Home > Articles > Operating Systems, Server > Solaris

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

Qualifying an Application

There are a number of characteristics used to determine whether an application can be made highly available using the Sun Cluster 3 framework. If the application does not possess these attributes, then making it highly available may require modification to the application itself rather than simply creating start and stop methods within the framework.

Here are the key points to consider when making an application highly available.

  • Data service access
  • Crash tolerance
  • Bounded recovery time after crash
  • File location independence
  • Absence of a tie to the physical identity of the node
  • Ability to work on multi-homed hosts
  • Ability to work with logical interfaces
  • Client recovery

Let's look at each of these in detail to see what's required.

Data Service Access

Generally speaking, the only applications that can be effectively made highly available under the Sun Cluster framework are client-server applications that receive discrete queries from an IP-network-based client. Examples of this type of service include databases and Web (HTTP) servers.

Applications requiring a persistent network connection are slightly less-suited to the HA model, since continuing the service after a failover requires a reconnection, which usually means you lose any state that was created up to the point of failure. Examples of this type of service include terminal connections, telnet, and FTP.

This does not mean that persistent-connection data services cannot get any advantage from being included in the HA framework. Because the HA failover emulates a very fast reboot of the server, the application becomes available again more quickly than it would on a stand-alone server. It does mean, however, that the client software or end user must be able to cope with this kind of break in the service. For example, an FTP client that automatically retries a download if the connection is broken and continues from where it left off would overcome this problem.

Some applications do not use any kind of network connection for client access, or may not have any kind of client access at all. In these cases, there may be some difficulty in making the application highly available using the Sun Cluster framework, particularly if some sort of fixed line (such as a serial terminal connection) is required. This is because the Sun Cluster framework has built-in mechanisms for migrating IP network addresses between nodes, but does not have a similar facility for other access methods. Fortunately, in modern computing environments, most applications subscribe to the IP client/server model or have limitations that can be worked around quite easily (see "Getting Around Requirements" later in this chapter).

Crash Tolerance

Since the Sun Cluster HA model essentially emulates the fast reboot of a failed system, the application must be able to put itself into a known state when it starts up. Generally, this means that the application state should be committed to persistent storage (on disk) rather than being held in RAM. Depending on the application, it may be necessary for some sort of automatic rollback recovery or consistency check to be performed each time the application starts, to ensure any partially completed transactions are properly taken care of. If an application uses techniques such as a two-phase commit to write data to disk, then it is usually more easily able to recover after a system failure.

Regardless of how they write persistent data, some applications still require manual intervention to start (classic examples include applications utilizing some form of security, and that require a passphrase to be entered at startup). For these applications, it may be possible to achieve a work-around (such as piping responses in from a file). Care must be taken, however, to pay attention to the security and procedural implications of this sort of work-around.

Typically, if an application is automatically started at boot time (with a script in /etc/rc3.d, for example) then it is usually safe to run as a cluster-controlled application without much (or any!) modification.

Bounded Recovery Time

When applications are restarted by the Sun Cluster framework, there needs to be some reasonable (and predictable) limit on how long it will take to recover. Obviously, the amount of time required will depend on what sort of application is involved; for example, a large OLTP database will almost certainly take longer to return to a consistent state than a small Web server due to the relative frequency of data changes.

Some limit is needed because the cluster framework needs to be able to determine when an application is not going to restart, which is particularly important if the cluster is attempting to restart the application on the same node. After the limit has expired, the cluster can take appropriate action (such as failing the application over to a different node).

File Location Independence

The location of files and data used by the data service is very important, since this information has to be shared by each node that will potentially run the application. For this reason, application and configuration data locations (or file paths) should not be hard-coded into the application itself.

Applications should store any changeable data, including configuration information, on the shared storage of the cluster—either on a globally mounted filesystem, a failover filesystem, or on globally accessible raw devices. This ensures that the data and behavior of the application are consistent across nodes of the cluster. The upshot of this is that there must be some way of defining to the application where the data and configuration files are stored in the filesystem, such as a command-line argument to the application program binary.

If the paths to data or configuration files are hard-coded into a program, then you can sometimes use symbolic links to overcome the problem. However, be aware that if an application completely removes and recreates a given file that has been redirected using a symbolic link, the behavior may not be as expected. For example, if the directory /var/myapp/data is actually a symbolic link to a globally mounted directory /global/myapp/data, then an application accessing /var/myapp/data/foo will get the correct file (see Figure 4.1). However, if the application unlinks the directory /var/myapp/data and recreates it, the symbolic link may be destroyed. This means that new data will be created in a directory /var/myapp/data that is not accessible to any other nodes in the cluster (see Figure 4.2).


