Backfill
Backfill puts new players into matches already in progress, filling seats that are empty because players left or because the match started short. It keeps a match playable and keeps paid-for server capacity occupied. The tension is that arriving mid-match can be a worse experience than starting one, so how aggressively to backfill is a judgement call.
Also called
matchmaking backfill, join in progress
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How Does Backfill Work?
Backfill starts on the game server, not in the queue. When a running match has empty seats, the server creates a backfill: a ticket the server owns, holding the details of the players already connected. Because it carries their data, a newcomer has to fit the same rules they matched on, such as skill, latency and mode.
In many matchmakers, the exchange runs in four steps:
The server creates one backfill for each team that is missing players.
It lists the group sizes it can take, up to the free seats. With 3 seats free, that's a party of 3, 2 or 1.
The matchmaker pairs the backfill with a waiting group that fits, often ahead of regular tickets, and can spread new players evenly across teams.
If the team still has room, the server repeats the process with the new players' tickets included.
The matched players receive the server's address and join the match where it stands.
In Practices - Insights from Edgegap's Blog
Article's key insights
This article is the practical companion to Edgegap's matchmaker documentation on fine-tuning expansions. It translates the configuration concepts into plain language and actionable decisions. For the latest syntax, configuration examples, and version-specific details, the documentation page remains the authoritative reference.
What Is Backfill Used For?
Replacing a player who left is the best-known use. It's one of several:
Replacing leavers, so a match doesn't have to restart a player short.
Topping up a match that started short. A matchmaker allowed to start below full size can fill the remaining seats once the match is running.
Letting friends or random players join a game in progress.
Letting spectators in, for tournaments or friends' matches.
Consolidating players into larger servers, so a quiet persistent world, such as an MMO zone, feels populated.
Each use asks the same thing of the server: know how many seats are free and who is already there. They differ in who arrives and what that player expects to find.
What Does a Game Server Do to Keep Backfill Running?
A backfill isn't a single request. The server manages backfills for as long as it runs:
Wait for arrivals. A matched player still has to connect. Edgegap's docs suggest allowing some time, 60 seconds for example, before treating them as gone, with a loading scene or progress bar while they arrive.
Replace whoever leaves. Each seat that opens during the match gets a new backfill.
Renew what expires. Backfills expire like any ticket, so the server recreates them while seats are still free.
Clean up at shutdown. Open backfills are deleted when the match ends or the server stops. Otherwise players can be matched into a server that's closing, and players assigned at that moment have to queue again.
When a matched player never arrives, the server has three options: replace them with an AI character, open another backfill, or play on short where the mode allows a variable player count.
Edgegap's matchmaker has SDKs for Unity and Unreal Engine. In Unity, a Server Agent creates, renews and cleans up backfills, so this lifecycle doesn't have to be written from scratch. For Unreal Engine, the docs name the engine callbacks where cleanup belongs, such as OnWorldDestroyed and PreExit.
When Should a Mode Use Backfill?
Backfill is a choice made per mode. Joining mid-match can be a worse experience than starting one: the new player arrives without the context of the score, sometimes on the losing side near the end.
Casual and co-op modes often gain the most, because a full team matters more than a fair start.
Competitive modes with fixed teams often skip it, since a replacement changes the balance the match started with (How to Optimize Session Fill Rate in Your Matchmaker). A tight skill band also slows it down.
A cutoff stops backfill late in a match, by time remaining or by score, when joining would mean arriving for the last moments.
Backfill also shows up on the hosting side. Every refilled seat raises session fill rate, and servers that keep taking players run longer. The two pages work together: fill rate shows where seats go empty, and backfill is one of the ways to refill them.
A word from our sponsor (ourselves!)
Every empty player slot is capacity you pay for and nobody uses. One studio on Edgegap's matchmaker averaged 91% of player slots filled across its sessions in Q2 2025, so fewer servers carried the same players.
Edgegap's Take (just our opinion, take it with a grain of salt!)
A Filled Seat Counts When the Player Stays
Backfill is easy to judge by the wrong number. A filled seat looks like success on the server, but if the new player quits a minute later, the match has lost a player twice.
A place to start: per mode, compare the share of backfilled players who finish the match with the share of players who started it. A wide gap usually points to a cutoff that comes too late, or to a mode where joining late doesn't work. Closing that gap tends to do more than backfilling more often.
Edgegap's matchmaker includes backfill, with SDKs for Unity and Unreal Engine that take on much of the server-side work, so a team's time goes to the cutoff and the per-mode decision.
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