Posts Tagged ‘Edit’

How to install and use memcached on Debian GNU / Linux to share php sessions between DNS round robined Apache webservers

Monday, November 9th, 2020

Reading Time: 8minutes


Recently I had to come up with a solution to make A bunch of websites hosted on a machine to be high available. For the task haproxy is one of logical options to use. However as I didn't wanted to set new IP addresses and play around to build a cluster. I decided the much more simplistic approach to use 2 separate Machines each running Up-to-date same version of Apache Webserver as front end and using a shared data running on Master-to-Master MySQL replication database as a backend. For the load balancing itself I've used a simple 2 multiple DNS 'A' Active records, configured via the Bind DNS name server an Round Robin DNS load balancing for each of the domains, to make them point to the the 2 Internet IP addresses (XXX.XXX.XXX.4 and YYY.YYY.YYY.5) each configured on the 2 Linux servers eth0.

So far so good, this setup worked but immediately, I've run another issue as I found out the WordPress and Joomla based websites's PHP sessions are lost, as the connectivity by the remote client browser reaches one time on XXX…4 and one time on YYY…4 configured listerner on TCP port 80 and TCP p. 443. In other words if request comes up to Front end Apache worker webserver 1 with opened channel data is sent back to Client Browser and the next request is sent due to the other IP resolved by the DNS server to come to Apache worker webserver 2 of course webserver 2 has no idea about this previous session data and it gets confused and returns soemething like a 404 or 500 or any other error … not exciting really huh …

I've thought about work around and as I didn't wanted to involve thirty party stuff as Privoxy / Squid  / Varnish / Polipo etc. just as that would add extra complexity as if I choose to use haproxy from the beginning, after short investigation came to a reason to use memcached as a central PHP sessions storage.


Why I choose memcached ?

Well it is relatively easy to configure, it doesn't come with mambo-jambo unreadable over-complicated configuration and the time to configure everything is really little as well as the configuration is much straight forward, plus I don't need to occupy more IP addresses and I don't need to do any changes to the already running 2 WebServers on 2 separate Linux hosts configured to be reachable from the Internet.
Of course using memcached is not a rock solid and not the best solution out there, as there is risk that if a memcached dies out for some reason all sessions stored in are lost as they're stored only in volatile memory, as well as there is a drawback that if a communication was done via one of the 2 webservers and one of them goes down sessions that were known by one of Apache's workers disappears.

So let me proceed and explain you the steps to take to configure memcached as a central session storage system.

1. Install memcached and php-memcached packages

To enable support for memcached besides installing memcached daemon, you need to have the php-memcached which will provide the used by Apache loaded php script interpretter module.

On a Debian / Ubuntu and other deb based GNU / Linux it should be:

webserver1:~# apt-get install memcached php-memcached

TO use php-memcached I assume Apache and its support for PHP is already installed with lets say:

webserver1:~# apt-get install php libapache2-mod-php php-mcrypt

On CentOS / RHEL / Fedora Linux it is a little bit more complicated as you'll need to install php-pear and compile the module with pecl


[root@centos ~]# yum install php-pear

[root@centos ~]# yum install php-pecl-memcache

Compile memcache

[root@centos ~]# pecl install memcache


2. Test if memcached is properly loaded in PHP

Once installed lets check if memcached service is running and memcached support is loaded as module into PHP core.


