Fwrite Fails w/ PHP Cron - php

Question

I have this issue where cron runs a php script every 5 minutes to update a list.
However, the list fails to update 5% of the time, and the list ends up blank. I don't believe it's related to cron, because I think I failed to manually generate the list twice out of like 100 tries.
What I believe it's related to is when the site has like 50+ people on it, it will fail to generate, perhaps being related to the server being busy. I added a check to make sure it's not MySQL not returning rows (which seems impossible) but it still does it leads me to believe fwrite is failing.
<?
$fileHandle = fopen("latest.html", 'w');
$links = array();
$query1 = $db_conn -> query("SELECT * FROM `views` ORDER BY `date` DESC LIMIT 0,20");
while ($result1 = $db_conn -> fetch_row($query1))
{
$result2 = $db_conn -> fetch_query("SELECT * FROM `title` WHERE `id` = '" . $result1['id'] . "'");
array_push($links, "<a href='/title/" . $result2['title'] . "'>" . $result2['title'] . "</a>");
}
if (count($links) > 0)
fwrite($fileHandle, implode(" • ", $links));
else
echo "Didn't work!";
fclose($fileHandle);
?>
Could there be a slight chance the file is in use so it ends up not working and writing a blank list?

Answer 1

$fileHandle = "latest.html", 'w');
I'm going to assume you mean
$fileHandle = fopen("latest.html", 'w');
the 'w' here opens the file, places the cursor at the start and truncates the file to zero length.
If you check count($links) before doing this you wont truncate the file when there is nothing to be written to it.
<?php
$links = "QUERY HERE AND HANDLE THE RESULTS (REMOVED)";
if (count($links) > 0)
{
$fileHandle = fopen("latest.html", 'w');
fwrite($fileHandle, implode(" • ", $links));
fclose($fileHandle);
}
else
{
echo "Didn't work!";
}
?>

Answer 2

Could there be a slight chance the file is in use so it ends up not
working and writing a blank list?
Well, yes. We don't know what other code you run that manipulates latest.html, so we can't really profile it.
Here are some suggestions:
Fix the syntax error in your file handler creation
You can acquire a fopen('w') handler to a file that has an existing fopen('r') process going on, so be sure to use PHP's flock while writing to the file to ensure other processes don't corrupt your list
Check to see what your logs have to say
Write to a string, then fwrite the entire string, so you spend less time in your inner loop with your file handler open (especially in this case where it doesn't eem that the string would be that long -- list of links)
Try outputting your links (datetamped) to a separate file besides latest.html; in the 5% chance when it fails, look back at the timetamped links and see how they compare. You can also include your query in that file so you can isolate if the issue is somthing to do with the DB or to do with writing to latest.html -- this will be especially useful in the case where your query (which isn't shown) possibly returns no results.

Answer 3

I think you are leaving yourself open to the possibility that the query is returning no data. The "removed" logic from your example may help shed light on what's going on. A good way of figuring this out is to write something to a log file, and check that log file after a few dozen iterations of your script. In the interest of having something in your latest.html file, I'd use file_put_contents over your current code.
<?php
$links = array();
$query = "SELECT links FROM tableA";
$result = mysql_query($links);
while ($row = mysql_fetch_row($result)) {
$links[] = $row[0];
}
if (count($links) > 0) {
file_put_contents('latest.html', implode(" * ", $links));
file_put_contents('linkupdate.log', "got links: " . count($links) . "\n", FILE_APPEND);
} else {
file_put_contents('linkupdate.log', "No links? [(" . mysql_errno() . ") " . mysql_error() . "]\n", FILE_APPEND);
}
?>
If we find no links, we won't overwrite the previous data file. If we encounter a MySQL error that might be causing the problem, it'll show up in the log output.
A read on the file shouldn't block a write, but switching to file_put_contents will help reduce the time the file is open and empty (there is some latency while you're performing the query and fetching the results).
Feel free to anonymize your query and post that as well - you definitely could have a problem with the result set since your code otherwise seems like it ought to work.