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Epic Games' released a postmortem yesterday detailing Fortnite's playground LTM (limited time) mode, which was initially released June 27 and gave players one hour to build, explore and hone their skills without being placed in an ongoing match.
However, the limited time mode closed shortly after launch, presumabely because of errors with the matchmaking system Fortnite uses.
The postmortem makes it clear now that the issue was related to how many matches the playground mode made, as opposed to the battle royale mode.
As the postmortem details, every node in the matchmaking group has a list of dedicated, available servers. Players connect to the matchmaking service, the service assigns the player to a corresponding node in their region, and then automatically selects one of its available servers for the player of that region.
While Fortnite's battle royale mode pits 100 players against each other in any given server, playground mode made matches for groups of one to four players at a time. This meant more matches were being made, requiring at least 15 times as many servers.
Because of this, the nodes were constantly asking to join or request extra servers, leading to the CPU holding “a backlog of pending requests resulting in a feedback loop," causing Epic to shut down playground and reassess the situation.
To fix playground mode, the matchmaking service was separated into its own service cluster. This stopped the server jam from affecting other modes, allowing developers to repair the service until it could get back online.
Playground's new matchmaking system now shifts regional capacity from nodes with extra space to those that are running low, ensuring nodes don't have to search outside of local region lists of servers as they had done previously.
After iterating and testing, Epic finally deemed playground mode to be working as intended. The company notes in the postmortem that they learned a lot about their own matchmaking service and it's "failure points."
For a full breakdown of how playground mode was fixed, click here.