From 768f6d42f53e371bc4f12205446b08989c7d3e5d Mon Sep 17 00:00:00 2001
From: Simon Sapin <simon@apollographql.com>
Date: Tue, 11 Feb 2025 15:43:03 +0100
Subject: [PATCH] Add $APOLLO_ROUTER_COMPUTE_THREADS environment variable
 (#6746)

---
 apollo-router/Cargo.toml                         |  1 +
 apollo-router/src/compute_job.rs                 | 16 ++++++++++++----
 apollo-router/src/executable.rs                  |  6 +++++-
 ...phql_with_safelist_log_unknown_true@logs.snap |  1 -
 4 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/apollo-router/Cargo.toml b/apollo-router/Cargo.toml
index fa0a427ff1..0f75137286 100644
--- a/apollo-router/Cargo.toml
+++ b/apollo-router/Cargo.toml
@@ -11,6 +11,7 @@ license = "Elastic-2.0"
 rust-version = "1.83.0"
 edition = "2021"
 build = "build/main.rs"
+default-run = "router"
 
 [[bin]]
 name = "router"
diff --git a/apollo-router/src/compute_job.rs b/apollo-router/src/compute_job.rs
index 64fe16b20d..a0bcab20a3 100644
--- a/apollo-router/src/compute_job.rs
+++ b/apollo-router/src/compute_job.rs
@@ -14,13 +14,21 @@ use crate::metrics::meter_provider;
 /// reaches `QUEUE_SOFT_CAPACITY_PER_THREAD * thread_pool_size()`
 const QUEUE_SOFT_CAPACITY_PER_THREAD: usize = 20;
 
-/// Let this thread pool use all available resources if it can.
+/// By default, let this thread pool use all available resources if it can.
 /// In the worst case, we’ll have moderate context switching cost
 /// as the kernel’s scheduler distributes time to it or Tokio or other threads.
 fn thread_pool_size() -> usize {
-    std::thread::available_parallelism()
-        .expect("available_parallelism() failed")
-        .get()
+    // This environment variable is intentionally undocumented.
+    if let Some(threads) = std::env::var("APOLLO_ROUTER_COMPUTE_THREADS")
+        .ok()
+        .and_then(|value| value.parse::<usize>().ok())
+    {
+        threads
+    } else {
+        std::thread::available_parallelism()
+            .expect("available_parallelism() failed")
+            .get()
+    }
 }
 
 type Job = Box<dyn FnOnce() + Send + 'static>;
diff --git a/apollo-router/src/executable.rs b/apollo-router/src/executable.rs
index 84ffb393d6..b1445014f3 100644
--- a/apollo-router/src/executable.rs
+++ b/apollo-router/src/executable.rs
@@ -327,12 +327,16 @@ pub fn main() -> Result<()> {
 
     let mut builder = tokio::runtime::Builder::new_multi_thread();
     builder.enable_all();
-    if let Some(nb) = std::env::var("APOLLO_ROUTER_NUM_CORES")
+
+    // This environment variable is intentionally undocumented.
+    // See also APOLLO_ROUTER_COMPUTE_THREADS in apollo-router/src/compute_job.rs
+    if let Some(nb) = std::env::var("APOLLO_ROUTER_IO_THREADS")
         .ok()
         .and_then(|value| value.parse::<usize>().ok())
     {
         builder.worker_threads(nb);
     }
+
     let runtime = builder.build()?;
     runtime.block_on(Executable::builder().start())
 }
diff --git a/apollo-router/src/services/layers/persisted_queries/snapshots/apollo_router__services__layers__persisted_queries__tests__pq_layer_freeform_graphql_with_safelist_log_unknown_true@logs.snap b/apollo-router/src/services/layers/persisted_queries/snapshots/apollo_router__services__layers__persisted_queries__tests__pq_layer_freeform_graphql_with_safelist_log_unknown_true@logs.snap
index f9a850f1c8..d8c2c9b330 100644
--- a/apollo-router/src/services/layers/persisted_queries/snapshots/apollo_router__services__layers__persisted_queries__tests__pq_layer_freeform_graphql_with_safelist_log_unknown_true@logs.snap
+++ b/apollo-router/src/services/layers/persisted_queries/snapshots/apollo_router__services__layers__persisted_queries__tests__pq_layer_freeform_graphql_with_safelist_log_unknown_true@logs.snap
@@ -1,7 +1,6 @@
 ---
 source: apollo-router/src/services/layers/persisted_queries/mod.rs
 expression: yaml
-snapshot_kind: text
 ---
 - fields:
     operation_body: "query SomeQuery { me { id } }"