Home > Articles

This chapter is from the book

25.3 The Basics of a Cluster

A cluster is a collection of at least two nodes and up to 16 nodes. Supported cluster configurations include:

  • Active/Active: This is where all nodes are running their own application package but can run additional application packages if necessary.

  • Active/Standby: This is where a single node is not actively running any application packages but is waiting for a failure to occur on any of the other nodes in the cluster, whereby it will adopt responsibility for running that nodes application package.

  • Rolling Standby: This is similar to Active/Standby in that we have a node that is waiting for a failure to occur on any node in the cluster. The difference here is that when a failure occurs, the failed node becomes the standby node after the initial problem is resolved. Should a second failure occur, the second failed node becomes the standby. In a purely Active/Standby configuration, if a second failure occurred, the original standby node would be running two application packages.

Cluster monitoring is performed by a number of Serviceguard processes. Serviceguard has three main management functions:

  • Cluster Manager: The management and coordination of cluster membership.

  • Network Manager: Monitoring network connectivity and activating standby LAN cards when necessary.

  • Package Manager: The management of starting, stopping, and relocating application packages within the cluster.

The main Cluster Management Daemon is a process called cmcld. This process is running on every node in the cluster, and one of its main duties is to send and receive heartbeat packets across all designated heartbeat networks. One node in the cluster will be elected the cluster coordinator that is responsible for coordinating all inter-node communication. The cluster coordinator is elected at cluster startup time and during a cluster reformation. A cluster will reform due to one of four events:

  • A node leaves the cluster, either "gracefully" or because the node fails

  • A node joins the cluster

  • Automatic cluster startup

  • Manual cluster startup

When we set up our cluster, we discuss this "election" in a bit more detail. A critical feature of the cluster coordinator is the detection of a node failure; in this, we mean either total LAN communication failure or total system failure. For every HEARTBEAT_INTERVAL, nodes are transmitting a heartbeat packet on all prescribed heartbeat interfaces. If this heartbeat packet does not reach the cluster coordinator, after a NODE_TIMEOUT interval the node is determined to have failed and a cluster reformation commences. It goes without saying that maintaining heartbeat communication is vitally important to all nodes in the cluster.

  • Cluster HEARTBEAT and STANDBY LAN interfaces: Because this is such a crucial part in determining the health of a cluster, the more LAN interfaces you prescribe as being heartbeat interfaces, the better. The only time this is not the case is if you are intending to use VERITAS Cluster Volume Manager (CVM). The design of CVM allows the CVM daemon vxclustd to communicate over a single IP subnet. You will realize when you try to run cmcheckconf and cmapplyconf. If more that one heartbeat LAN is configured in a CVM configuration, both commands will fail. LAN interfaces can either be designated as a HEARTBEAT_IP (carries the cluster heartbeat) or a STATIONARY_IP (does not carry the cluster heartbeat). Even if a LAN interface is configured as a HEARTBEAT_IP, it can carry normal application data as well. The designation STATIONARY_IP simply means that no heartbeat packets are transmitted over that interface; it does not mean the IP address cannot be moved to a redundant, standby LAN card. The use of a redundant standby LAN interface for all interfaces is highly recommended. If you are going to use only one standby LAN card for all LAN interfaces, it must be bridged to all the networks for which it is being a standby. Figure 25-1 shows a good setup where we have a standby LAN card for each active network.

    25fig01.jpgFigure 25-1 The use of standby LAN cards.

In Figure 25-1, you will also notice that the HEARTBEAT_IP is not being utilized by any clients for data traffic. This is an ideal scenario because HEARTBEAT packets are not contending with data packets for access to the network. You can use a HEARTBEAT_IP for data traffic as well, although you should note that if data traffic becomes particularly heavy, then the heartbeat packet may not reach the cluster coordinator, and this could cause a cluster reformation because some nodes "appear" to have "disappeared." You should also note that the standby LAN cards are bridged with the active LAN card. This is absolutely crucial. Serviceguard will poll standby/active LAN cards every NETWORK_POLLING_INTERVAL to ensure that they can still communicate. The bridge/switch/hub that is used should support the 802.1 Spanning Tree Algorithm (most of them do). The Quorum Server is currently attached to the main corporate data LAN. This is not a requirement. It just shows that all nodes in the cluster must be able to communicate with it, and it could be "any" machine in your organization running HP-UX. Many customers I know have the Quorum Server attached to the dedicated Heartbeat network. I think this is a good idea because all nodes in the cluster need access to the Heartbeat network and when we need to communicate with the Quorum Server, we are not competing with other users for access to our corporate data LAN.

When I first looked at Serviceguard I wondered, "How many LAN cards do I need?" The simple answer is two. Serviceguard is designed to fit into the whole philosophy of high availability. If Serviceguard "allowed" you to run with just one LAN card, it would be an immediate SPOF. So you need two LAN cards, with one acting as a STANDBY. Well, if I am really honest, you can get away with one LAN card. In a simple two-node cluster similar to the one you can see in Figure 25-1, you could use only one LAN card as long as you used an RS-232 null modem cable as a "serial heartbeat" between the two nodes. The one LAN card needs to be used as a HEARTBEAT_IP, i.e., in this case for data plus heartbeat packets. The serial heartbeat will be used as a last-ditch means for nodes to communicate in the event of network saturation. In that instance, both nodes will use the serial link to determine who was the "best candidate" to reform the cluster on their own. (Note: The use of serial heartbeats is being viewed as "less than ideal" and may be phased out in the near future.) In essence, the serial heartbeat is adding a little intelligence into the cluster reformation process, but only when we have a two-node cluster with only one LAN card in each node. This leads me to my next point about High Availability Clusters: the "split-brain" syndrome.

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