Availability
Availability Availability of a site shows if a site is accessible from the Internet.

Availability of a site shows if a site is accessible from the Internet. This term is also about access to a server, database or another entity. As all objects in the Internet are based on comlicated infrastructure, nothing can be available always. The question is how to measure it, what is normal and if we may affect it anyway. The measurement of availability is connected to terms Uptime and Downtime, which show for how long a site is unavailable. However, these values can not create the full view of the situation. There could be many reasons of unavailability - server or connection problems, application error, overload of the site and so on. To avoid or shorter the failures, it is needed to supervise the site and analyze the errors, if they happen. For these purposes there are special monitoring tools, which help to increase to availability of the site, collect statistics and provide necessary information about the errors for fast repair. These tools are called Website Monitoring services.

  • CM.Glossary.WebsiteMonitoring
  • CM.Glossary.Downtime
  • CM.Glossary.Uptime
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"To have a record of up-time of a Web site seems a very important concept. So what you offer seems interesting."
- Gerardo
Host-Tracker under Windows Azure
Those, who actively involved with the Web, should know Host Tracker, a company from Ukraine, which has been supporting one of the leading global web monitoring services since 2004. Its goal is to monitor site health and accessibility in near-real-time access. Using alert message system, Host Tracker allows to reduce downtimes, to improve quality of service for users, to quickly localize troubles...
Those, who actively involved with the Web, should know Host Tracker, a company from Ukraine, which has been supporting one of the leading global web monitoring services since 2004. Its goal is to monitor site health and accessibility in near-real-time access. Using alert message system, Host Tracker allows to reduce downtimes, to improve quality of service for users, to quickly localize troubles, and etc.
Architecturally, Host Tracker includes a server-based hub, acting both as a data collector and control center, and a series of software agents, launched in various regions – typically using the equipment operated by major providers, hosters and affiliates. The geographically distributed architecture provides common system reliability and also allows collecting data in terms of access speed, bandwidth and other key performance characteristics on regional level – a critically important feature for the international business.
The first version of Host Tracker, which is still functioning and providing services for tens of thousands of customers, was Linux based. Today, it is supported by nine control servers, located and organized in two DPCs on collocation principle, and few dozens of agents. Considering that the final objective of web monitoring is focused on increasing the uptime of client-based web resources – whereas 95% of Host Tracker customers were able to increase it up to 99% – then, performance and accessibility of the service itself are not just critical, but rather fundamental parameters that influence the whole business. Theoretically, Host Tracker should demonstrate accessibility close to 100%. However, an extensive growth of the service made this task hard to solve.
Host Tracker was facing constantly increasing network traffic – a problem for seamless operation of the service. Inability to add new control servers on-the-fly, difficulties when maintaining not uniform and multiple-aged hardware was another limiting factor. Moreover, the desire to develop the service through wider protocol and network service support was meeting certain obstacles. “Unfortunately, for Linux there was a limited choice of ready-to-use solutions and libraries, while inventing something completely new was difficult”, says Artem Prisyazhnyuk, Host Tracker director. “We had an idea of reviewing the stack of technologies we used for a more sophisticated one and after taking a closer look at the .NET platform, its potential in terms of scalability and network support, I realized that was exactly the thing we had been looking for.”
It was sure that migrating to a completely different platform should be a complex task – the project extended over three years. However, it was like blessing in disguise: during this period, the world has seen the cloud computing that seemed an ideal tool for solving both the scalability problem and putting aside one’s own whole infrastructure. Besides, the PaaS model allowed to remove most of the effort in terms of administering the solution and to control the application as a self-contained entity, to the extent of complete automation, and thus, Windows Azure had in fact no alternatives.
As a result, the second version of Host Tracker, commercial operation of which started in May 2012, is already functioning under Windows Azure. Its central ingredient is realized as Web Role and associated with SQL Azure Database – it provides external portal, analytics and report generation, control of monitoring applications. The latter are ensured with instances of Worker Role, which also use SQL Azure Database to store their data and to provide the service scalability depending on the network loading. Agents are functioning as they did before, with the viability of their transfer to Windows Azure being considered.
Now, Host Tracker uses HTTP/HTTPS and ICMP protocols to monitor specific ports, including various methods (HEAD/POST/GET), and etc.



Alarm reporting is available via email, SMS and instant messages. The customer can receive reports with statistics about resources being controlled and their performances. You can spend only 6 minutes to make monitoring settings for five sites, while the average response time in case of failure is limited by a couple of minutes, and it takes 1-3 minutes more to inform the customer about the problem. Using this service, anyone can check any site, including access from various regions.
As a result, if on the one side the transfer to the .NET platform itself gave us the potential to modernize Host Tracker, to optimize the application architecture and realize new internal functions, then, on the other side, the migration to the cloud allowed to refuse from less important, though time consuming activities such as administering the solution, and, first of all, to reach necessary performance indicators. Microsoft, for all basic Windows Azure services, declares 99,9% accessibility and guarantees monthly refunds, should this indicator be lower. This creates a firm ground for operating such services like Host Tracker, as the accessibility is the most critical parameter for these applications. Using the cloud infrastructure also provides a better protection for the service: unauthorized access to the application and many types of attacks are effectively excluded, while the data safety is ensured by triple reservation.  
Host Tracker received another advantage from abandoning its own infrastructure. The service’s performance characteristics are also rather critical, for they directly affect the failure reporting system operation. In this respect, Windows Azure is virtually a drainless source of computing power. This means that by timely starting additional monitoring instances you can support Host Tracker functioning parameters on the necessary level. Moreover, the cloud environment is exactly what you need in order to make this process almost fully automatic, excluding further need for direct control.
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