When accessing or allocating a resource, be sure to check its availability. For example, if the program must work with a file, then after opening, check the resulting pointer. The file containing the error will not be found. It is not necessary to hope that the files will always be in their place, because the user can intentionally or accidentally delete or move them, they can be "corrupted" due to a malfunction on the hard drive, etc.
The same applies to memory. When it is allocated, it is necessary to check the received pointer. It may happen that you requested too much memory or some kind of failure occurred. In Windows 2000/XP, such problems do not arise due to good "paging" and dynamic memory allocation, but the user can also work in Win9x.
If the program needs to work with devices such as a printer or sound card, then make sure you have them before attempting to use them. If you think that a computer without sound is now unthinkable, then you are greatly mistaken. My users often send in their configurations when they have problems and computers without sound are still encountered. It doesn't happen among Russian users, and abroad.
I'm not an analyst, and I can't even give an approximate ratio of computers with and without sound. But if without sound card will be at least 10%, it turns out that you ignore them all, because the program for such users will be unstable. Ten percent is not such a small number. If you count how many people in the world have a computer, you get a number with large quantity zeros.
I once wrote an economic reporting program (it was 1995 and I wrote in Pascal language) for large enterprise and did not follow the memory. Of course, then it was MS DOS and a computer only 386 DX with 8 MB random access memory. A very small stack was allocated for the program, and memory was severely limited. The report was small, so I hoped that there would be enough memory. There really were no problems for two months, and then the program issued one report in which the data was overestimated by 100 times, but I did not notice this and handed over the documents. When a company owns only a million, and the reporting has reached trillions, the director may have a heart attack. This one had a heart. Having carried out the calculation again, I achieved the correct results. It's hard to say what caused the crash, but after I added resource checking to the program, the problems never happened again.
After this incident, I always check for any allocated resources and do not rely on the "swap" files, because they can also run out if there is not enough free space on the disk.
Before each access to the file, be sure to check for its existence. There is no need to handle an error on open when there is a FileExists function that can be used to prevent future errors in advance. In Delphi, many lists (for example, TStrings) can be saved and loaded from disk using the SaveToFile and LoadFromFi1 e methods. When saving, an error can occur if the path is incorrect, and when loading, if the file is missing.
Consider an example. Let's say that the program has a TListBox list. We want all the elements of the list to be saved in a file when exiting the program, and loaded at startup. Let's look at the following OnShow event handler:
procedure TMainForm.FormShow(Sender: TObject): begin ListBoxl.Iterns.LoadFromFile("c:\list.txt"); // initialization
In this example, the lines from the file are loaded first, and then the remaining steps to initialize the program are performed. If the file does not exist, then an error will occur, after which the procedure will interrupt its execution and initialization will stop. All this can affect the operation of the program and the stability of the system.
It is possible to solve the problem in two ways:
To enclose the code for loading data from a file in a try. .except;
in advance to check the existence of the file.
Procedure TMainForm.FormShowCSender: TObject); begin if Fi1eExists("C:\list.txt") then try ListBoxl.Items.LoadFromFi1e("c-\lTSt.txt"): except
ShowMessage("File upload failed"):
end; // initialization end;
Website accessibility checker, primarily aimed at developers and owners of web resources. The service for checking the availability (status) of sites allows, in real time, to send a standard request to check the availability on user specified site (domain, blog, etc.), get and decode the answer. The site availability check service will help users determine the host name (Host), User-Agent and browser name, HttpRequest, the language preferences of the specified site, the parameters Accept, Accept-Language, Accept-Encoding, DNT, X-Requested-With , the value of the Referer field, track the type and parameters of Cookies used by the analyzed site.
In addition, the site accessibility check service allows users to receive and view content in textual form (in the form of pure HTML code) specified page site.
Checking the availability (status) of the site is a useful and popular option. Only on the technology portal site, the site availability check is performed in real time, contains the current one (current at the time of the check), is not cached and is not added to the database. The number of accessibility check requests from one IP address is unlimited, which allows our users to comfortably control the data provided by the site.
Please note that checking the availability (status) of the site, at the current time, is carried out from one of the 32 distributed access points. In turn, access points are randomly selected by the scanbot, without taking into account the geographic proximity and the current location of the site being tested.
The site accessibility check service is publicly available, the service is provided free of charge.
Every webmaster sooner or later faces the inaccessibility of the site. First of all, the site owner (or administrator) contacts the hosting provider's support service to find out the reasons. It's not quite the right approach, because there are many ways to independently find out the cause of the problem. Let's look at the main ones.
1. Whois check. First of all, I check the whois data, namely the NS records of the domain and the status (domain delegated or not). If problems arise already at this stage, it is worth contacting the domain registrar and restoring the functioning of the domain name. For whois checks you can use the resource http://who.is
2. Ping (ping) - this is the easiest way to check the availability of a resource. Open a command prompt on your computer (Start - Run - cmd.exe; if you have Linux or MacOS, use a standard terminal) and type ping command example.com , where example.com is your domain name. As a result of executing the command, we will get the ip of the server that is being accessed, the server response time and the number of transmitted / lost packets:
If there is no response from the server, the response takes more than 100-200 ms, a high percentage of losses, it makes sense to find out in which section the problem occurred. Perhaps the matter is not at all in the hoster, but, for example, in your provider. It may turn out that access problems did not affect the majority of site visitors, there is no reason to blame the host and prepare for the move.
There are services with which you can check the ping to the domain from different points. For example, http://just-ping.com
On this site, you only need to enter the domain name or ip-address, the availability of which you want to check. The availability of the resource will be checked from 50 different locations around the world. This method is more accurate than pinging from your computer, because it eliminates the possibility of problems on the side of your ISP.
