A question over the way we use maxSeqNoOfUpdatesOrDeletes for optimization

Currently, I see that we are using the following check to decide if we can optimize indexing of new documents on replicas (ApendOnly Optimization for non-primary).

if (maxSeqNoOfUpdatesOrDeletes <= localCheckpointTracker.getProcessedCheckpoint()) {
    assert maxSeqNoOfUpdatesOrDeletes < index.seqNo() : index.seqNo() + ">=" + maxSeqNoOfUpdatesOrDeletes;
    plan = IndexingStrategy.optimizedAppendOnly(index.version());

However, I think we can perhaps loosen the constraint using maxSeqNoOfUpdatesOrDeletes < index.seqNo() based on my analysis below.

For an operation O, if it's an index update on exiting doc or a delete, the primary would update its MSU to some value >= seqno(O) before replicating this operation O and its latest MSU to other replicas. Thus when replicas see that seqno(O) <= its maintained MSU, they know that append-only optimazation can not be applied since O could be an UPDATE. On the other hand, if they see seqno(O) > their latest MSU, it can be sure that index operation O is not an UPDATE but add a new doc.

For the case LCP < MSU, if a future operation O' with seqno(O') > seqno(O) is processed before operation O on replica, we should have seqno(O) < seqno(O') <= MSU(O') <= MSU. Thus we can also avoid creating duplicated doc by not applying optimization for O when out-of-order happens.

Not sure if it's reliable enough to use maxSeqNoOfUpdatesOrDeletes < index.seqNo() to decide to use optimization or not. It would be helpful if some cases/examples can be used to illustrage why this doesn't work.


Hi @iamorchid,

an example where this would go wrong is if the previous op were a delete. Consider:

seqNo(O') < seqNo(O)

where O' is a deletion. When replicating O, MSU would be seqNo(O'). If O arrives on replica before O', O would be added, not updated, leading to a duplicate.

By checking MSU <= LCP, we only apply the optimization if we processed every update or delete on replica already, avoiding this issue.

Hi @HenningAndersen

Sorry, not quite understand why the out-of-order would cause duplicate. Let's say we have two cases here for deletion O' and index operation O.

1. The same doc doesn't exist in Lucene before we execute O' and O
a) if we execute O' and O in order, we would end up with a soft-deleted doc and a new doc in Lucene。
b) if we execute O (MSU=seqno(O')) before O'(MSU=seqno(O')), if the optimizaiton rule I suppose is used, O would add a new doc into Lucene and then O' would be marked as stale, which also creates a soft-deleted doc in Lucene.
So I think a) and b) would end up with the same state for this case.

2. The same doc exists in Lucene before we execute O' and O
a) if we execute O' and O in order, O' would create two soft-deleted doc (one for the previous existing doc and the other for the doc in O') in Lucene while O would add a new doc.
b) if we execute O (MSU=seqno(O')) before O', if the optimizaiton rule I suppose is used, O would convert the existing doc into a soft-deleted one and add a new doc in Lucene while O' would add another soft-deleted doc in Lucene since O' now is stale.
For this case, it looks a) and b) seems still to end up with the same state.

Maybe I have misunderstood something here.

Hi @iamorchid,

this is the interesting part:

If I understand your rule correct, you would check seqNo(O) > MSU and if so, use the append only optimization. In that case, indexing O would only do the "add" and not do the soft-delete of the existing doc. Meaning this part of the 2b does not hold true:

O would convert the existing doc into a soft-deleted one

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Ah, right. The optimization doesn't check version map. Got it. Thanks @HenningAndersen so much.

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