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While working with log filters i've noticed interesting behavior of logs during chain reorganizations. Sometimes same event (log) can have different logIndex in transaction after chain reorganizations.

My question is: can logIndex change in the same block? For example: it happens that sometimes you can get duplicate events from listening eth_getFilterChanges, can it be that event you get as Duplicate (has same tx hash and blocknum) can have another logIndex than before?

Second question is: why do logIndex change at all?!

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In a chain reorganization the order of the transaction can change, some transactions can be removed, and new ones added replacing others.

Since the log order follows the order of the transactions a chain reorganization can cause the logIndex to be different.

Moreover it is possible some events disappear from a block and appear again later if the transaction that generated the event was removed from a block.

The reverse also happens, a new event can appear if a new transaction was added to the block.

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  • Thanks for your answer, but can logIndex change in same block while i'm listening for logChanges from geth?
    – Coder
    Commented Jan 26, 2018 at 10:11
  • And also how many blocks later can happen reorganization?
    – Coder
    Commented Jan 26, 2018 at 10:14
  • In a chain reorg a block can be replaced by another, so the transaction order can be different, and the logindex will change accordingly. A chain reorg has no limit on how many blocks it will affect. Most wallets use a 12 blocks as confirmation, meaning a chain reorg of more than 12 blocks is unlikely.
    – Ismael ♦
    Commented Jan 26, 2018 at 14:31
  • Thanks for your reply, so there is no guarantee that there wont be any chain reorganizations even after 12 blocks?
    – Coder
    Commented Jan 26, 2018 at 14:34
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    There aren't any guarantee. A reorg can have an arbitrary length, although it is very unlikely because you need to have more computing power than the whole network.
    – Ismael ♦
    Commented Nov 28, 2019 at 14:22

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