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From the question below I can see the circumstances under which a transaction will be removed from the Bitcoin mempool :-

How do transactions leave the memory pool?

Whilst a transaction is still unconfirmed, it shows up in the Blockchain.info Explorer as an 'Unconfirmed Transaction!', within a red colored button. This button then turns to blue with "1 Confirmations" when the transaction is confirmed.

However if the transaction remains unconfirmed for a long time and the button remains at red, and if the transaction is then eventually "evicted" from the mempool, will the Blockchain Explorer then just report "Transaction Not Found" when I search for the transaction's id ?

I would find this behaviour useful as it would allow me to tell when a transaction has effectively been cancelled. If the red button is still showing, I know my transaction may still have a chance of confirmation, and I may be prepared to wait a few days in some cases for non-urgent transfers, especially when fees are at very high levels. If the transaction disappears altogether then I know I would have to submit a new transaction entirely. (I would actually prefer that a wallet did not automatically resubmit a failed transaction, but rather let me know it has failed).

Many thanks

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First of all, there is no "the Bitcoin mempool". Each node has its own mempool, so if the transaction is evicted from one node's mempool, there is no guarantee that it has been evicted from other node's mempools.

If most nodes have evicted a transaction, it still may appear on a blockchain explorer because they may have other configuration settings which has them have a larger mempool and hold onto transactions for longer. Generally blockchain explorers have less restrictive mempool configurations than most nodes, so if a transaction disappears from multiple block explorer's mempools, then it is likely that most nodes on the network have evicted the transaction.

  • So would I be right in saying that after a reasonable time frame (say 4 weeks, ie twice the default transaction expiry time for a node), a transaction that has still not been mined must, with very high likelihood, no longer be visible in any Blockchain Explorer, and not in any node's mempool? And in fact it might less than 4 weeks in most cases? – Tangent Plane Jan 3 '18 at 15:09

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