Ethereum conducted its first major test towards PoS yesterday. The developers successfully tested the proof-of-stake algorithm on the mainnet via shadow fork. The test was largely successful, with a few minor glitches.
Shadow fork is a true version of the mainnet. Although the algorithm has been extensively tested on test networks; it needs to be tested in a real build like shadow fork to see if it works on a real network. In this structure called shadow fork, some validating nodes leave the mainnet and perform the update themselves. The rest of the network is unaffected by that event. Tim Beiko, one of the Ethereum developers, said in a statement on the subject:
Ethereum developers reported that the test was largely successful. Shadow fork was able to generate blocks and terminate transactions entirely using the PoS algorithm.