WordPress Write Cache Problem with Multiple Sessions

Question

I'm working on a content dripper custom plugin in WordPress that my client asked me to build. He says he wants it to catch a page view event, and if it's the right time of day (24 hours since last post), to pull from a resource file and output another post. He needed it to also raise a flag and prevent other sessions from firing that same snippet of code. So, raise some kind of flag saying, "I'm posting that post, go away other process," and then it makes that post and releases the flag again.

However, the strangest thing is occurring when placed under load with multiple sessions hitting the site with page views. It's firing instead of one post -- it's randomly doing like 1, 2, or 3 extra posts, with each one thinking that it was the right time to post because it was 24 hours past the time of the last post. Because it's somewhat random, I'm guessing that the problem is some kind of write caching where the other sessions don't see the raised flag just yet until a couple microseconds pass.

The plugin was raising the "flag" by simply writing to the wp_options table with the update_option() API in WordPress. The other user sessions were supposed to read that value with get_option() and see the flag, and then not run that piece of code that creates the post because a given session was already doing it. Then, when done, I lower the flag and the other sessions continue as normal.

But what it's doing is letting those other sessions in.

To make this work, I was using add_action('loop_start','checkToAddContent'). The odd thing about that function though is that it's called more than once on a page, and in fact some plugins may call it. I don't know if there's a better event to hook. Even still, even if I find an event to hook that only runs once on a page view, I still have multiple sessions to contend with (different users who may view the page at the same time) and I want only one given session to trigger the content post when the post is due on the schedule.

I'm wondering if there are any WordPress plugin devs out there who could suggest another event hook to latch on to, and to figure out another way to raise a flag that all sessions would see. I mean, I could use the shared memory API in PHP, but many hosting plans have that disabled. Can't use a cookie or session var because that's only one single session. About the only thing that might work across hosting plans would be to drop a file as a flag, instead. If the file is present, then one session has the flag. If the file is not present, then other sessions can attempt to get the flag. Sure, I could use the file route, but it's kind of immature in my opinion and I was wondering if there's something in WordPress I could do.

Answer 1

The problem goes like this with page requests even from the same session (same visitor), but also can occur with page requests from separate visitors. It works like this:

• If you are doing content dripping, then a page request is probably what you intercept with add_action('wp','myPageRequest'). From there, if a scheduled post is due, then you create the new post.

• The post takes a little bit of time to write to the database. In that time, a query on get_posts() may not see that new record yet. It may actually trigger your piece of code to create a new post when one has already been placed.

The fix is to force WordPress to flush the write cache appears to be this:

try {
  $asPosts = array();
  $asPosts = @ wp_get_recent_posts(1);
  foreach($asPosts as $asPost) {break;}
  @ delete_post_meta($asPost['ID'], '_thwart');
  @ add_post_meta($asPost['ID'], '_thwart', '' . date('Y-m-d H:i:s'));
} catch (Exception $e) {}
$asPosts = array();
$asPosts = @ wp_get_recent_posts(1);
foreach($asPosts as $asPost) {break;}   
$sLastPostDate = '';
@ $sLastPostDate = $asPost['post_date'];
$sLastPostDate = substr($sLastPostDate, 0, strpos($sLastPostDate, ' '));
$sNow = date('Y-m-d H:i:s');
$sNow = substr($sNow, 0, strpos($sNow, ' '));
if ($sLastPostDate != $sNow) {
  // No post today, so go ahead and post your new blog post.
  // Place that code here.
}

The first thing we do is get the most recent post. But we don't really care if it's not the most recent post or not. All we're getting it for is to get a single Post ID, and then we add a hidden custom field (thus the underscore it begins with) called

_thwart

...as in, thwart the write cache by posting some data to the database that's not too CPU heavy.

Once that is in place, we then also use wp_get_recent_posts(1) yet again so that we can see if the most recent post is not today's date. If not, then we are clear to drip some content in. (Or, if you want to only drip in like every 72 hours, etc., you can change this a little here.)

Answer 2

The key may be to create a semaphore record in the database for the "drip" event.

Warning - consider the following pseudocode - I'm not looking up the functions.

When the post is queried, use a SQL statement like

$ts = get_time_now(); // or whatever the function is
$sid = session_id();

INSERT INTO table (postcategory, timestamp, sessionid)
VALUES ("$category", $ts, "$sid")
WHERE NOT EXISTS (SELECT 1 FROM table WHERE postcategory = "$category"
    AND timestamp < $ts - 24 hours)

Database integrity will make this atomic so only one record can be inserted. and the insertion will only take place if the timespan has been exceeded.

Then immediately check to see if the current session_id() and timestamp are yours. If they are, drip.

SELECT sessionid FROM table WHERE postcategory = "$postcategory" AND timestamp = $ts AND sessionid = "$sid"