01.16.09
Posted in Web Development at 12:16 pm by BrianS
When designing the school reports I decided to use a GET request on form submission so that it would be easier to bookmark or share a report. I knew at the time that using GET might cause problems down the road, but the utility/programming trade-off seemed worthwhile.
Long URLs
The main problem with GET is that the total size a URL is allowed to be (and thus the amount of data passed) is fairly limited when compared to POST. The exact limit varies by browser and web server, but a practical limit is the lowest limit across platforms. In this case that is 2083 characters in Internet Explorer.
Since each report has to pass the list of included packets and items, the URL could easily surpass this limit. Recently one user generated a report that went over the IE limit. Figuring this out, though, proved to be somewhat difficult.
Held up at security
The user in question was using Firefox to access the utility. The report in question was generating and displaying just fine. However, when the user tried to download the report an error would result. It seemed odd that the browser would display the report on screen without issue then error out when attempting to download the same report.
I tried many options, but no matter what I did I was unable to download the report in Firefox. Next I tried the same report in IE and the situation started to clarify. IE would not even display the report. So now I was beginning to think that maybe the GET request was causing problems. Still it was difficult to fathom Firefox displaying the report on screen but refusing to download it.
A little web research finally clarified everything. Firefox attempts to honor the Windows Internet security zone settings for downloaded files. In order to apply the security zone settings Windows has to pass the URL to the IE engine for analysis. Firefox allows URLs of significantly larger size than IE, so the same URL that displays correctly on Firefox will thus cause IE to error out. Since Firefox attempts to honors the security settings, when IE coughs up a hairball due to the URL length the download is aborted.
Solving the problem
Initially I thought to just shorten the request by modifying the variable names on the form. This worked well enough for the problem at hand, but I could easily see having to address the problem again in the future. In the end I decided to modify the code behind the page so that the report parameters are submitted by POST instead of GET. It required more work up front, but will prevent this problem from cropping up in the future.
Plus, the utility has an option to save reports, so bookmarking/sharing is not as much of an issue.
References
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01.15.09
Posted in CakePHP, PHP, Web Development at 2:44 pm by BrianS
I’m still learning CakePHP. My most recent experimentation is with forms and URL redirecting. In the process I noticed that if the URL includes an ampersand, even in URL-encoded form, CakePHP will read in the URL incorrectly.
In my case, I was taking a form submission and redirecting it to a new page, appending the submitted data as URL parameters (e.g. of the form /controller/action/name:value). CakePHP handles ampersand in POST-submitted forms just fine, but during the redirect I noticed that the value of the parameter was cut off at the ampersand. For example, I’m creating a search form for a list of items (located at /items/index). When you submit the form with a value of “ball & chain” the controller sees it just fine. The form is submitted to the search action (/items/search) which takes the values, constructs a new URL, and redirects back to the item list (/items/index/Search.keywords:ball+%26+chain). However, the index action only sees “ball ” … the rest of the value is assumed to be additional key/value pairs in the querystring.
This problem seems like a pretty major issue to me. A bug report has already been filed, but I don’t believe it will be addressed anytime soon. And for good reason, the bug is actually in Apache’s mod_rewrite module rather than in CakePHP. When mod_rewrite processes a URL it unescapes any escaped characters (newer releases unescape to two levels). What this means for CakePHP is that any escaped characters in the URL become normal characters in the querystring (%26 becomes &).
The best overview of the problem has a good work-around, but it requires modification of the core CakePHP scripts. I’ll leave that to the experts. On our install I’ve found that a triple-escaped value will only be unescaped twice, resulting in the proper escaping after being parsed by mod_rewrite.
References
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11.21.08
Posted in Web Development at 3:27 pm by BrianS
Float styling is a useful aspect of CSS … until it’s not. When a floated element occurs inside a container and you want it to be wholly in that container you have to resort to hacks to do so. Or do you? Never content to assume that a question has been resolved, I decided to see if anyone had come up with a new method of clearing a float. Lo and behold a solution has emerged, and the solution is so simple as to defy logic: apply overflow: auto to the container element. I’ll leave the full explanation (with examples, alternatives, and warnings) to the experts.
Of course, there can be advantages to the default float rendering. Most notably I found it useful for creating some nice formatting around blockquotes, such as in our textbook analysis report.
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Posted in MySQL, PHP, Web Development at 2:28 pm by BrianS
With a large set of test packets in need of transcription, little time to do so, and even less desire to spend the money on temporary workers, I needed to think of some other way to perform the task. Luckily I was able to come up with a method using a tool from Amazon that proved relatively easy to implement. Mechanical Turk .
Read the rest of this entry »
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10.10.08
Posted in Adobe Illustrator, Web Development at 2:50 pm by BrianS
Elements with opacity styling are stacked independent of their containing object. As a result, any positioned elements that do not have a z-index declaration and that appear in the HTML code prior to an opacity-styled element will appear underneath the opacity-styled element (see example). This is the defined behavior in the CSS specification, though currently only gecko- and WebKit-based browsers (Firefox, Safari, Chrome) exhibit the “quirk” in question. From the CSS3 Working Draft:
… implementations must create a new stacking context for any element with opacity less than 1.