webserver1:~# ps -efa  | egrep memcached
nobody   14443     1  0 Oct23 ?        00:04:34 /usr/bin/memcached -v -m 64 -p 11211 -u nobody -l -l

root@webserver1:/# php -m | egrep memcache

To get a bit more verbose information on memcache version and few of memcached variable settings:

root@webserver1:/# php -i |grep -i memcache
memcached support => enabled
libmemcached version => 1.0.18
memcached.compression_factor => 1.3 => 1.3
memcached.compression_threshold => 2000 => 2000
memcached.compression_type => fastlz => fastlz
memcached.default_binary_protocol => Off => Off
memcached.default_connect_timeout => 0 => 0
memcached.default_consistent_hash => Off => Off
memcached.serializer => php => php
memcached.sess_binary_protocol => On => On
memcached.sess_connect_timeout => 0 => 0
memcached.sess_consistent_hash => On => On
memcached.sess_consistent_hash_type => ketama => ketama
memcached.sess_lock_expire => 0 => 0
memcached.sess_lock_max_wait => not set => not set
memcached.sess_lock_retries => 5 => 5
memcached.sess_lock_wait => not set => not set
memcached.sess_lock_wait_max => 150 => 150
memcached.sess_lock_wait_min => 150 => 150
memcached.sess_locking => On => On
memcached.sess_number_of_replicas => 0 => 0
memcached.sess_persistent => Off => Off
memcached.sess_prefix => memc.sess.key. => memc.sess.key.
memcached.sess_randomize_replica_read => Off => Off
memcached.sess_remove_failed_servers => Off => Off
memcached.sess_sasl_password => no value => no value
memcached.sess_sasl_username => no value => no value
memcached.sess_server_failure_limit => 0 => 0
memcached.store_retry_count => 2 => 2
Registered save handlers => files user memcached

Make sure /etc/default/memcached (on Debian is enabled) on CentOS / RHELs this should be /etc/sysconfig/memcached

webserver1:~# cat default/memcached 
# Set this to no to disable memcached.

As assured on server1 memcached + php is ready to be used, next login to Linux server 2 and repeat the same steps install memcached and the module and check it is showing as loaded.

Next place under some of your webservers hosted websites under check_memcached.php below PHP code

if (class_exists('Memcache')) {
    $server = 'localhost';
    if (!empty($_REQUEST[‘server’])) {
        $server = $_REQUEST[‘server’];
    $memcache = new Memcache;
    $isMemcacheAvailable = @$memcache->connect($server);

    if ($isMemcacheAvailable) {
        $aData = $memcache->get('data');
        echo '<pre>';
        if ($aData) {
            echo '<h2>Data from Cache:</h2>';
        } else {
            $aData = array(
                'me' => 'you',
                'us' => 'them',
            echo '<h2>Fresh Data:</h2>';
            $memcache->set('data', $aData, 0, 300);
        $aData = $memcache->get('data');
        if ($aData) {
            echo '<h3>Memcache seem to be working fine!</h3>';
        } else {
            echo '<h3>Memcache DOES NOT seem to be working!</h3>';
        echo '</pre>';

if (!$isMemcacheAvailable) {
    echo 'Memcache not available';


Launch in a browser, the browser output should be as on below screenshot:


3. Configure memcached daemons on both nodes

All we need to set up is the listen IPv4 addresses

On Host Webserver1
You should have in /etc/memcached.conf


webserver1:~# grep -Ei '\-l' /etc/memcached.conf 

On Host Webserver2



webserver2:~# grep -Ei '\-l' /etc/memcached.conf


4. Configure memcached in php.ini

Edit config /etc/php.ini (on CentOS / RHEL) or on Debians / Ubuntus etc. modify /etc/php/*/apache2/php.ini (where depending on the PHP version you're using your php location could be different lets say /etc/php/5.6/apache2/php.ini):

If you wonder where is the php.ini config in your case you can usually get it from the php cli:

webserver1:~# php -i | grep "php.ini"
Configuration File (php.ini) Path => /etc/php/7.4/cli
Loaded Configuration File => /etc/php/7.4/cli/php.ini


! Note: That on on PHP-FPM installations (where FastCGI Process Manager) is handling PHP requests,path would be rather something like:


in php.ini you need to change as minimum below 2 variables

session.save_handler =
session.save_path =

By default session.save_path would be set to lets say session.save_path = "


To make php use a 2 central configured memcached servers on webserver1 and webserver2 or even more memcached configured machines set it to look as so:


