Depending on who you ask, Bitcoin Unlimited is either the future of bitcoin or a broken implementation of the software.
One thing's for certain, though, the controversial alternative to Bitcoin Core (the technology's standard software implementation) has drifted to the heart of bitcoin's raging scaling debate over the past year.
To summarize an admittedly complex debate, each side wants to increase capacity of the network, but they want to do so by different methods.
In today's climate, running or supporting Bitcoin Unlimited is basically synonymous with desiring one such method: a tweak to bitcoin's block size parameter, set at 1MB today.
What is Bitcoin Unlimited?
Bitcoin's code is public for anyone to read or copy ('fork') for their own project. As such, it's possible for different versions of bitcoin to run side by side on the network.
Bitcoin Unlimited differs from Bitcoin Core in that the block size parameter is not hard-coded – nodes and miners flag support for the size that they want. Then, it relies on an idea called 'emergent consensus'.
"An emergent consensus will thus arise based on free-market economics as the nodes/miners converge on consensus focal points, creating in the process a living, breathing entity that responds to changing real-world conditions in a free and decentralized manner," the website reads.
In the case of the block size, the idea is that via the free market, miners will come to agreement on a block size. Though, users can 'vote' on other parameters as well.
While a larger community has cropped up around Bitcoin Unlimited, there are a few key players.
Investor Roger Ver has been one particularly eager supporter, even launching a small mining pool, Bitcoin.com, around the effort.
Other bitcoin mining pools, such as ViaBTC and Antpool, operated by mining giant Bitmain, are also now signaling support.
As far as the technical community goes, Bitcoin Unlimited chief scientist Peter Rizun is possibly one of the better-known individuals to generate ideas for the Core alternative. From outside the community, others, such as former Bitcoin Core maintainer, Gavin Andresen, have said positive things about it too.
What's the debate?
There are many in the technical community who feel the Bitcoin Unlimited might not be a safe replacement for Bitcoin Core.
Some developers, such as Luke Dashjr and David Vorick, argue that the approach doesn't work at a technical level.
One reason they give is that the software hands miners too much control over protocol decisions. (Some even argue that this is a key reason that some big mining pools back the effort.)
Another, is that many developers think that 'emergent consensus', in practice, would lead to blockchain forks (the creation of different and competing versions of the network).
Meanwhile, there has been at least one problem with the software. A month back, one buggy Bitcoin Unlimited upgrade led Ver's pool Bitcoin.com to lose 13 bitcoins when it created a block that was not accepted by the network.
Antpool is the latest of several bitcoin mining pools to signal for Bitcoin Unlimited, but its support (while not new) is all that significant.
Bitmain, its operator, has an outsized position in China's bitcoin mining scene (it's one of the only major manufacturers of hardware in the region), meaning its support is likely to carry through its business partners as well.
Currently, approximately 2.5% of users are now running a Bitcoin Unlimited node, but some think Bitmain may have the power to influence this figure.
Right now, this method of voting isn't active. There isn't a activation date or hashrate threshold put in place yet, but if users and miners begin using the new Bitcoin Unlimited rules, it could split off into another coin and begin living by these new rules.
Some in the ecosystem fear that such a fork – if implemented poorly – could create two bitcoin blockchains, and two publicly traded bitcoins operating on blockchains with different rule sets.
Correction: An earlier version of this article misstated Bitcoin Unlimited's activation threshold. This has been corrected.
Image via Bitcoin Unlimited