Redis keys are not expiring - Laravel, Predis

Question

I am using Laravel 5.4 , with Predis and the latest Redis (or Redis for Windows ).

The keys are being saved without issue. So, I doubt it's a configuration issue.

The problem is that they are not expiring. The key is reused until it expires...similar to how a session works.

I create the key once, if it does not exist. In that same logic I then set the expiration time.

In the Controller, I have

use Illuminate\Support\Facades\Redis;

In one of the functions, Get the connection instance :

$redis = Redis::connection();

Before I create the key, I check the existence (simplified) then create and set expiration.

if(!$redis->exists($some_unique_key))
{
   //set the key
   $redis->set($some_unique_key, 'Some Value'));
   //set the expiration
   //I understand this means expire in 60s.
   $redis->expire($some_unique_key,60); 
}

Why could it not be expiring the key?

As I mentioned, everything else works. I see the key updating without issue if I monitor, and can query it.

For the record, I have read:

• Why Redis keys are not expiring?
• Laravel flash or session messages not expiring [ not maintained Updated ]
• my redis keys do not expire

There is nothing on expiration on the Laravel documentation:

• https://laravel.com/docs/5.4/redis

UPDATE 1

Investigating a possible cause where setting(updating) the key resets the expiry

UPDATE 2

Used @for_thestack's reasoning (in REDIS commands) to come up with the solution. See my answer with the code. Feel free to upvote @for_thestack :)

Answer 1

For those who uses Laravel, it is possible to use EX param (expire resolution) + ttl. In below example EX means that TTL is expressed in seconds (see Redis docs: https://redis.io/commands/set ). So the result of below will be that $val stored under $key will be removed after 35 seconds.

Redis::set($key, $val, 'EX', 35);

In predis u can use the same, actually Laravel uses predis under the hood.

Answer 2

Some other process might call SET to update the key-value pair, in this case, the expiration will be removed.

// set expiration
EXPIRE key expiration_in_seconds
// update key-value pair with no expiration
SET key new_value
// now, expiration has been reset, and the key won't be expired any more

In order to keep the expiration, when you update the key-value pair, you should call SET with expiration parameters.

// get TTL, i.e. how much time left before the key will be expired
TTL key
// update with expiration parameter
SET key new_value EX ttl

You can wrap the two commands into a lua script to make it atomic. And you also need to take care of the case that key doesn't exist when you call TTL . See the document for details.

Answer 3

Since @for_stack provided me with the logic(in REDIS commands & logic), I accepted his contribution as the answer.

My problem was that I did not know that set ing the key, resets the expiry. So making it work, as explained by @for_stack involves:

1. getting the TTL if the key exists
2. after updating the key, set the expiration to the TTL that I got from (1)

It means that the overall TTL is not precise. There is a margin of milli or micro seconds that it takes between the time I got the TTL value in (1) to the time I update it.... which is fine by me!

So for my Laravel(PHP), Predis scenario, I do the following:

At some relevant point, higher up in the code:

//get ttl - time left before expiry
$ttl = $redis->ttl($some_unique_key);

Then wherever I have to update the value, I set the expiry after setting the value. The logic for creating the key(in my question) remains correct and unchanged.

//***note that I am UPDATING a key. Checking if it exists then I update
if($redis->exists($some_unique_key))
{
   //set/up the key
   $redis->set($some_unique_key, 'Some New, Updated, Value'));

   //Do some work   

   //set the expiration with the TTL value from (1)
   $redis->expire($some_unique_key,$ttl); 
}

Works perfectly!

Answer 4

If you're using Laravel and the Redis Fassade, you can also do

Redis::setex('yourkey', 120, 'your content'); // 120 seconds

instead of

Redis::set('yourkey', 'your content', 'EX', 120);

I'm not sure if it was already possible in Laravel 5.4. But definitely with Laravel 8 and Predis 1.1.