Also modify set

session.save_handler = memcache

Overall changed php.ini configuration on Linux machine 1 ( webserver1 ) and Linux machine 2 ( webserver2 ) should be:

session.save_handler = memcache


Below is approximately how it should look on both :

webserver1: ~# grep -Ei 'session.save_handler|session.save_path' /etc/php.ini
;; session.save_handler = files
session.save_handler = memcache
;     session.save_path = "N;/path"
;     session.save_path = "N;MODE;/path"
;session.save_path = "/var/lib/php7/sessions"
;       (see session.save_path above), then garbage collection does *not*


webserver2: ~# grep -Ei 'session.save_handler|session.save_path' /etc/php.ini
;; session.save_handler = files
session.save_handler = memcache
;     session.save_path = "N;/path"
;     session.save_path = "N;MODE;/path"
;session.save_path = "/var/lib/php7/sessions"
;       (see session.save_path above), then garbage collection does *not*

As you can see I have configured memcached on webserver1 to listen on internal local LAN IP and on Local LAN eth iface on TCP port 11211 (this is the default memcached connections listen port), for security or obscurity reasons you might choose another empty one. Make sure to also set the proper firewalling to that port, the best is to enable connections only between and on each of machine 1 and machine 2.


5. Enable Memcached for session redundancy

Next step is to configure memcached to allow failover (e.g. use both memcached on 2 linux hosts) and configure session redundancy.
Configure /etc/php/7.3/mods-available/memcache.ini or /etc/php5/mods-available/memcache.ini or respectively to the right location depending on the PHP installed and used webservers version.

webserver1 :~#  vim /etc/php/7.3/mods-available/memcache.ini

; configuration for php memcached module
; priority=20
; settings to write sessions to both servers and have fail over


webserver2 :~# vim /etc/php/7.3/mods-available/memcache.ini

; configuration for php memcached module
; priority=20
; settings to write sessions to both servers and have fail over


memcache.session_redundancy directive must be equal to the number of memcached servers + 1 for the session information to be replicated to all the servers. This is due to a bug in PHP.
I have only 2 memcached configured that's why I set it to 3.

6. Restart Apache Webservers

Restart on both machines webserver1 and webserver2 Apache to make php load

webserver1:~# systemctl restart httpd

webserver2:~# systemctl restart httpd


7. Restart memcached on machine 1 and 2


webserver1 :~# systemctl restart memcached

webserver2 :~# systemctl restart memcached


8. Test php sessions are working as expected with a php script

Copy to both website locations to accessible URL a file test_sessions.php:


echo "Sessions is ".$_SESSION[‘georgi’]."!\n";
echo "Session ID: ".session_id()."\n";
echo "Session Name: ".session_name()."\n";
echo "Setting 'georgi' to 'cool'\n";


Now run the test to see PHP sessions are kept persistently:

hipo@jeremiah:~/Desktop $ curl -vL -s 2>&1 | grep 'Set-Cookie:'
< Set-Cookie: PHPSESSID=micir464cplbdfpo36n3qi9hd3; expires=Tue, 10-Nov-2020 12:14:32 GMT; Max-Age=86400; path=/

hipo@jeremiah:~/Desktop $ curl -L –cookie "PHPSESSID=micir464cplbdfpo36n3qi9hd3"
Session is cool!
Session is cool!


Copy to the locations that is resolving to both DNS servers some sample php script such as sessions_test.php  with below content:

    header('Content-Type: text/plain');
        echo "This is the first time you're visiting this server\n";
        $_SESSION[‘visit’] = 0;
            echo "Your number of visits: ".$_SESSION[‘visit’] . "\n";


    echo "Server IP: ".$_SERVER[‘SERVER_ADDR’] . "\n";
    echo "Client IP: ".$_SERVER[‘REMOTE_ADDR’] . "\n";

Test in a Web Opera / Firefox / Chrome browser.

