How to lock database for Laravel's `firstOrCreate`? - php

Question

We currently encounter a Duplicate entry QueryException when executing the following code:
Slug::firstOrCreate([
Slug::ENTITY_TYPE => $this->getEntityType(),
Slug::SLUG => $slug
], [
Slug::ENTITY_ID => $this->getKey()
]);
Since the firstOrCreate method by Laravel first checks if the entry with the attributes exist before inserting it, this exception should never occur. However, we have an application with million of visitors and million of actions every day and therefore also use a master DB connection with two slaves for reading. Therefore, it might be possible that some race conditions might occur.
We currently tried to separate the query and force the master connection for reading:
$slugModel = Slug::onWriteConnection()->where([
Slug::SLUG => $slug,
Slug::ENTITY_TYPE => $this->getEntityType()
])->first();
if ($slugModel && $slugModel->entity_id !== $this->getKey()) {
$class = get_class($this);
throw new \RuntimeException("The slug [{$slug}] already exists for a model of type [{$class}].");
}
if (!$slugModel) {
return $this->slugs()->create([
Slug::SLUG => $slug,
Slug::ENTITY_TYPE => $this->getEntityType()
]);
}
However the exception still occurs sometimes.
Our next approach would be to lock the table before the reading check and release the lock after the writing to prevent any inserts with the same slug from other database actions between our reading and our writing. Does anyone know how to solve this? I don`t really understand how Laravel's Pessimistic Locking can help solving the issue. We use MySql for our database.

Answer 1

I would not recommend to lock the table, especially if you have millions of viewers.
Most race-conditions can be fixed by locks, but this is not fixable with locks, because you cannot lock a row that does not exist (there is something like gap locking, but this won't help here.).
Laravel does not handle race-conditions by itself. If you call firstOrCreate it does two queries:
SELECT item where slug=X and entity_type=Y
If it does not exists, create it
Now because we have two queries, race condition is possible, meaning two user in parallel reach step 1, then both try to create the entry in step 2 and your system will crash.
Since you already have a Duplicate Key error, it means you aleady put a unique constrain on the tuple on the two columns that identify your row, which is good.
What you could do now, is to catch the duplicate key error like this:
try{
$slug = Slug::firstOrCreate([
Slug::ENTITY_TYPE => $this->getEntityType(),
Slug::SLUG => $slug
], [
Slug::ENTITY_ID => $this->getKey()
]);
}
catch (Illuminate\Database\QueryException $e){
$errorCode = $e->errorInfo[1];
if($errorCode == 1062){
$slug = Slug::where('slug','=', $slug)->where('entity_type','=', $this->getEntityType())->first();
}
}

Answer 2

one solution for this is to use Laravel queue and make sure that it runs one job at a time, in this way you will never have 2 identical queries at the same time.
for sure this will not work if you want to return back the result in the same request.