The http://host-tracker.com resource has a wider functionality, which works on the same principle, but, in addition to the ping results, it also shows the server's http response, page size, time and page load speed. Since ICMP packets may be blocked by the server or router, there may be no ping to the domain, but the site will function without problems. In such situations, http://host-tracker.com is the best tool to check the availability of the site.
3. Tracing - this is a check of the signal route from the working PC to the site or server. In order to check the trace route to any resource, you need to command line Your PC to call the appropriate utility. If you have Windows, go to Start - Run - cmd.exe and in the menu that opens, enter tracert example.com. If you are on Linux or MacOS, run the traceroute example.com command in a standard terminal. The result of executing the command will be a list of all hosts on the route between your PC and the example.com domain
If any network node loses signal, a message is displayed that the node is unavailable or that the timeout has been exceeded. Look at whose area the problem arose, and draw conclusions.
4. Mtr (MyTraceroute) - utility that combines ping and traceroute. The utility displays in real time the route of packets from your PC to a domain or IP, displaying all nodes on the network, the response time from each node and lost packets. On Linux and MacOS the utility is called mtr, on Windows it is called WinMTR.
5. Proxy servers, anonymizers. To this type include resources that allow you to hide your real IP address or view how a resource is displayed for the region in which the proxy server is physically located. An example of such a resource is http://anonymouse.org. You just need to enter the address of the site you are interested in, and the resource will display the requested site. This method of checking the availability of the site is used when the Internet provider blocks access to the site, or the provider's DNS server cannot process the request.
Here are five basic ways to check the availability of a resource. It is recommended to perform a comprehensive analysis rather than using any one method. In this way, you can save time finding out the reasons for the inaccessibility of the site, because this is not always the fault of the hosting provider.
By creating their own Internet resource, each owner wants to know what his performance is after some time. This is especially useful and important for those who order the optimization and promotion of their resource, investing a lot of money in it.
How to know if your web resource is indexed search engines? Where can I find information about the IP of a domain or host? What is a ping or pinger and how will it benefit your website? How to get the domain dns? These and many other questions today are of interest to both beginners and experienced site owners, and each of them is looking for an answer to his question on various portals of the World Wide Web.
Checking ping, traceroute and server headers
Our tool is universal system site checker, which will help you find all the information you need in one place. Literally in a matter of minutes on our site you can get an up-to-date report on the status of your site, which will include the following indicators: traceroute and domain IP, ping IP and HTTP headers .. "What is needed for this?" - you ask. You just need to enter the address of your site in the appropriate field.
is the first assistant of any optimizer who will provide detailed information about domain dns and ping report.
Checking with tracert, trace, traceroute
When creating a report, we use a diagnostic tool such as tracert. Thanks to tracert, you can find out the route of requests and set the destination, which will correspond to the IP address you specified. Along with tracert, we use the trace method, which is widely used for debugging applications. Another router that our service uses is traceroute. At the same time, traceroute helps to determine if there are problems with the delivery of data along one or another intermediate route.
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For each site owner, it is important that his resource is always available to users. Otherwise, it may come out in big problems with search engines and site traffic. So let's start with, what is server uptime.
Uptime (uptime) is the time of continuous operation of the server (computer or hosting) from the moment it is turned on until it is turned off (as well as rebooting or freezing). The opposite concept of Downtime is the time during which the server is in the off state, and your site, accordingly, is not working.
At this time, when your site is down, it is still visited by users who see the inscription "Request timed out. Please, try again later" and leave disappointed. And also at this time the site can bypass search robot, which also sees the unavailability of the resource. As a result, your site is downgraded to search results and part of the pages flies out of the index.
Most high value uptime is 99.9% - you should strive for this ideal, you should not take it below 99%, otherwise it will result in the problems described above. Some hosting providers you will find an indication of uptime, some do not indicate it.
How to check website availability online
There are many different services, with which it is possible to carry out website availability monitoring.
- Host tracket - the advantage of the service is that it has a Russian interface. Although the service is paid, but free version Enough to track the work of 2 sites. The frequency of the check can be set independently from 1 to 60 minutes. Server failure messages will be sent to your email or phone.
- Uptime Robot - this service completely free of charge allows you to check the availability of the site every 5 minutes, and the number of sites here is up to 50 pieces. However, the downside is the frequency of monitoring - in these 5 minutes, server failures can happen, and you won't know about it.
- Montastic - the service is very simple, error messages are sent to email. The downside of the service is the frequency of checks - the minimum frequency is 30 minutes. The service is more suitable for those who just want to know when the site is working and when it is not.
- Pingdom is the most popular service uptime. With it, you can check the accessibility of the site from various parts peace, different countries or regions. You can set the check frequency from 1 to 60 minutes, failure messages will be sent to your email.
- Monitor is an American service, the advantage of which is that it provides all information about server operation in the form of tables, graphs and diagrams. In the free version, you can monitor only 1 site with a 30-minute check interval.
- Yandex Metrica - to determine the health of the site, the metric also checks server uptime, this is especially important when you use contextual advertising. You can use email or SMS to notify you of accessibility issues.
These are perhaps the main services that monitor the availability of servers, although there are a number of others, less well-known:
- LoadImpact
- BrowserMob
- Alertra
- You get a signal
- Wipmania
- Syslab
- Uptimerobot
To test the site you can choose any service you like and track the quality of your server. If its uptime is below 99%, then you should seriously think about changing your hosting provider.
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