PHP session過期機制和配置/session回收

Posted by feng on 2010/05/11 Leave a comment (0)Go to comments

使用PHP session時會遇到明明超過了session過期時間，但session依然完好無損的活著，讓人頭大。其實仔細(xì)看一下php.ini關(guān)于PHP session回收機制就一目了然了。

session 回收機制：

PHP采用Garbage Collection process對過期session進(jìn)行回收，然而并不是每次session建立時，都能夠喚起 ‘garbage collection’ process ，gc是按照一定概率啟動的。這主要是出于對服務(wù)器性能方面的考慮，每個session都觸發(fā)gc，瀏覽量大的話，服務(wù)器吃不消，然而按照一定概率開啟 gc，當(dāng)流覽量大的時候，session過期機制能夠正常運行，而且服務(wù)器效率得到節(jié)省。細(xì)節(jié)應(yīng)該都是多年的經(jīng)驗積累得出的。

三個與PHP session過期相關(guān)的參數(shù)(php.ini中)：

session.gc_probability = 1

session.gc_divisor = 1000

session.gc_maxlifetime = 1440

gc啟動概率 = gc_probability / gc_divisor = 0.1%

session過期時間 gc_maxlifetime 單位：秒

建議：

自己測試時，當(dāng)然把這個session過期概率設(shè)置越大越好，過期效果才明顯。

當(dāng)web服務(wù)正式提供時，session過期概率就需要根據(jù)web服務(wù)的瀏覽量和服務(wù)器的性能來綜合考慮session過期概率。為每個 session都開啟gc，顯然是很不明智的。

變通方法：

手動在代碼中為session設(shè)定上次訪問時間，并且每次訪問的時候判斷訪問間隔是否超過時間限制，超過手動銷毀session，未超過則更新上次 訪問時間。這種方法還可以限制兩次訪問間隔。

配置文件摘錄（來自php.ini），英文的原汁兒原味兒

; Defines the probability that the ‘garbage collection’ process is started

; on every session initialization. The probability is calculated by using

; gc_probability/gc_divisor. Where session.gc_probability is the numerator

; and gc_divisor is the denominator in the equation. Setting this value to 1

; when the session.gc_divisor value is 100 will give you approximately a 1% chance

; the gc will run on any give request.

; Default Value: 1

; Development Value: 1

; Production Value: 1

; http://php.net/session.gc-probability

session.gc_probability = 1

; Defines the probability that the ‘garbage collection’ process is started on every

; session initialization. The probability is calculated by using the following equation:

; gc_probability/gc_divisor. Where session.gc_probability is the numerator and

; session.gc_divisor is the denominator in the equation. Setting this value to 1

; when the session.gc_divisor value is 100 will give you approximately a 1% chance

; the gc will run on any give request. Increasing this value to 1000 will give you

; a 0.1% chance the gc will run on any give request. For high volume production servers,

; this is a more efficient approach.

; Default Value: 100

; Development Value: 1000

; Production Value: 1000

; http://php.net/session.gc-divisor

session.gc_divisor = 1000

; After this number of seconds, stored data will be seen as ‘garbage’ and

; cleaned up by the garbage collection process.

; http://php.net/session.gc-maxlifetime

session.gc_maxlifetime = 1440

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