Home > Articles > Operating Systems, Server

  • Print
  • + Share This
This chapter is from the book

Quick and Dirty Clustering

As previously mentioned, clusters are made up of two or more different machines (or processors) that serve the same application. Even though it takes a little more to make a full-fledged clustered solution for most applications, it's easy to bring basic services to most networks.

All Linux actually needs to be involved in a cluster is to talk to other machines in a network, which includes most of the machines out there. Put two boxes together that talk to each other, and you have a cluster—not a very effective cluster, mind you, but one that works. It's a simple matter, really, to enable basic clustering features to existing servers.

Dial Ping for Service

A basic HA cluster can be achieved with nothing more than straight ping. If you were to set up a script that would enable a failover server to ping a primary application server every minute or so, the failover server could determine when the primary failed. As soon as the secondary couldn't find the other server, it would assume that it was down and launch scripts to enable its own services. After all, if the failover server can't reach the primary, neither can any other service or user.

You need to consider some options before you start planning for your basic HA cluster. Are you going to have the failover server in standby mode with an application already running, ready to go? Is the standby server already in production with other elements running? Here's what you need to set up a basic HA cluster:

  • Script that will ping the other server.

  • Crontab entry to call the script.

  • If the other server responds, then sleep.

  • If the other server doesn't respond, ping again.

  • If the other server still doesn't respond, assume failover sequence.

  • Take over IP address of failed server.

The first step when implementing such a cluster is to assign a virtual IP to the servers. This serves as the floating address that resolves to a DNS entry. Initially, the primary server owns the IP and then the secondary assumes it after failover.

IP aliasing has to either have been compiled in the kernel or have been loaded as a module. The kernel configuration for 2.2.x kernels is under Networking Options, IP: Aliasing Support, and is unchecked by default. Module support is only available in 2.0.x kernels. 2.2.x kernels either have support compiled in or not.

The module that you'll be looking for is ip_alias. To insert it into the 2.0.x kernel, use the insmod command.

RedHat 7.1 default install allows for an IP alias right out of the box. A stock 6.2 does as well, yet Debian 2.2 doesn't, so check your distribution.

After IP aliasing is available, an extra alias (or several) can be added with the same ease as a regular IP. The format is

/sbin/ifconfig <interface>:<virtual number> <ip> netmask <netmask> up

For example,

/sbin/ifconfig eth0:1 172.16.0.1 netmask 255.255.0.0 up

brings up virtual interface one and attaches it to adapter eth0 with an IP of 172.16.0.1 and a class B /16 netmask. Assign the virtual interface to the interface of the primary server and make sure that this entry is resolved by DNS.

The next step includes setting up the failover script. This script, placed on the failover server, initially pings the primary server and exits if a response is given. If a response is not given, the program waits for 20 seconds and pings again. If the failover server doesn't receive a ping response from the primary a second time, the server then sets the virtual IP on itself and optionally sends out a notification. Don't forget to set the script executable.

#!/bin/bash
# Poor Man's Cluster
# Script to Test for Failover
# 
host="172.16.0.2"
netmask="255.255.0.0"
while true; do
/bin/ping -c 5 -i 3 $host > /dev/null 2>&1
if [ $? -ge 1 ]; then
    #This doesn't look good, let's test again.
    /bin/ping -c 5 -i 3 $host > /dev/null 2>$1
        if [ $? -ge 1 ]; then
        # We need to assume host is really down. Assume
        # failover sequence
        /sbin/ifconfig eth0:1 $host netmask $netmask up
        echo "$host is down. Please check immediately." > /bin/mail
-s "$host is down!" user@host.com else #second ping returned a value. Whew. : else : fi done

The script should be able to run in the background and mail the designated user if a connectivity problem occurs. The script can easily be adapted to ping more than one host, if you're creative enough.

Load Balancing Using DNS Records

A quick and dirty way to enable load balancing can be achieved with simple A records in DNS. Although it presents itself as a dirty hack, enabling the same name with two or more different IP addresses does offer a measure of load balancing, albeit limited in scope. The same effect does not work, however, when enabling different IP addresses in /etc/hosts.

To illustrate, we have the following zone enabled:

[root@matrix named]# cat zone.dns
;
; Zone file for zone.com
;
@    IN   SOA   zone.com. postmaster.zone.com. (
            19990913    ; serial
            10800       ; refresh
            3600        ; retry
            604800      ; expire, seconds
            86400 )     ; minimum, seconds
IN NS   ns1.zone.com.
IN NS   ns2.zone.com.
host1     IN A  10.2.2.10
host1     IN A  10.2.2.11
host1     IN A  10.2.2.12

In this example, each host carries the same domain name, yet the A record points to a different address. Pings of the machine show the following:

[root@matrix named]# ping host1

PING host1.zone.com (10.2.2.10) from 10.2.2.100 : 56(84) bytes of data.
[root@matrix named]$ ping host1

PING host1.zone.com (10.2.2.11) from 10.2.2.100 : 56(84) bytes of data.
[root@matrix named]$ ping host1

PING host1.zone.com (10.2.2.10) from 10.2.2.100 : 56(84) bytes of data.
[root@matrix named]$ ping host1

PING host1.zone.com (10.2.2.12) from 10.2.2.100 : 56(84) bytes of data.

As you can see, it's not a foolproof situation, but it does offer some method of load balancing between hosts.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020