Traffic Splitting
Directing a defined percentage or subset of requests to a different version of a service, configured at the routing layer rather than in application code.
The capability that makes canary releases, A/B tests and dark launches operational rather than bespoke. The mesh or gateway routes 1% of traffic to the new version, then 5%, then more, with automated rollback on metric regression.
Beyond percentages, routing can be header-based (internal users, a specific tenant, a test cohort) — which enables dark launching to staff before any customer sees the change.
Two caveats that make percentage splitting less precise than it appears. Sticky sessions mean a user who lands on the canary stays there, so 1% of traffic may be 1% of users seeing 100% of the new behaviour. And asynchronous work is not split at all: a canary consuming from a shared queue processes whatever it pulls, regardless of the HTTP split.
Splitting is only useful with metrics that distinguish the canary from the baseline, which must be labelled at emission.