Signet has been announced and discussed previously on the mailing list so I won't repeat what Signet is and its motivation.

(For more background we recently had a Socratic Seminar with Kalle Alm and AJ Towns on Signet. Transcript, reading list and video are available.)

The first (of multiple) Signet PR 18267 in Bitcoin Core is at an advanced stage of review and certainly additional code review and testing of that PR is encouraged.

However there are some meta questions around Signet(s) that are best discussed outside of the Bitcoin Core repo and it would be good to ensure everyone's testing needs are being met. I will put forward my initial thoughts on some of these questions. These thoughts seem to be aligned with Kalle's and AJ's initial views but they have not reviewed this post and they can chime in if they feel I am misrepresenting their perspectives.

1) Should there be one "default" Signet that we use for specific purpose(s) or should we "let a thousand ships sail"? 

To be clear there will be multiple custom Signets. Even if we wanted to prevent them we couldn't. But is there an argument for having a "default" Signet with a network effect? A Signet that a large proportion of the community is drawn to using with tooling and support? I would say yes. Especially if we see Signet as a staging ground for testing proposed soft fork(s). Otherwise there will be lots of splintered Signet networks all with different combinations of proposed soft forks enabled and no network effect around a particular Signet. I think this would be bewildering for say Taproot testers to have to choose between Person A's Signet with Taproot enabled and Person B's Signet with Taproot enabled. For this to work there would have to be a formal understanding of at what stage a proposed soft fork should be enabled on "default" Signet. It would have to be at a sufficiently mature stage (e.g. BIP number allocated, BIP drafted and under review, PR open in Bitcoin Core repo under review etc) but early enough so that it can be tested on Signet well in advance of being considered for activation on mainnet. This does present challenges if soft forks are enabled on Signet and then change/get updated. However there are approaches that AJ in particular is working on to deal with this, one of which I have described below.

2) Assuming there is a "default" Signet how many people and who should have keys to sign each new "default" Signet block? If one of these keys is lost or stolen should we reset Signet? Should we plan to reset "default" Signet at regular intervals anyway (say every two years)?

Currently it is a 1-of-2 multisig with Kalle Alm and AJ Towns having keys. It was suggested on IRC that there should be at least one additional key present in the EU/US timezone so blocks can continue to be mined during an Asia-Pacific outage. (Kalle and AJ are both in the Asia-Pacific region). Kalle believes we should keep Signet running indefinitely unless we encounter specific problems and personally I think this makes sense.

3) Kalle has also experienced concern from some in the community that testnet will somehow be replaced by Signet. This is not the case. As long as someone out there is mining testnet blocks testnet will continue. However, there is the question of whether testnet needs to be reset. It was last reset in 2012 and there are differing accounts on whether this is presenting a problem for users of testnet. Assuming Signet is successful there will be less testing on testnet but what testing use cases will still prefer testnet? It has been argued that testnet should be a large chain to stress test certain IBD, P2P scenarios in which case it may be the case that we don't want to reset testnet. All other testing use cases would not be impacted by the downsides of a large chain as they would gravitate towards Signet regardless.

If you have thoughts, feedback, questions it would be great to hear them. Certainly we should seek to make sure everybody's testing needs are being considered.

There is a closed issue on the Bitcoin Core repo if you seek to review some of the prior conversation. Ideally though we would have discussion that isn't directly impacting Bitcoin Core here on the mailing list or on IRC rather than in the Bitcoin Core repo.

Thanks

Michael

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