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Yep. OOMs are the most common cause. It's definitely low frequency. On the order of one in a billion. For some systems that's once a year. For us that's once a week. If that's an important job and it just gets dropped, then you've got a problem.

With the paid features to keep it from getting dropped things still can be painful. We have a lot of different workers, all with different concurrency settings and resource limits. A memory heavy worker might need a few GB of memory and be capped at concurrency of 2 while a lightweight worker might only need 512MB and have concurrency of 20. If the big memory worker crashes, its jobs might get picked up by the lightweight worker (and possibly hours later), which will then OOM and all its 19 other in flight jobs all end up in the orphanage. And now your alerts are going off saying are saying the lightweight worker is OOMing and your team is scratching their heads because that doesn't make any sense. It just gets messy.

Sidekiq probably works great outside of containerized environment. Many swear to me they've never encountered any of these problems. And maybe we should be questioning the containerization rather than sidekiq, but ultimately our operations have been much simpler as we've moved off of sidekiq.