PHP implements multi-server SESSION sharing

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PHP realizes multi-server SESSION sharing

Why session sharing is required

Nowadays, slightly larger websites basically have several subdomains, such as www.feiniu.com, search.feiniu.com, member.feiniu.com, if these websites need to share user login information, they need to share sessions. Of course, the premise is that they have the same main domain.

PHP session principle

The client accesses the php page, executes session_start, and generates the session_id. Generally, we store the session_id on the cookie, and the session content is saved on the server. The client access is different. All pages will pass the session_id to the server, and obtain the session content through the session_id.

The process is like this, but different servers will generate different session_ids for the same client, so different servers cannot get the same session content. Moreover, PHP's default SESSION data is stored separately in the file system of this server.

So if we want to solve session sharing, we must solve two problems:

1. Multiple servers use the same session_id

这个比较容易解决，只要在php中设置存session_id的cookie域名为网站主域就可以了
打开PHP.ini， 设置session.cookie_domain = .feiniu.com, 
当然也可以在php代码当中设置ini_set("session.cookie_domain","feiniu.com");

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2. Multiple servers use the same session_id to access the same session content

要实现这点，就必须把session内容存储到让所有服务器都能访问到的地方，php的session内容是默认存储到本服务器的文件中的，一般的解决方案是存入数据库，memcache或者redis这种缓存服务器，当然用默认的文件存储方式也可以，用NFS统一存储。

如何修改session存储引擎，参考这篇文章：http://blog.csdn.net/yagas/article/details/7593415

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3. How to choose a storage engine

Default file storage: This method of session destruction relies on the PHP garbage collector, in the case of high concurrency or long destruction time , a large number of files are generated in the SESSION directory. Of course, you can set up a hierarchical directory to save SESSION files. This will cause two problems: first, it is slow to find files; second, the number of files that can be accommodated in each directory is limited, which may cause the new SESSION to fail to be stored.
Database storage: Storing the session in the database can prevent the session data from being deleted by the garbage collector, and can solidify and store the session data. However, using a database to synchronize sessions will increase the IO of the database and increase the burden on the database. Moreover, the database reading and writing speed is slow, which is not conducive to timely synchronization of sessions.
memcache storage:

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# Synchronizing sessions in this way will not increase the burden on the database, and The security is relatively high. Putting the session in the memory is much faster than reading from the file.
But memcache divides the memory into storage blocks of many specifications. Each block has a size. This method also determines that memcache cannot fully utilize the memory and will produce memory fragmentation. If Insufficient storage blocks may cause memory overflow

• Memcached will not bring any benefits to those applications that do not need to be "distributed", do not need to be shared, or are simply small enough to have only one server. On the contrary, it will slow down the system efficiency. Because network connections also require resources.

• redis storage: Compared with memcache, redis access is slightly slower. The benefits are:
  

• redis supports many data structures and can store arrays or objects, while memcache can only store strings

• In the case of restarting the session machine, all users of memcache must regain the session , and redis will not

• When a large number of users suddenly come in and generate a lot of data, which fills up the memory of the machine storing the session, memcache will go on strike and all keys will not expire. It will continuously overwrite the last written data, and redis will only slow down and will not affect the logic of the program

Related recommendations:

PHP implementation Method of NFS sharing for multi-server session sharing_PHP tutorial

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