You should get an output in the browser similar to:

Your number of visits: 15
Server IP:
Client IP:
    [_ga] => GA1.2.651288003.1538922937
    [__utma] => 238407297.651288003.1538922937.1601730730.1601759984.45
    [__utmz] => 238407297.1571087583.28.4.utmcsr=google|utmccn=(organic)|utmcmd=organic|utmctr=(not provided)
    [shellInABox] => 467306938:1110101010
    [fpestid] => EzkIzv_9OWmR9PxhUM8HEKoV3fbOri1iAiHesU7T4Pso4Mbi7Gtt9L1vlChtkli5GVDKtg
    [__gads] => ID=8a1e445d88889784-22302f2c01b9005b:T=1603219663:RT=1603219663:S=ALNI_MZ6L4IIaIBcwaeCk_KNwmL3df3Z2g
    [PHPSESSID] => mgpk1ivhvfc2d0daq08e0p0ec5

If you want to test php sessions are working with text browser or from another external script for automation use something as below PHP code:

// save as "session_test.php" inside your webspace  
ini_set('display_errors', 'On');


$sessionSavePath = ini_get('session.save_path');

echo '<br><div style="background:#def;padding:6px">'
   , 'If a session could be started successfully <b>you should'
   , ' not see any Warning(s)</b>, otherwise check the path/folder'
   , ' mentioned in the warning(s) for proper access rights.<hr>';
echo "WebServer IP:" . $_SERVER[‘SERVER_ADDR’] . "\n<br />";
if (empty($sessionSavePath)) {
    echo 'A "<b>session.save_path</b>" is currently',
         ' <b>not</b> set.<br>Normally "<b>';
    if (isset($_ENV[‘TMP’])) {
        echo  $_ENV[‘TMP’], ‘” ($_ENV[“TMP”]) ';
    } else {
        echo '/tmp</b>" or "<b>C:\tmp</b>" (or whatever',
             ' the OS default "TMP" folder is set to)';
    echo ' is used in this case.';
} else {
    echo 'The current "session.save_path" is "<b>',
         $sessionSavePath, '</b>".';

echo '<br>Session file name: "<b>sess_', session_id()
   , '</b>".</div><br>';

You can download the test_php_sessions.php script here.

To test with lynx:

hipo@jeremiah:~/Desktop $ lynx -source ''
<br><div style="background:#def;padding:6px">If a session could be started successfully <b>you should not see any Warning(s)</b>, otherwise check the path/folder mentioned in the warning(s) for proper access rights.<hr>WebServer IP:
<br />The current "session.save_path" is "<b>tcp://, tcp://</b>".<br>Session file name: "<b>sess_5h18f809b88isf8vileudgrl40</b>".</div><br>

Improve Apache Load Balancing with mod_cluster – Apaches to Tomcats Application servers Get Better Load Balancing

Thursday, March 31st, 2016

Reading Time: 6minutes


Earlier I've blogged on How to set up Apache to to serve as a Load Balancer for 2, 3, 4  etc. Tomcat / other backend application servers with mod_proxy and mod_proxy_balancer, howeverthough default Apache provided mod_proxy_balancer works fine most of the time, If you want a more precise and sophisticated balancing with better load distribuion you will probably want to install and use mod_cluster instead.


So what is Mod_Cluster and why use it instead of Apache proxy_balancer ?

Mod_cluster is an innovative Apache module for HTTP load balancing and proxying. It implements a communication channel between the load balancer and back-end nodes to make better load-balancing decisions and redistribute loads more evenly.

Why use mod_cluster instead of a traditional load balancer such as Apache's mod_balancer and mod_proxy or even a high-performance hardware balancer?

Thanks to its unique back-end communication channel, mod_cluster takes into account back-end servers' loads, and thus provides better and more precise load balancing tailored for JBoss and Tomcat servers. Mod_cluster also knows when an application is undeployed, and does not forward requests for its context (URL path) until its redeployment. And mod_cluster is easy to implement, use, and configure, requiring minimal configuration on the front-end Apache server and on the back-end servers.