CSS Color Module Level 3: Working Draft 21 July 2008
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10.07.08
Posted in MySQL at 2:38 pm by BrianS
I was trying to find a way to update some rows in a MySQL table where the condition for performing the update was based on a column in another table. Normally this isn’t an issue, just run UPDATE table ... WHERE key_column IN (SELECT key_column FROM ...).
I was having some problems, however, because the table that needed to be updated uses a mutli-column primary key. Normally I would select the relevant rows using a JOIN. Unfortunately, you can’t update the results of a JOIN. How then to select the relevant rows?
Turns out it’s easier than I thought. You can still use the WHERE ... IN clause, you just specify all of the columns that satisfy the condition: UPDATE table ... WHERE (key_column1, key_column2) IN (SELECT key_column1, key_column2 FROM ...).
I did not see any information about this functionality in the MySQL Reference Manual (1, 2).
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10.03.08
Posted in Apache, System Administration at 1:57 pm by BrianS
A note way past due.
I updated the log format for flora.p2061.org (but not the other sites on flora). Some information is not included in the default “combined” format that could help with debugging (such as user cookies). I set up a new format that is similar to the IIS W3C extended format. Two values not included that could prove useful are bytes received (%I) and bytes sent (%O). The logio_module has to be enabled to record this information. It would probably be worthwhile to add this functionality once we start placing more public content on the server. For now the following format suffices:
#Fields: date time c-ip cs-host cs-version cs-method cs-uri-stem cs-uri-query cs(Referer) cs(User-Agent) cs(Cookie) sc-status time-taken
LogFormat "%{%Y-%m-%d %H:%M:%S}t %h %V %H %m %U %q %{Referer}i %{User-Agent}i %{Cookie}i %>s %D" webapp
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08.14.08
Posted in PHP, System Administration at 4:32 pm by BrianS
I needed to do a mass resampling of around 280,000 images. There are a number of ways to do this, but I settled on doing it via PHP because the images are stored on our web server, the total size of the images is large (~10GB), and I didn’t want to kill my machine trying to get it done.
PHP is ideal for a task such as this: parsing directories and subdirectories for images is easy; resampling using the built-in library (GD) is a breeze; specifying the destination as a subdirectory is simple. The one minus was processor usage. Performing image manipulation eats up the CPU in a big way.
Luckily linux systems have a built-in utility for addressing a situation like this: nice. nice will “run a program with modified scheduling priority.” I’m running the image manipulation script using the following command:
nice --adjustment=19 php script.php
If nothing else is going on the script will use whatever resources are available. When anything with a higher priority executes, that program will take precedence over the script with regard to system resources. The script should thus not affect the responsiveness of the web server. This is the reason I was searching for this kind of functionality.
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07.25.08
Posted in Adobe Illustrator, MySQL, Web Development at 9:54 am by BrianS
After creating a new structure for the current year’s piloting data there has been a bit of disconnect in development between the two data sets. There’s just not enough time to ensure that everything works across both data sets. There were basically two choices to moving forward: 1) construct a view of the new data structure that mimics the old data structure; 2) port the old data structure to the new one and continue redevelopment of the scripts. I chose the latter, mainly because there are some improvements I’d like to make to the interface in the process.
I created a series of SQL statements to run in MySQL that will convert the data from the packet_item_records and miscon_packet_refs tables to the packet_students, packet_data, and miscon_packetdata_refs tables. After the conversion I updated any pages that referenced the old data structure. So far in testing the data seems to have converted perfectly.
Minus one issue. Multiple selections for the answer choice questions (A, B, C, D) from 2006 were recorded as a generic Multiple rather than Y+NS, N+NS, etc. This value is not represented in the updated data format or on the data entry forms or summary tables. Rather then spend too much time addressing this issue I’m going to leave these values empty for now. I don’t expect this to be a problem since the researchers are focused on data for the current pilot and field tests. Also, I’m keeping the old version of the piloting data and scripts that interact with it online in case it’s needed.
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06.19.08
Posted in PHP, Web Development at 3:13 pm by BrianS
I was tasked with creating a demonstration of a dynamically generated graph for one of the grant proposals being worked on at present. This isn’t something we’ve really done before in PHP, but I had a feeling it would not be a unique requirement. A quick search revealed that a PEAR package exists for just this purpose: Image_Graph.
Installation was not too difficult, though I did have to ensure that a few dependencies were installed before PEAR would load the package. Documentation for the module is not that great, but there are a number of samples and a fairly active discussion forum.
I did not find the graphing object very intuitive. It seems to me that a lot of the properties and functions are abstracted from the objects they affect. I suspect this was a design decision to allow for greater flexibility, but it may also be due to an older coding style. The current branch appears to have been developed in earnest starting in 2005, and it’s been a few years since a new release. Even so, my initial exposure leads me to believe the package may be stable enough for even a production-level project so long as sufficient QA has been performed.
It took me only a couple of days to hack together the demonstration. Though the requirements of the script were few, I decided to push the limits of my object-oriented PHP and create the graphing component as a class. Overall it was a good first-try at producing something of this nature. You can find the demonstrator at http://flora.p2061.org/weather_data/.
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