Riot outlined exactly how random League of Legends‘ All Random All Mid (ARAM) mode is.
Senior software designer Mojibake discussed the chaotic mode in today’s Ask Riot blog, breaking down the inner workings of ARAM champion select.
ARAM’s champion select uses TeamBuilder, a service that knows what champs each player has rolled and what’s on the bench. And it prevents players from getting a champ that’s already been rolled by someone else.
“In other words, the whole Champ Select process is server-authoritative,” Mojibake said. “TeamBuilder is in charge. And every roll, reroll, trade, bench swap, and lock-in is transactional. That means it is impossible for two players to, say, reroll the same champion at the same time; TeamBuilder will carefully process one reroll, then the other.”
Whether the process is “totally random,” however, is up in the air. Each time you’re assigned a champion, it takes many different factors into account. There are sets of always-free-to-play ARAM champs, the current free-to-play rotation, the champs you own, and the champs rolled by both teams. Despite all the variables, “you will always receive a random champion that isn’t already on a team or bench,” according to Riot.
The champ select process also resets after every lobby. So it’s entirely possible to get the same champ from a previous game or a queue-dodged lobby in consecutive matches.
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Source: https://dotesports.com/league-of-legends/news/riot-discusses-the-randomness-of-champion-select-in-leagues-aram-mode
