Aug 25

session_start: Cannot send session cookie – headers already sent by

By jason PHP World No Comments »

my session test code:

<html>
<head>
<title>PHP SESSION TEST CODE</title>
</head>
<body>
<?
session_start();
session_register("MVAR");
$MVAR="hello world";
echo "The content of sess variable is $MVAR";
?>
<a href="call_session.php">Next page</a>
</body>
</html>

Error Tips

Warning: session_start(): Cannot send session cookie – headers already sent by (output started at d:\www\session.php:7) in d:\www\session.php on line 8

Warning: session_start(): Cannot send session cache limiter – headers already sent (output started at d:\www\session.php:7) in d:\www\session.php on line 8
The content of sess variable is hello worldNext page

Solution:

1.Modify your php.ini file. Find the output_buffering variable and varlue is on,as output_buffering = On.

2.Find your session save path, you can find it from php.ini of session.save_path = "c:\tmp"
   Please ensure php can write the “c:\tmp” directory.

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Jun 05

How to load balance with apache

By jason Apache Server No Comments »

deployment overview

web server
site1.yourdomain.com
192.168.1.35

web server
siste2.yourdomain.com
192.168.1.36

data server
db1.yourdomain.com
192.168.1.37

load balancer
balance-1.yourdomain.com
192.168.1.45

install and enable apache and proxy_balancer

1.create a dedicated server for load balancing. install apache2 and then install mod proxy_balancer and proxy_http with dependencies.

2.enable mod_proxy in httpd.conf. note that i’m leaving ProxyRequests off since we’re only using the ProxyPass and ProxyPassReverse directives. this keeps the server secure from spammers trying to use your proxy to send email.

<IfModule mod_proxy.c>
        ProxyRequests Off

        <Proxy *>
                AddDefaultCharset off
                Order deny,allow
                Allow from all
                #Allow from .example.com
        </Proxy>

        ProxyVia On
</IfModule>

configure mod_proxy and mod_proxy_balancer

mod_proxy and mod_proxy balancer serve as a very functional load balancer. however mod_proxy_balancer makes slightly unfortunate assumptions about the format of the cookie that you’ll use for sticky session handling. one way to work around this is to create your own session cookie (very easy with apache). the examples below describe how to do this

first create a virtual host or use the default  and add this configuration to it:

<Location /balancer-manager>
SetHandler balancer-manager
Order Deny,Allow
Deny from all
Allow from 192.168
</Location>
<Proxy balancer://mycluster>
  # cluster member 1
  BalancerMember http://site1.yourdomain.com:80 route=lb1
  # cluster member 2
  BalancerMember http://site2.yourdomain.com:80 route=lb2
</Proxy>
ProxyPass /balancer-manager !
ProxyPass / balancer://mycluster/ lbmethod=byrequests stickysession=BALANCEID
ProxyPassReverse / http://site1.yourdomain.com/
ProxyPassReverse / http://site2.yourdomain.com/

Note:

  • i’m allowing access to the balancer manager from any IP matching 192.168.*.*
  • i’m load balancing between 2 servers (site1.yourdomain.com, site2.yourdomain.com) on port 80
  • i’m defining two routes for these servers called lb1 and lb2
  • i’m excluding (!) the balancer-manager directory fro the ProxyPass to allow access to the manager ui on the load balancing server
  • i’m expecting a cookie called BALANCEID to be available to manage sticky sessions

  • this is a simplistic load balancing configuration. apache has many options to control timeouts, server loading, failover etc. too much to cover but read more in the apache documentation

    configure the web servers to write a session cookie

    on each of the web servers, add this code to your vhost configuration:

    RewriteEngine On
    RewriteRule .* - [CO=BALANCEID:balancer.lb1:.yourdomain.com]

    making sure to specify the correct route e.g. lb1 on site1.yourdomain.com etc.

    you also probably want to setup your cookie domain properly in drupal, i.e. modify drupal/sites/default/settings.php as follows:

    # $cookie_domain = 'example.com';
    $cookie_domain = 'yourdomain.com';

    important urls

    useful urls for testing are:

    References:

  • apache’s mod_proxy_balancer documentation
  • apache’s mod_proxy documentation

     

    About Load Banlance topic you can read my old post:

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    preload preload preload