Tham khảo tài liệu 'practical mod_perl-chapter 18:mod_perl data-sharing techniques', công nghệ thông tin, kỹ thuật lập trình phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Page 550 Thursday November 18 2004 12 44 PM CHAPTER 18 mod_perl Data-Sharing Techniques In this chapter we discuss the ways mod_perl makes it possible to share data between processes or even between different handlers. Sharing the Read-Only Data in and Between Processes If you need to access some data in your code that s static and will not be modified you can save time and resources by processing the data once and caching it for later reuse. Since under mod_perl processes persist and don t get killed after each request you can store the data in global variables and reuse it. For example let s assume that you have a rather expensive function get_data which returns read-only data as a hash. In your code you can do the following use vars qw CACHE CACHE get_data unless CACHE my foo CACHE bar This code creates a global hash CACHE which is undefined when the code is executed for the first time. Therefore the get_data method is called which hopefully populates CACHE with some data. Now you can access this data as usual. When the code is executed for the second time within the same process the get_ data method will not be called again since CACHE has the data already assuming that get_data returned data when it was called for the first time . Now you can access the data without any extra retrieval overhead. If for example get_data returns a reference to a list the code will look like this use enum qw FIRST SECOND THIRD use vars qw RA_CACHE 550 Page 551 Thursday November 18 2004 12 44 PM RA_CACHE get_data unless RA_CACHE my second RA_CACHE- SECOND Here we use the enum pragma to create constants that we will use in accessing the array reference. In our example FIRST equals 0 SECOND equals 1 etc. We have used the RA_ prefix to indicate that this variable includes a reference to an array. So just like with the hash from the previous example we retrieve the data once per process cache it and then access it in all subsequent code re-executions . HTTP .