So what is the advantage of mod_cluster vs mod proxy_balancer ?

Well here is few things that turns the scales  in favour for mod_cluster:


  •     advertises its presence via multicast so as workers can join without any configuration
  •     workers will report their available contexts
  •     mod_cluster will create proxies for these contexts automatically
  •     if you want to, you can still fine-tune this behaviour, e.g. so as .gif images are served from httpd and not from workers…
  •     most importantly: unlike pure mod_proxy or mod_jk, mod_cluster knows exactly how much load there is on each node because nodes are reporting their load back to the balancer via special messages
  •     default communication goes over AJP, you can use HTTP and HTTPS


1. How to install mod_cluster on Linux ?

You can use mod_cluster either with JBoss or Tomcat back-end servers. We'll install and configure mod_cluster with Tomcat under CentOS; using it with JBoss or on other Linux distributions is a similar process. I'll assume you already have at least one front-end Apache server and a few back-end Tomcat servers installed.

To install mod_cluster, first download the latest mod_cluster httpd binaries. Make sure to select the correct package for your hardware architecture – 32- or 64-bit.
Unpack the archive to create four new Apache module files:,,, and We won't need; it advertises the location of the load balancer through multicast packets, but we will use a static address on each back-end server.

Copy the other three .so files to the default Apache modules directory (/etc/httpd/modules/ for CentOS).
Before loading the new modules in Apache you have to remove the default proxy balancer module ( because it is not compatible with mod_cluster.

Edit the Apache configuration file (/etc/httpd/conf/httpd.conf) and remove the line


LoadModule proxy_balancer_module modules/


Create a new configuration file and give it a name such as /etc/httpd/conf.d/mod_cluster.conf. Use it to load mod_cluster's modules:




LoadModule slotmem_module modules/
LoadModule manager_module modules/
LoadModule proxy_cluster_module modules/

In the same file add the rest of the settings you'll need for mod_cluster something like:

And for permissions and Virtualhost section



        Order deny,allow
        Allow from all 192.168

    ManagerBalancerName mymodcluster

ProxyPass / balancer://mymodcluster/

The above directives create a new virtual host listening on port 9999 on the Apache server you want to use for load balancing, on which the load balancer will receive information from the back-end application servers. In this example, the virtual host is listening on IP address, and for security reasons it allows connections only from the network.
The directive ManagerBalancerName defines the name of the cluster – mymodcluster in this example. The directive EnableMCPMReceive allows the back-end servers to send updates to the load balancer. The standard ProxyPass and ProxyPassReverse directives instruct Apache to proxy all requests to the mymodcluster balancer.
That's all you need for a minimal configuration of mod_cluster on the Apache load balancer. At next server restart Apache will automatically load the file mod_cluster.conf from the /etc/httpd/conf.d directory. To learn about more options that might be useful in specific scenarios, check mod_cluster's documentation.

While you're changing Apache configuration, you should probably set the log level in Apache to debug when you're getting started with mod_cluster, so that you can trace the communication between the front- and the back-end servers and troubleshoot problems more easily. To do so, edit Apache's configuration file and add the line LogLevel debug , then restart Apache.

2. How to set up Tomcat appserver for mod_cluster ?

Mod_clusterworks with Tomcat version 6, 7 and 8, to set up the Tomcat back ends you have to deploy a few JAR files and make a change in Tomcat's server.xml configuration file.
The necessary JAR files extend Tomcat's default functionality so that it can communicate with the proxy load balancer. You can download the JAR file archive by clicking on "Java bundles" on the mod_cluster download page. It will be saved under the name mod_cluster-parent-1.2.6.Final-bin.tar.gz.