Figure 4.1 Using symbolic links to access global data


Figure 4.2 Symbolic links after directory recreation

You may also want to consider where to store your application binaries, and there are good arguments each way as to whether application binaries should be installed on shared storage or installed individually on the local storage of each node. With binaries stored locally, it is possible to perform rolling upgrades of the application software by performing the upgrade on the standby node and then manually switching control to that new node as the original node is upgraded. This maintains the availability of the data service to clients, but introduces possible management problems if a failure occurs partway through an upgrade or if the data format is different between application software releases. With binaries installed on shared storage, there is only one copy of the software to manage and an upgrade is performed by scheduling downtime for that service as the installation occurs. This consideration comes down to operational preference, rather than any technical reasoning.

Absence of a Tie to Physical Identity of Node

Central to the understanding of Sun Cluster systems is the concept that a highly available IP address (otherwise known as a logical host) may operate on one of a number of physical computers at any time, and may in fact move to another computer under certain circumstances. For this reason it is important that applications be able to use the logical hostname rather than the physical name of the host on which it is running. In short, the question to ask yourself is: Can the application provide its service using a hostname that is not the physical hostname of the node?

If an application's configuration is dependent upon the physical hostname, then it almost certainly cannot be made highly available, since failing the application over to a node with a different physical hostname would render it inoperable.

If a program binds its network connection to the special address INADDR_ANY—a wildcard term that means the application will bind to all available IP addresses on the node simultaneously (see in(3HEAD))—then this usually satisfies the requirement.

Ability to Work on Multi-Homed Hosts

A multi-homed host is a host that is connected to more than one public network. Each node may have multiple interfaces, allowing the cluster and its data services to appear on more than one network and also allowing for hardware redundancy. An application should not assume that it must bind itself to the first network interface it can find because this may not be the correct one. In short, the question to ask is: Can the application cope with a host that has more than one network interface?

A data service that binds to a host's IP address must be flexible enough to bind to any and all of the IP addresses specified by its related logical host resource. The simplest way to do this is to have the application bind to INADDR_ANY (most modern network applications do).

In some circumstances, however, this approach is not desirable. Consider the case where two data services provide some service on different IP addresses but on the same IP port. If at some point both services are mastered by the same physical node, having either service bind to all IP addresses would mean the other could not bind to that address and the service will fail. In this case, or in the case where binding to INADDR_ANY is not possible, the application must have some configuration option to specify which IP address or port to bind to.

Ability to Work with Logical Interfaces

In some instances, a cluster node may control more logical hosts (and therefore IP addresses) than it has physical network interfaces. To cope with this, the network interfaces are assigned more than one IP address, a technique sometimes called IP aliasing. IP addresses are dynamically added to and removed from physical hosts as they master the logical hosts, by adding logical interfaces to the physical network interfaces already on the node. Logical network interfaces are labelled the same as physical interfaces, but with an additional number part, for example:

hme1, qfe3

physical interfaces

hme1:1, qfe3:2

logical interfaces

The data service should be able to deal with a given physical interface having more than one IP address. Again, INADDR_ANY usually makes this an easy task, but occasionally an application will try to manipulate network traffic in particular ways that make it unable to manage more than one IP address per interface. In particular, an application may not recognize the logical portion of an interface (for example, :1), and incorrectly perform operations on the physical interface instead (for example, hme1). In these cases, it may not be possible to make the application highly available. In short, the question to ask is: Can the application cope with more than one IP address on a single network interface?

Client Recovery

As indicated in the previous discussion of the data service access, the most effective HA data services include some capacity in the client to automatically retry a query when the first one is cut off or times out. If an automated retry facility is not feasible, then the end user at least has to be comfortable with the concept of manually retrying a query, such as when an HTTP query from a WWW browser fails.

Requirements for Scalable Services

If you want to make your application into a scalable service (that is, a service that operates on multiple nodes at the same time), you will have to consider a number of additional requirements. We'll leave these until we investigate scalable services in detail in Chapter 11, "Writing Scalable Services."

  • + 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.


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.


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.


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.


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


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


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.


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.


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