The code so far is fairly limited in scope, essentially making it 
possible to change the values in consensus/params.h based on block 
height. The actual code to interpret those values does need to be 
provided ahead of time, of course, so there's still developer time to 
implement, it just moves consensus arguments to the users.

Loading the values from disk rather than hard-coding them is a 
reasonably straight forward extension to the code, in theory. The 
existing work has application-specific changes that would need stripping 
out, but you can get an idea of what this would look like from 
https://github.com/rnicoll/dogecoin/commit/949b1ccd88ff13c74a3c1a7b9faa7f36c1085904

Ross

On 23/07/2015 01:43, Eric Lombrozo via bitcoin-dev wrote:
>> On Jul 22, 2015, at 5:34 PM, Cory Fields <lists@coryfields.com> wrote:
>>
>> On Wed, Jul 22, 2015 at 8:13 PM, Eric Lombrozo <elombrozo@gmail.com> wrote:
>>>> On Jul 22, 2015, at 5:05 PM, Cory Fields <lists@coryfields.com> wrote:
>>>>
>>>> On Wed, Jul 22, 2015 at 7:53 PM, Eric Lombrozo <elombrozo@gmail.com> wrote:
>>>>> FWIW, I had worked on something similar a while back:
>>>>> https://github.com/CodeShark/bitcoin/tree/coinparams_new/altconf
>>>>>
>>>>> I like the idea in principle…but we should require a new genesis block,
>>>>> different magic bytes, and a different network port at the very least. :)
>>>>>
>>>> Not sure if serious, so I'll assume you are :)
>>> Only being partly serious - I strongly am in favor of a sufficiently modularized codebase that swapping out consensus rules is fairly straightforward and easy to test. I’m not in favor of encouraging forking an existing blockchain without having mechanisms in place to gracefully merge back without significant network disruptions. We do not have this yet.
>>>
>> Again, why? If someone wants to create a scamcoin, they can. If
>> someone wants to burn money on a scamcoin, equally, they can. I'm not
>> sure how this is any different. If someone manages to garner realistic
>> support for a hard-fork, I don't see the benefit in forcing them to
>> use forked software.. that only leaves Core in the middle because it's
>> forced to choose a side (not choosing is unfortunately a side as
>> well). It doesn't remove the reality of the split.
> In general, new consensus rules are not trivial to implement. Block size limits are exceptional in being so simple to change in the code. So what you’re proposing sounds more like a plugin model supporting dynamic linking than a configuration file.
>
>>>> Why? The idea in this case would be to allow the user to decide
>>>> between (say) "./bitcoind -1mbchain" and "./bitcoind -2mbchain" at
>>>> runtime rather than the likely alternative of "./bitcoind" vs
>>>> "./bitcoin-fork”.
>>> That’s exactly what my coinparams_new branch does. Adding a parameter for maximum block size would be straightforward.
>>>
>>>> Chain params may be identical other than the value of some future
>>>> event (miner vote for example), in which case the configs would run
>>>> identically until that point.
>>> Yes, indeed - this would be a special case.
>>>
>>>> If your concern is about nodes with different configs communicating
>>>> with eachother, I'd like to reiterate: the idea really is no different
>>>> than suggesting that someone fork the codebase and implement their own
>>>> changes, it just cuts out most of the work required.
>>> I do not encourage anyone to try to fork an existing blockchain without first securing overwhelming (near unanimous) consensus…or without having yet built a mechanism that can merge divergent chains gracefully.
>> Well of course. It would be a terrible idea. People would try it and
>> fail, and lose money. But for those crying foul at Core for being the
>> consensus/policy gatekeeper, it seems to me that user-selectable
>> params is the only logical solution.
> The real problem isn’t so much the difficulty of creating forks of the codebase - but the fact that unless a fork has overwhelming support, blockchains cannot guarantee irreversibility of transactions…which defeats their entire purpose.
>
>
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