Consistent hashing: consistently retry another host #119
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This implements a retry policy to consistently retry another host within the SRV set if we get a failure talking to the original host. We still use the original fallback strategy if this retry fails. (Should we?)
tempusfrangit
approved these changes
Dec 15, 2023
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Approve with the caveat that I have not tested the code directly in the validation environment. It looks sane. I largely agree with @nickstenning 's comments.
This extracts the consistent hashing code into a new package and writes more tests for it. This removes a bunch of duplication from ConsistentHashingMode. As a consequence this jumbles the consistent hashing algorithm (because I've embedded the Key into a top-level CacheKey struct which includes Attempt for retry support). At this stage this is safe because nothing live is using it.
tempusfrangit
approved these changes
Dec 15, 2023
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| func HashBucket(key any, buckets int, previousBuckets ...int) (int, error) { | ||
| if len(previousBuckets) >= buckets { | ||
| return -1, fmt.Errorf("No more buckets left: %d buckets available but %d already attempted", buckets, previousBuckets) | ||
| } | ||
| // we set IgnoreZeroValue so that we can add fields to the hash key | ||
| // later without breaking things. | ||
| // note that it's not safe to share a HashOptions so we create a fresh one each time. | ||
| hashopts := &hashstructure.HashOptions{IgnoreZeroValue: true} | ||
| hash, err := hashstructure.Hash(cacheKey{Key: key, Attempt: len(previousBuckets)}, hashstructure.FormatV2, hashopts) | ||
| if err != nil { | ||
| return -1, fmt.Errorf("error calculating hash of key: %w", err) | ||
| } | ||
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| // jump is an implementation of Google's Jump Consistent Hash. | ||
| // | ||
| // See http://arxiv.org/abs/1406.2294 for details. | ||
| bucket := int(jump.Hash(hash, buckets-len(previousBuckets))) | ||
| slices.Sort(previousBuckets) | ||
| for _, prev := range previousBuckets { | ||
| if bucket >= prev { | ||
| bucket++ | ||
| } | ||
| } | ||
| return bucket, nil | ||
| } |
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This is so nice to have it extracted!
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| if err != nil { | ||
| // return origErr so that we can use our regular fallback strategy | ||
| return nil, origErr |
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I think we're going to want this for the foreseeable future. As reliability is the real goal, I want to be durable for "cache disappears" but we still serve inference.
That said, I hear the earlier comments about "do we need this".
nickstenning
approved these changes
Dec 15, 2023
| // retry, you can pass previousBuckets, which indicates buckets which must be | ||
| // avoided in the output. HashBucket will modify the previousBuckets slice by | ||
| // sorting it. | ||
| func HashBucket(key any, buckets int, previousBuckets ...int) (int, error) { |
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This is really nice. I like the derived/wrapped key a lot.
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This implements a retry policy to consistently retry another host within the SRV set if we get a failure talking to the original host.
We still use the original fallback strategy if this retry fails. (Should we?)