|Initial release||August 15, 2015|
Release I / 22 April 2018
Bitcoin XT is a fork of Bitcoin Core, the reference client for the bitcoin network. In mid-2015, the concept achieved significant attention within the bitcoin community amid a contentious debate among core developers over increasing the block size cap. The current reference implementation for bitcoin contains a computational bottleneck. This averages to a daily maximum of around 300,000 transactions.
It was proposed that the block size increase to eight megabytes, and from then onwards to automatically increase it exponentially, doubling every two years. The proposal did not gain the necessary support to go into effect on the Bitcoin network by early 2016, the earliest possible switchover date. Its use has been in steady decline from March 2016 onwards.
On June 10, 2014 Mike Hearn published a Bitcoin Improvement Proposal (BIP 64), calling for the addition of "a small P2P protocol extension that performs UTXO lookups given a set of outpoints." On December 27, 2014 Hearn released version 0.10 of the client, with the BIP 64 changes. It was intended to support queries for his Lighthouse crowdfunding platform project.
On June 22, 2015, Gavin Andresen published BIP 101 calling for an increase in the maximum block size. The changes would activate a fork allowing eight MB blocks (doubling in size every two years) once 75% of a stretch of 1,000 mined blocks is achieved after the beginning of 2016. The new maximum transaction rate under XT would have been 24 transactions per second.
On August 6, 2015 Andresen's BIP101 proposal was merged into the XT codebase. On August 15, 2015 version 0.11A was released to the public. Bip 101 was reverted and the 2-MB block size bump of Bitcoin Classic was applied instead.
According to bitcoin statistics website coin.dance the software peaked at more than 1,000 nodes in August 2015 before declining to a second peak in January 2016 of around 670 nodes. From March 2016 the program was seen a consistent decline in its user base to less than 30 instances in January 2017.
The August 2015 release of XT received widespread media coverage. The Guardian wrote that "bitcoin is facing civil war". Reason wrote that "Bitcoin XT represents a technical and philosophical divergence." Wired wrote that "Bitcoin XT exposes the extremely social — extremely democratic — underpinnings of the open source idea, an approach that makes open source so much more powerful than technology controlled by any one person or organization." Developer Adam Back was critical of the 75% activation threshold being too low and that some of the changes were insecure.
In any event, the XT hard fork stalled. On 11 January 2016, the earliest possible date for the transition to take effect, only approximately 10% of Bitcoin miners were using the XT protocol to sign blocks, whereas 75% would have been required to make the new protocol the de facto standard. The software lacked the support of some of China's biggest mining pools. The Bitcoin community response to the software has been attributed to the adherence to gradual change based on consensus.
Bitcoin Cash support
On August 25, 2017, Bitcoin XT published Release G, which was a Bitcoin Cash client by default. Subsequently, Release H was published, which supported the November 2017 Bitcoin Cash protocol upgrade, followed by Release I, which supported the May 2018 Bitcoin Cash protocol upgrade.
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