[question] is it possible to configure a queue to allow to schedule a specific job only once?

[fcevado]

i want to allow a job(same class, args and queue) to be schedule only once... is that possible? i found out that there isn't any easi api to get scheduled jobs or to verify running jobs... i kinda use Verk.SortedSet.range/4 for that but idk if that is consistent.... i've seen duplicate jobs scheduling nevertheless

[edgurgel]

Even if you implement something with Verk.SortedSet there's always a chance for a race condition:

Suppose that the job just came out of the scheduled set.

• Check if this job does not exist on the schedule set
• Job is not scheduled but is running
• Add new job to the schedule set

In this case your code will add a new job to the scheduled set.

I'm not sure what's the best approach here 🤔

[keyan]

If the goal is simply to prevent rescheduling, one possible solution:

1. create a new set scheduled
2. serialize every Job struct to a string
3. before scheduling, attempt to write the serialized Job to the scheduled set using an atomic read/update command, possibly HSETNX
4. only schedule the Job if HSETNX returns 1, otherwise we discard it as a duplicate

The main issue that arises: how we do garbage collect the set? The maximum set size is ~14mil (2^32), so if you are doing normal-ish production volume then you can't persist the set forever. How much of a problem this is depends on the requirements you had in mind @fcevado.