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Comments (6)

leonoel avatar leonoel commented on September 26, 2024 1

OK, I understand the problem completely now.

First, here is a workaround for your use case - increase maximum parallelism to 3 :

(defn fetch-missing-items [cache-response-flow]
  (m/ap (let [[hit? flow] (m/?> 3 (m/group-by #(contains? % :result) cache-response-flow))]
          (m/?> (if hit?
                  flow
                  (->> flow
                       (m/eduction (map :key) (partition-all 100))
                       fetch-item-details
                       (m/eduction cat (partition-all 25))
                       store-item-details
                       (m/eduction cat)))))))

Now, the issue. Currently, when group-by's input crashes, the error is redirected on output. When the error is consumed, the active group consumers are terminated along with the main process. This ordering of events is problematic when the consumer backpressure prevents transfers (in this case, ?> when max parallelism has been reached), because then the group consumers are kept alive and never terminate, which stalls the rest of the pipeline indefinitely.

Proposed fix : when group-by's input crashes, propagate the error on output and terminate all active group consumers immediately (i.e don't wait for the main consumer to transfer the error).

from missionary.

lowecg avatar lowecg commented on September 26, 2024 1

Thanks for the explanation and workaround - I've hammered the loop for the last 30 minutes, and it's holding up perfectly.

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leonoel avatar leonoel commented on September 26, 2024 1

Fixed in b.28

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lowecg avatar lowecg commented on September 26, 2024

Repro project

https://github.com/lowecg/missionary-deadlock

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leonoel avatar leonoel commented on September 26, 2024

Thank you for reporting this !

I've managed to reduce it further.

(m/? (m/reduce conj
       (m/ap (m/?> (val (m/?> 2 (->> (m/seed (range))
                                  (m/eduction (map (fn [x] (assert (< x 2)) x)))
                                  (m/group-by identity))))))))

This doesn't terminate, it should fail immediately. This is a bug in either group-by or ap, related to input failure.

from missionary.

lowecg avatar lowecg commented on September 26, 2024

You're very welcome, Leo. Thank you for looking into this so quickly.

Fantastic news that you're able to isolate the issue.

from missionary.

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