What is Hard Fork Combinator?

A Hard Fork Combinator is a tool used in the Cardano ecosystem to combine protocols after a hard fork event.

A hard fork splits one blockchain into two distinguishable protocols which run parallel to each other. The new blockchain branch loses the history of the prior blockchain. A hard fork occurs when new upgrades are implemented into the blockchain. It can modify some of the protocol parameters.

Hard forks may also occur on the live blockchain, which can compromise the integrity of the network. In 2017, a hard fork occurred on the Bitcoin blockchain, leading to the chain split. The event had a significant impact on the interests of Bitcoin investors and caused the formation of Bitcoin Cash (BCH).

Hard forks often cause downtime in the blockchain ecosystem which can negatively impact the user experience and disrupt any ongoing transactions. If a hard fork occurs, the network nodes are unable to update to the newly set parameters and block headers simultaneously, without experiencing system interruptions. The blockchain then has to be taken into downtime.

Although a hard fork may occur on any blockchain, IOHK developed the hard fork combinator as a solution specifically for the Cardano ecosystem. The hard fork combinator merges both the pre and post-upgrade protocols together to eliminate the possible interruption or restart issues.

With the hard fork combinator, several protocols can appear as a single ledger. This prevents the network nodes from having to update to the new blockchain simultaneously.

In July 2020, Cardano received upgrades, moving from the wallet-only Byron protocols to proof-of-stake-supported Shelley protocols. During the upgrade, the utility of the blockchain was altered by the hard fork.

This was a risky procedure, as Cardano contained over 1 million wallets at the time. Any issues with the hard fork execution could have been detrimental to the Cardano ecosystem as a whole.

The Byron-to-Shelley transition was successfully completed on 29 July 2020, at 21:45 UTC. Users received notice about the ongoing upgrade. However, the use of the Cardano function was not restricted and the hard fork processes were completed in the background. This allowed the network nodes to transition between protocols sequentially.

Thanks to the hard fork combinator, the Cardano blockchain has been able to maintain both Byron and Shelley blocks. The tool will be implemented in future upgrades to support the transitional process.