Communication between processes in PHP

Written at February 13, 2001 - 9:02 | by storm | 2,671 readings

PHP has several features that allow different processes communicate with each PHP but most programmers know their use. The paper explores how to use shared memory and semaphores from PHP and a new world of possibilities becomes reality.

Communication between processes in PHP

Garland Foster

"Preoptimization is the root of all evil (Donald Knuth)"

Preparation

In this article we will explain how to handle shared memory and semaphores from PHP, this requires having the php compiled with the following options:


--enable-shmop
--enable-sysvsem

The two extensions that we use are the extension of shared memory and semaphore extension, we aver we do with each course ..

What is IPC?

IPC (Inter-process communication) mechanism is a standard Unix processes to communicate with each other. Processes residing, it is clarification, all on the same machine. Basically IPC defines 3 different forms of communication: message queues, shared memory segments and semaphores. In this article we will discuss the use of semaphores and shared memory.

Using shared memory from PHP

The use of shared memory segments is a very good way of communicating processes together. Basically a segment of memory that can be shared by multiple processes, if a process writes to the segment others can view this data processes defined. In PHP we can control with this technique the number of running, or put data in memorua preventing other processes have to generate the same information that was already generated by other processes processes. In critical applications using cache memory as a temporary storage is widely more efficient than using a file-system file

To create a shared memory segment used:

  $ Shm_id = shmop_open ($ key, $ mode, $ perm, $ size); 

Where:

  • $ Key is the number that idnetifica the shared memory segment, all processes that want to access this same segment must know this number as key to access the segment.
  • $ Mode is the mode of creation, "c" is used to create a segment while "a" is used to access a segment already created
  • $ Perm segment defines the permissions according to the mechanism of Unix permissions
  • $ Size The size of the segment defined

The function returns an id that debermos used to read / write to the segment. It's not like a file handle as the functions used to manipulate shared memory are different.

Instance

  $ Shm_id = shmop_open (0xff3, "c", 0644, 100); 

In the example we create a shared memory segment 100 bytes.

If we want to access an existing segment shmop_open use by setting to 0 the third and fourth parameters of the function. If you want to create or access a segment whichever is applicable proceed as if we were creating it, if the segment already exists simply accede to it.

We did?

There are some Unix utilities that can be called from the command line to monitor and control IPC resources, for example we use "ipcs" to check what resources are created and some parameters thereof.

Instance

  Shared Memory Segments ------ -------- key perms shmid owner bytes nattch status0x00280267 0 root 644 1 1048576 30x00000000 nobody dest0x00000000 2 600 46084 10 600 46084 8 nobody nobody dest0x00000ff3 131 644 100 0

 Semaphore Arrays ------ -------- key perms semid owner nsems root status0x00280269 0 666 14

 Message Queues ------ -------- key perms msqid owner used-bytes messages 

As we can see we have 4 shared memory segments created, the last with (key 0x0000ff3) is that we believe in our example. We can see the properties of the segments, as the creator, permissions, size and number of processes that are using the segment in the output of ipcs command.

Returning to PHP

If we remove a shared memory segment we can use the following instruction from PHP:

  shmop_delete ($ id); 

The $ id we use is that the function shmop_open promptly returned it to create or access the segment.

If we remove the segment from the command line you can use the 'ipcrm shmid' eg 'ipcrm 131' command (in our example) would eliminate the segment created from PHP.

WARNING: If you use ipcrm as "root" must be very careful checking that segment as the segment that will be really clear that we want to eliminate. If we eliminate other segments belonging to other processes we can produce unexpected results and generally are pretty bad ...

Reading and writing data

To write data to a shared memory segment used:

  shmop_write int (int shmid, string data, int offset) 

shmid is the id that remanded shmop_open, data is the data (what else?) and the offset is the offset within the segment (0 to start writing at the beginning of it)

To read use:

  shmop_read string (int shmid, int start, int count) 

We indicate the position (offset) from which we read (0 for the beginning of the segment) and the number of bytes to read. Using shared memory data does not have any meaning beyond being strips of bytes, in most implementations is good to build classes that handle shared memory taking charge of providing interfaces for guaradar strings, objects or other types of data. The class is then responsible for handling the lengths and abstract the concept of strip bytes in memory programmer.

Two processes

We will have no problem creating, accessing, reading and writing a shared memory segment from a single isolated process. But we can find some problems when more than one process at the same time try to use the segment, we are about to enter the fascinating world of concurrent processes and the problem of mutual exclusion.

Suppose we have two processes reading and writing data to a single shared memory segment. If two processes attempt to write data to the same position memory at the same time we can get inconsistencies, this is known as the "problem of mutual exclusion" There is a lot of information and literature on the subject primarily of books on operating systems. A simple way to achieve mutual exclusion is using semaphores, and is the mechanism that will study below.

Semaforos

The traffic lights are another IPC resources, we can see information about the traffic lights system with ipcs and remove them with ipcrm. To create a semaphore from Php use:

  sem_get int (int key, int [max_acquire [, int perm]]) 

The semaphore is created if necessary or accessed if already existed. The function returns a handle that is used to manipulate the semaphore. max_acquiere indicates the maximum number of processes that can acquire the semaphore without releasing it, the default is 1. Permits are setean as constumbre.

Once we have the traffic light we can do two things with it: acquire and release. When we acquire a traffic light might think we are increasing and if we try to increase it beyond the amount set will with max_acquire the process is blocked until another process releases the semaphore locked and enable the process to acquire it. In binary semaphore, the most common in 90 \% of cases only one process can acquire the semaphore, other processes that want to acquire the semaphore will remain blocked until the process that has the semaphore releases it.

To acquire and release semaphore used:

  sem_acquire int (int sem_identifier) 
  sem_release int (int sem_identifier) 

Wherein the identifier is what we got with sem_get

A simple protocol for mutual exclusion.

We can use the following protocol to achieve mutual exclusion from PHP:

  $ Semid = sem_get (0xee3,1,0666); $ shm_id = shmop_open (0xff3, "c", 0644, 100); sem_acquire ($ semid); / * If we are here we are alone!  * / SHARED WRITE TO THE SEGMENT HEREsem_release ($ semid); 

As we can see the mechanism is simple, the semaphore is acquired, does something and then the semaphore is released. We can be sure that no two processes simultaneously writing this because once you acquire the semaphore others are blocked until the semaphore is released. The lines between sem_acquire and sem_release are known as "critical section" ie areas where we do not want to attend two processes simultaneously.

An interesting point is that in PHP semaphore implementation process that releases the semaphore should be the same as acquired. A process can not release a semaphore acquired by another process. This is not common in other languages ​​that use IPC so we must take this into account.

Importantly, we only have to worry about the mutual exclusion between writers, that more than one process reads the same shared memory segment while not present any problem but if there is a process "writer" then the process must be the only one access memory. This is a known problem of concurrency known as the problem of readers-writers.

Applications

There are several applications for IPC, for some simple optimization recursoso as stored in shared memory configuration files parsed to advanced search engines, authentication systems or cache.

Garland Foster

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