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Improved description as suggested by Frederic.
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Signed-off-by: Bartlomiej Plotka <[email protected]>
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bwplotka committed May 11, 2020
1 parent cea5c06 commit 8d5b0a2
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16 changes: 16 additions & 0 deletions pkg/query/iter.go
Original file line number Diff line number Diff line change
Expand Up @@ -596,6 +596,22 @@ func (it *dedupSeriesIterator) Err() error {

// counterDedupAdjustSeriesIterator is used when we deduplicate counter.
// It makes sure we always adjust for the latest seen last counter value for all replicas.
// Let's consider following example:
//
// Replica 1 counter scrapes: 20 30 40 Nan - 0 5
// Replica 2 counter scrapes: 25 35 45 Nan - 2
//
// Now for downsampling purposes we are accounting the resets so our replicas before going to dedup iterator looks like this:
//
// Replica 1 counter total: 20 30 40 - - 40 45
// Replica 2 counter total: 25 35 45 - - 47
//
// Now if at any point we will switch our focus from replica 2 to replica 1 we will experience lower value than previous,
// which will trigger false positive counter reset in PromQL.
//
// We mitigate this by taking always adjusting for the "behind" replica value to be not smaller than highest sample seen.
// This is also what is closest to the truth (last seen counter value on this target).
//
// This short-term solution to mitigate https://github.com/thanos-io/thanos/issues/2401.
// TODO(bwplotka): Find better deduplication algorithm that does not require knowledge if the given
// series is counter or not: https://github.com/thanos-io/thanos/issues/2547.
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