Create a new directory such as /root/java_bundles and extract the files from mod_cluster-parent-1.2.6.Final-bin.tar.gzthere. Inside the directory /root/java_bundlesJBossWeb-Tomcat/lib/*.jar you will find all the necessary JAR files for Tomcat, including two Tomcat version-specific JAR files – mod_cluster-container-tomcat6-1.2.6.Final.jar for Tomcat 6 and mod_cluster-container-tomcat7-1.2.6.Final.jar for Tomcat 7. Delete the one that does not correspond to your Tomcat version.

Copy all the files from /root/java_bundlesJBossWeb-Tomcat/lib/ to your Tomcat lib directory – thus if you have installed Tomcat in


run the command:


cp -rpf /root/java_bundles/JBossWeb-Tomcat/lib/* /srv/tomcat/lib/ .


Then edit your Tomcat's server.xml file


After the default listeners add the following line:


<listener classname="org.jboss.modcluster.container.catalina.standalone.ModClusterListener" proxylist=""> </listener>

This instructs Tomcat to send its mod_cluster-related information to IP on TCP port 9999, which is what we set up as Apache's dedicated vhost for mod_cluster.
While that's enough for a basic mod_cluster setup, you should also configure a unique, intuitive JVM route value on each Tomcat instance so that you can easily differentiate the nodes later. To do so, edit the server.xml file and extend the Engine property to contain a jvmRoute, like this:



<engine defaulthost="localhost" jvmroute="node2" name="Catalina"></engine>

Assign a different value, such as node2, to each Tomcat instance. Then restart Tomcat so that these settings take effect.

To confirm that everything is working as expected and that the Tomcat instance connects to the load balancer, grep Tomcat's log for the string "modcluster" (case-insensitive). You should see output similar to:

Mar 29, 2016 10:05:00 AM org.jboss.modcluster.ModClusterService init
INFO: MODCLUSTER000001: Initializing mod_cluster ${project.version}
Mar 29, 2016 10:05:17 AM org.jboss.modcluster.ModClusterService connectionEstablished
INFO: MODCLUSTER000012: Catalina connector will use /

This shows that mod_cluster has been successfully initialized and that it will use the connector for, the configured IP address for the main listener.
Also check Apache's error log. You should see confirmation about the properly working back-end server:

[Tue Mar 29 10:05:00 2013] [debug] proxy_util.c(2026): proxy: ajp: has acquired connection for (
[Tue Mar 29 10:05:00 2013] [debug] proxy_util.c(2082): proxy: connecting ajp:// to
[Tue Mar 29 10:05:00 2013] [debug] proxy_util.c(2209): proxy: connected / to
[Tue Mar 29 10:05:00 2013] [debug] mod_proxy_cluster.c(1366): proxy_cluster_try_pingpong: connected to backend
[Tue Mar 29 10:05:00 2013] [debug] mod_proxy_cluster.c(1089): ajp_cping_cpong: Done
[Tue Mar 29 10:05:00 2013] [debug] proxy_util.c(2044): proxy: ajp: has released connection for (

This Apache error log shows that an AJP connection with was successfully established and confirms the working state of the node, then shows that the load balancer closed the connection after the successful attempt.

You can start testing by opening in a browser the example servlet SessionExample, which is available in a default installation of Tomcat.
Access this servlet through a browser at the URL http://balancer_address/examples/servlets/servlet/SessionExample. In your browser you should see first a session ID that contains the name of the back-end node that is serving your request – for instance, Session ID: 5D90CB3C0AA05CB5FE13121E4B23E670.node2.

Next, through the servlet's web form, create different session attributes. If you have a properly working load balancer with sticky sessions you should always (that is, until your current browser session expires) access the same node, with the previously created session attributes still available.

To test further to confirm load balancing is in place, at the same time open the same servlet from another browser. You should be redirected to another back-end server where you can conduct a similar session test.
As you can see, mod_cluster is easy to use and configure. Give it a try to address sporadic single-back-end overloads that cause overall application slowdowns.