Backfill
Backfill is the process of filling empty player slots in an ongoing match — adding new players to a session that has already started because others disconnected or dropped out.
Without backfill, a 5v5 match where one player disconnects becomes a permanent 5v4. With backfill, the session signals that it has an open slot, the matchmaker finds an eligible replacement, and the new player joins the running game — ideally with minimal disruption to the players already in it.
How backfill works
When a player leaves a session, the game server notifies the orchestrator or matchmaker that a slot is available. The matchmaker then searches its queue for a compatible player — someone with the right skill range, low enough latency to the server’s region, and ideally one who has been waiting long enough that joining mid-match is preferable to continuing to wait.
The new player connects to the already-running server. The server sends them the current game state so they can join without starting from scratch. Depending on game design, the incoming player might spawn immediately or wait for the next safe entry point (end of a round, respawn timer, etc.).
When backfill is worth implementing
Not every game should backfill. The decision depends on match structure and player experience.
Backfill makes sense when matches are long, leaving mid-match is common due to disconnections, and a missing player significantly degrades the experience for everyone else. Battle royale games, objective-based team games, and long co-op sessions all benefit.
Backfill is often skipped for short matches (under five minutes), ranked competitive modes where joining mid-match is considered unfair to the incoming player, or game modes where team composition is fixed at match start for balance reasons.
Some games implement backfill for casual modes but disable it for ranked, treating the tradeoff explicitly: casual players get fuller lobbies, competitive players get consistent match conditions.
Implementation complexity
Backfill is more technically involved than initial matchmaking. The matchmaker must track live session state — which sessions are running, how many open slots each has, how far into the match they are. It must also decide how old a match can be before backfilling it is pointless (joining with 30 seconds left is usually worse than waiting for a fresh game).
The server must handle mid-match player joins cleanly: sending state to the new player, notifying existing players, and handling the edge case where the new player disconnects before fully loading. Any persistent state accumulated during the match — score, inventory, progress — needs to be correctly initialised for the incoming player.
Backfill and server lifetime
Backfill changes how long servers stay alive. Without it, a session ends when players leave or the match completes. With it, a session can be extended by player replacement — the server’s expected lifetime becomes harder to predict. Orchestrators managing large fleets need to account for this when making decisions about which servers to release versus extend.
See also: Matchmaker · Server Allocation · Session Hosting · Orchestrator