A handler adapter declares the way the Mixer should interact with handlers. For the previous logentry, we can have a handler definition that looks as follows:

apiVersion: config.istio.io/v1alpha2
kind: handler
metadata:
  name: fluentd
  namespace: istio-system
spec:
  compiledAdapter: fluentd
  params:
    address: fluentd-aggegater-svc.logging:24224

From this code snippet, Mixer knows a destination that can receive the logentry. The capabilities of every type of adapter differ significantly. For example, the fluentd adapter can only accept the logentry template, and Prometheus is only able to deal with the metric template, while the Stackdriver can take metric, logentry, and tracespan templates. All supported adapters are listed here: https://istio.io/docs/reference/config/policy-and-telemetry/adapters/.