refactor(app/outbound): simplify backend metrics layer (#4299)

cratelyn · web-flow · commit f170c513af32 · 2025-11-17T13:18:45.000-05:00
this branch makes some small preliminary changes to the outbound proxy's backend metrics layer. this will simplify the introduction of additional middleware layers. --- * refactor(app/outbound): simplify backend layer construction this is akin to #3334, which cleaned up the equivalent expression in `linkerd/app/outbound/src/http/logical/policy/route/metrics.rs`. Signed-off-by: katelyn martin <kate@buoyant.io> * refactor(app/outbound): type aliases in backend metrics layer before we introduce additional layers to our backend stack, we should define type aliases as prophylaxis against: <https://rust-lang.github.io/rust-clippy/stable/index.html#type_complexity> Signed-off-by: katelyn martin <kate@buoyant.io> --------- Signed-off-by: katelyn martin <kate@buoyant.io>
diff --git a/linkerd/app/outbound/src/http/logical/policy/route/backend/metrics.rs b/linkerd/app/outbound/src/http/logical/policy/route/backend/metrics.rs
@@ -1,9 +1,9 @@
 use crate::{BackendRef, ParentRef, RouteRef};
 use linkerd_app_core::{metrics::prom, svc};
 use linkerd_http_prom::{
-    body_data::response::{BodyDataMetrics, NewRecordBodyData, ResponseBodyFamilies},
-    count_reqs::{NewCountRequests, RequestCount, RequestCountFamilies},
-    record_response::{self, NewResponseDuration},
+    body_data::response::{BodyDataMetrics, ResponseBodyFamilies},
+    count_reqs::{RequestCount, RequestCountFamilies},
+    record_response,
     stream_label::{LabelSet, StreamLabel},
 };
 
@@ -30,18 +30,19 @@ type ResponseMetrics<L> = record_response::ResponseMetrics<
     <L as StreamLabel>::StatusLabels,
 >;
 
+type Instrumented<T, N> = NewRecordBodyData<NewCountRequests<NewResponseDuration<T, N>>>;
+type NewRecordBodyData<N> =
+    linkerd_http_prom::body_data::response::NewRecordBodyData<ExtractRecordBodyDataParams, N>;
+type NewCountRequests<N> = linkerd_http_prom::count_reqs::NewCountRequests<ExtractRequestCount, N>;
+type NewResponseDuration<T, N> = linkerd_http_prom::record_response::NewResponseDuration<
+    T,
+    ExtractRecordDurationParams<ResponseMetrics<<T as MkStreamLabel>::StreamLabel>>,
+    N,
+>;
+
 pub fn layer<T, N>(
     metrics: &RouteBackendMetrics<T::StreamLabel>,
-) -> impl svc::Layer<
-    N,
-    Service = NewRecordBodyData<
-        ExtractRecordBodyDataParams,
-        NewCountRequests<
-            ExtractRequestCount,
-            NewResponseDuration<T, ExtractRecordDurationParams<ResponseMetrics<T::StreamLabel>>, N>,
-        >,
-    >,
-> + Clone
+) -> impl svc::Layer<N, Service = Instrumented<T, N>> + Clone
 where
     T: MkStreamLabel,
     T::DurationLabels: LabelSet,
@@ -56,12 +57,13 @@ where
 
     svc::layer::mk(move |inner| {
         use svc::Layer;
-        NewRecordBodyData::layer_via(ExtractRecordBodyDataParams(body_metrics.clone())).layer(
-            NewCountRequests::layer_via(ExtractRequestCount(requests.clone())).layer(
-                NewRecordDuration::layer_via(ExtractRecordDurationParams(responses.clone()))
-                    .layer(inner),
-            ),
-        )
+
+        let record = NewRecordDuration::layer_via(ExtractRecordDurationParams(responses.clone()));
+        let count = NewCountRequests::layer_via(ExtractRequestCount(requests.clone()));
+        let body_data =
+            NewRecordBodyData::layer_via(ExtractRecordBodyDataParams(body_metrics.clone()));
+
+        body_data.layer(count.layer(record.layer(inner)))
     })
 }
 
