From df29960f472d7c69916e2f51ebb1d1018408f350 Mon Sep 17 00:00:00 2001
From: katelyn martin <kate@buoyant.io>
Date: Fri, 1 Nov 2024 00:00:00 +0000
Subject: [PATCH 1/2] refactor(app): Simplify backend metrics layer bounds

this commit changes the backend-level metrics layer function, so that it
invokes `linkerd_app_core::svc::layers()` rather than
`linkerd_app_core::svc::layer::mk()`.

this has a nice effect in that it simplifies the bounds of the `layer`
function.

Signed-off-by: katelyn martin <kate@buoyant.io>
---
 .../logical/policy/route/backend/metrics.rs   | 32 ++++++-------------
 linkerd/http/prom/src/body_data/response.rs   |  2 +-
 2 files changed, 10 insertions(+), 24 deletions(-)

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
index af15735757..8854a1cd66 100644
--- a/linkerd/app/outbound/src/http/logical/policy/route/backend/metrics.rs
+++ b/linkerd/app/outbound/src/http/logical/policy/route/backend/metrics.rs
@@ -38,20 +38,6 @@ pub fn layer<T, N>(
 > + Clone
 where
     T: MkStreamLabel,
-    N: svc::NewService<T>,
-    NewRecordBodyData<
-        ExtractRecordBodyDataParams,
-        NewCountRequests<
-            ExtractRequestCount,
-            NewResponseDuration<T, ExtractRecordDurationParams<ResponseMetrics<T::StreamLabel>>, N>,
-        >,
-    >: svc::NewService<T>,
-    NewCountRequests<
-        ExtractRequestCount,
-        NewResponseDuration<T, ExtractRecordDurationParams<ResponseMetrics<T::StreamLabel>>, N>,
-    >: svc::NewService<T>,
-    NewResponseDuration<T, ExtractRecordDurationParams<ResponseMetrics<T::StreamLabel>>, N>:
-        svc::NewService<T>,
 {
     let RouteBackendMetrics {
         requests,
@@ -59,15 +45,15 @@ where
         body_metrics,
     } = metrics.clone();
 
-    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 requests = NewCountRequests::layer_via(ExtractRequestCount(requests.clone()));
+    let responses = NewRecordDuration::layer_via(ExtractRecordDurationParams(responses.clone()));
+    let body_metrics =
+        NewRecordBodyData::layer_via(ExtractRecordBodyDataParams(body_metrics.clone()));
+
+    svc::layers()
+        .push(responses)
+        .push(requests)
+        .push(body_metrics)
 }
 
 #[derive(Clone, Debug)]
diff --git a/linkerd/http/prom/src/body_data/response.rs b/linkerd/http/prom/src/body_data/response.rs
index f7999a9f4d..a83095ed1c 100644
--- a/linkerd/http/prom/src/body_data/response.rs
+++ b/linkerd/http/prom/src/body_data/response.rs
@@ -34,7 +34,7 @@ impl<X: Clone, N> NewRecordBodyData<X, N> {
     ///
     /// This uses an `X`-typed [`ExtractParam<P, T>`] implementation to extract service parameters
     /// from a `T`-typed target.
-    pub fn layer_via(extract: X) -> impl Layer<N, Service = Self> {
+    pub fn layer_via(extract: X) -> impl Layer<N, Service = Self> + Clone {
         svc::layer::mk(move |inner| Self {
             extract: extract.clone(),
             inner,

From 4f1c33b7e31e8e46649e8f5976d3e1950a4f24a1 Mon Sep 17 00:00:00 2001
From: katelyn martin <kate@buoyant.io>
Date: Fri, 1 Nov 2024 00:00:00 +0000
Subject: [PATCH 2/2] refactor(app): Define layer type aliases

this isn't a "simplification" so much as an outlining of the chunky
types in this function signature.

that said... it's subjectively nice to outline the definition of our
layers into a `MetricsLayer<T, N>` that we can use as shorthand in the
function signature of `layer(..)`.

Signed-off-by: katelyn martin <kate@buoyant.io>
---
 .../logical/policy/route/backend/metrics.rs   | 20 +++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

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
index 8854a1cd66..6851ad56f8 100644
--- a/linkerd/app/outbound/src/http/logical/policy/route/backend/metrics.rs
+++ b/linkerd/app/outbound/src/http/logical/policy/route/backend/metrics.rs
@@ -24,18 +24,18 @@ type ResponseMetrics<L> = record_response::ResponseMetrics<
     <L as StreamLabel>::StatusLabels,
 >;
 
+type MetricsLayer<T, N> = BodyDataLayer<RequestLayer<ResponseLayer<T, N>>>;
+type BodyDataLayer<N> = NewRecordBodyData<ExtractRecordBodyDataParams, N>;
+type RequestLayer<N> = NewCountRequests<ExtractRequestCount, N>;
+type ResponseLayer<T, N> = 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 = MetricsLayer<T, N>> + Clone
 where
     T: MkStreamLabel,
 {