[BIP Proposal] Mempool Validation and Relay Policies via User-Defined Scripts

In a recent exchange on the Bitcoin Development Mailing List, an innovative concept was discussed regarding the management of mempool size and the prioritization of transactions with higher fee rates through scripting. The original intent behind this idea was to enhance the efficiency of handling transactions by implementing a function (evict()) and incorporating a minFeerate parameter into the API. This approach aimed at providing a systematic way to manage the mempool by evicting lower priority transactions in favor of those offering higher fees, thereby optimizing transaction processing based on economic incentives.

However, concerns were raised about the feasibility of achieving such functionality purely through scripting. It was pointed out that the desired outcomes might not be attainable without introducing new opcodes to the Bitcoin scripting language. The introduction of additional opcodes for this purpose could complicate matters, as it is recognized that such changes would only be relevant in very specific scenarios, potentially limiting their applicability and desirability.

The discussion led to a reconsideration of whether the dynamic management of the mempool size and transaction prioritization needs to be integrated into scripts directly. This reflection opens up avenues for rethinking how best to accomplish these goals within the constraints of existing technologies and without necessarily expanding the scripting language. The dialogue underscores the importance of balancing innovation with practicality, especially in the context of cryptocurrency infrastructure where changes can have far-reaching implications.