usleep() in while loop causing issues - whole process freezes - php

Question

I am using php, Laravel, Redis, and SQL on an Ubuntu localhost server. I have made a bunch of methods that return results from API searches after some processing. I am calling 5 of these methods which will be very slow if done synchronously, so I've been experimenting with async approaches (which I know php isn't optimised for). After a few approaches I have found some success with pcntl_fork(), but I'm running into some nasty problems.
Edit: After some messing around I have found that if I remove the while loop then the code afterward executes properly, I have removed the while loop and placed it in the second 'search' method. However it still causes a freeze of the system. This makes no sense as there shouldn't be an infinite loop as if I manually query the Redis db, all 5 results are there.
This is my code: (I have a few custom classes for making and processing the API calls, fyi these methods work flawlessly)
//this caches the individual api results to a Redis list
public static function cacheAsyncApiSearch(string $searchQuery, int $maxResults = 20)
{
$key = "search:".$searchQuery; //for Redis
if(!Redis::client()->exists($key)) {
for ($i = 0; $i < 5; $i++) {
// Create a child process
$pid = pcntl_fork();
if ($pid == -1) {
// Fork failed
exit(1);
} elseif ($pid) {
// This is the parent process
// I have tried many versions of pcntl_wait, none work! They all still don't allow code to be ran afterwards (even within this elseif block), and the best it does is cache the 1st api case (YouTube)
// while (!pcntl_wait($status, WNOHANG)) {
// $exitStatus = pcntl_wexitstatus($status);
// // Do something with the exit status of the child process
// }
// dd($pid);
// pcntl_waitpid($pid, $status, WUNTRACED);
} else {
//child processes
switch ($i) {
case 0:
$results = YouTube::search($searchQuery, $maxResults)['results'];
Redis::client()->rPush($key,SearchResultDTO::jsonEncodeArray($results));
SearchResultDTO::convertResultDTOToModels($results);
break;
case 1:
$results = Dailymotion::search($searchQuery, $maxResults)['results'];
Redis::client()->rPush($key,SearchResultDTO::jsonEncodeArray($results));
SearchResultDTO::convertResultDTOToModels($results);
break;
case 2:
$results = Vimeo::search($searchQuery, $maxResults)['results'];
Redis::client()->rPush($key,SearchResultDTO::jsonEncodeArray($results));
SearchResultDTO::convertResultDTOToModels($results);
break;
case 3:
$results = Twitch::search($searchQuery, 2)['results'];
Redis::client()->rPush($key,SearchResultDTO::jsonEncodeArray($results));
SearchResultDTO::convertResultDTOToModels($results);
break;
case 4:
$results = Podcasts::getPodcastsFromItunesResults(Podcasts::search($searchQuery, 2)["response"]->results);
Redis::client()->rPush($key,SearchResultDTO::jsonEncodeArray($results));
SearchResultDTO::convertResultDTOToModels($results);
break;
}
$i = 10000;
exit(0);
}
}
// for noting the process id of the given process that gets to this point
Redis::client()->lPush("search_pid:".$searchQuery, $pid);
// sets a time out for the redis cache
Redis::client()->expire($key, 60*60*4);
while (is_numeric( Redis::client()->lLen($key)) && Redis::client()->lLen($key) < 5) {
usleep(500000); // 0.5 seconds
// pcntl_waitpid(-1, $status); //does this even do anything? not for me
}
return false; // not already cached
}
return true; // already cached
}
This code somewhat works, It performs the api calls and caches the Redis perfectly. However when the method is ran, no code will be ran after it (unless redis has found a cached version and the process is not forked).
This made me think that all processes are being exited (possibly true? if so i dont know why), so I tried writing a version without the exit(0) line. This works, I can then perform code after the method call, however I noticed (when getting SQL race conditions) that all 6 (5 child, 1 parent) processes continued to run their own version of the code after this method (e.g. some database writes)
public static function search(string $searchQuery, int $maxResults = 20): array
{
$key = "search:".$searchQuery;
$results = [];
// the quoted method above
self::cacheAsyncApiSearch($searchQuery, $maxResults);
foreach (Redis::client()->lRange($key,0,-1) as $result){
$results = array_merge($results, SearchResultDTO::jsonDecodeArray($result));
}
$creatorDTOs = [];
$videoDTOs = [];
$streamDTOs = [];
$playlistDTOs = [];
$podcastDTOs = [];
/** #var SearchResultDTO $result */
foreach ($results as $result) {
match ($result->kind) {
Kind::Creator => $creatorDTOs[] = $result,
Kind::Video => $videoDTOs[] = $result,
Kind::Stream => $streamDTOs[] = $result,
Kind::Playlist => $playlistDTOs[] = $result,
Kind::Podcast => $podcastDTOs[] = $result,
};
}
// did this to test how many times the code was being ran (the list has 6 1's in it)
Redis::client()->lPush("here", '1');
// I know this code isn't completely efficient since I already called these conversion methods before, however I am just trying to get the forking stuff to work right now.
return [
"creators" => SearchResultDTO::convertResultDTOToModels($creatorDTOs),
"videos" => SearchResultDTO::convertResultDTOToModels($videoDTOs),
"streams" => SearchResultDTO::convertResultDTOToModels($streamDTOs),
"playlists" => SearchResultDTO::convertResultDTOToModels($playlistDTOs),
"podcasts" => SearchResultDTO::convertResultDTOToModels($podcastDTOs)
];
}
These DTO's (Data Transfer Objects) are being used to populate a UI. So for example, when I make a search (that isn't cached), the page is blank forever. But if I refresh the page (after the search is cached) then the results show just fine.
This is the most bizarre problem I have ever ran into and I really appreciate any help.
Edit please read:
After some messing around I have found that if I remove the while loop then the code afterward executes properly, I have removed the while loop and placed it in the second 'search' method. However it still causes a freeze of the system. This makes no sense as there shouldn't be an infinite loop as if I manually query the Redis db, all 5 results are there. And the dd("two") can never be excecated unless the usleep() is removed. Hopefully this narrows the problem down.
Edit 2 please read:
I have figured out that I can get the dd("two") to work when usleep() is reduced to 0.05s from 0.5 seconds, but it still doesnt seem to run long enough for it to work.
if(!self::cacheAsyncApiSearch($searchQuery, $maxResults))
{
// make sure Redis is properly returning a number not object
$len = Redis::client()->lLen($key);
while(!is_numeric($len)){
usleep(500000); // 0.5 seconds
$len = Redis::client()->lLen($key);
}
//dd($len); //this dd() works
while ($len < 5) {
dd("one"); // this dd() works
usleep(500000); // 0.5 seconds
dd("two"); **//$this does not work, why?**
$len = Redis::client()->lLen($key);
}
}