key).
setMulti()  : array<string|int, bool>
Store multiple values at once.
touch()  : bool
Updates an item's expiration time without altering the stored value.

Methods

__construct()

public __construct(KeyValueStore $cache[, int $sla = 1000 ]) : mixed
Parameters
$cache : KeyValueStore

The real cache we'll buffer for

$sla : int = 1000

Stampede protection time, in milliseconds

add()

Adds an item under new key.

public add(mixed $key, mixed $value[, mixed $expire = 0 ]) : bool
Parameters
$key : mixed
$value : mixed
$expire : mixed = 0

Time when item falls out of the cache: 0 = permanent (doesn't expires); under 2592000 (30 days) = relative time, in seconds from now; over 2592000 = absolute time, unix timestamp

Return values
bool

cas()

Replaces an item in 1 atomic operation, to ensure it didn't change since it was originally read, when the CAS token was issued.

public cas(mixed $token, mixed $key, mixed $value[, mixed $expire = 0 ]) : bool
Parameters
$token : mixed

Token received from get() or getMulti()

$key : mixed
$value : mixed
$expire : mixed = 0

Time when item falls out of the cache: 0 = permanent (doesn't expires); under 2592000 (30 days) = relative time, in seconds from now; over 2592000 = absolute time, unix timestamp

Return values
bool

decrement()

Decrements a counter value, or sets an initial value if it does not yet exist.

public decrement(mixed $key[, mixed $offset = 1 ][, mixed $initial = 0 ][, mixed $expire = 0 ]) : int|bool
Parameters
$key : mixed
$offset : mixed = 1

Value to decrement with

$initial : mixed = 0

Initial value (if item doesn't yet exist)

$expire : mixed = 0

Time when item falls out of the cache: 0 = permanent (doesn't expires); under 2592000 (30 days) = relative time, in seconds from now; over 2592000 = absolute time, unix timestamp

Return values
int|bool

New value or false on failure

delete()

Deletes an item from the cache.

public delete(mixed $key) : bool
Parameters
$key : mixed
Return values
bool

deleteMulti()

Deletes multiple items at once (reduced network traffic compared to individual operations).

public deleteMulti(array<string|int, mixed> $keys) : array<string|int, bool>
Parameters
$keys : array<string|int, mixed>
Return values
array<string|int, bool>

flush()

Clears the entire cache (or the everything for the given collection).

public flush() : bool
Return values
bool

get()

Retrieves an item from the cache.

public get(mixed $key[, mixed &$token = null ]) : mixed|bool
Parameters
$key : mixed
$token : mixed = null

Will be filled with the CAS token

Return values
mixed|bool

Value, or false on failure

getCollection()

Returns an isolated subset (collection) in which to store or fetch data from.

public getCollection(mixed $name) : KeyValueStore
Parameters
$name : mixed
Return values
KeyValueStore

A new KeyValueStore instance representing only a subset of data on this server

getMulti()

Retrieves multiple items at once.

public getMulti(array<string|int, mixed> $keys[, array<string|int, mixed> &$tokens = null ]) : array<string|int, mixed>
Parameters
$keys : array<string|int, mixed>
$tokens : array<string|int, mixed> = null

Will be filled with the CAS tokens, in [key => token] format

Return values
array<string|int, mixed>

[key => value]

increment()

Increments a counter value, or sets an initial value if it does not yet exist.

public increment(mixed $key[, mixed $offset = 1 ][, mixed $initial = 0 ][, mixed $expire = 0 ]) : int|bool
Parameters
$key : mixed
$offset : mixed = 1

Value to increment with

$initial : mixed = 0

Initial value (if item doesn't yet exist)

$expire : mixed = 0

Time when item falls out of the cache: 0 = permanent (doesn't expires); under 2592000 (30 days) = relative time, in seconds from now; over 2592000 = absolute time, unix timestamp

Return values
int|bool

New value or false on failure

replace()

Replaces an item.

public replace(mixed $key, mixed $value[, mixed $expire = 0 ]) : bool
Parameters
$key : mixed
$value : mixed
$expire : mixed = 0

Time when item falls out of the cache: 0 = permanent (doesn't expires); under 2592000 (30 days) = relative time, in seconds from now; over 2592000 = absolute time, unix timestamp

Return values
bool

set()

Stores a value, regardless of whether or not the key already exists (in which case it will overwrite the existing value for that key).

public set(mixed $key, mixed $value[, mixed $expire = 0 ]) : bool
Parameters
$key : mixed
$value : mixed
$expire : mixed = 0

Time when item falls out of the cache: 0 = permanent (doesn't expires); under 2592000 (30 days) = relative time, in seconds from now; over 2592000 = absolute time, unix timestamp

Return values
bool

setMulti()

Store multiple values at once.

public setMulti(array<string|int, mixed> $items[, mixed $expire = 0 ]) : array<string|int, bool>
Parameters
$items : array<string|int, mixed>

[key => value]

$expire : mixed = 0

Time when item falls out of the cache: 0 = permanent (doesn't expires); under 2592000 (30 days) = relative time, in seconds from now; over 2592000 = absolute time, unix timestamp

Return values
array<string|int, bool>

touch()

Updates an item's expiration time without altering the stored value.

public touch(mixed $key, mixed $expire) : bool
Parameters
$key : mixed
$expire : mixed

Time when item falls out of the cache: 0 = permanent (doesn't expires); under 2592000 (30 days) = relative time, in seconds from now; over 2592000 = absolute time, unix timestamp

Return values
bool

        
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