feat: rework max connections for OkHttp engine (#795)

Author: ianbotsf

.changes/73e8719a-3ed4-4316-a1a6-d673f1c85282.json

@@ -0,0 +1,5 @@
+{
+    "id": "73e8719a-3ed4-4316-a1a6-d673f1c85282",
+    "type": "bugfix",
+    "description": "Correctly apply `maxConnections` configuration setting to OkHttp engines"
+}

.changes/f71de744-44c0-4404-8d93-04ecda57650b.json

@@ -0,0 +1,5 @@
+{
+    "id": "f71de744-44c0-4404-8d93-04ecda57650b",
+    "type": "feature",
+    "description": "Add new `maxConnectionsPerHost` configuration setting for OkHttp engines"
+}

runtime/protocol/http-client-engines/http-client-engine-okhttp/jvm/src/aws/smithy/kotlin/runtime/http/engine/okhttp/OkHttpEngine.kt

@@ -70,20 +70,29 @@ private fun OkHttpEngineConfig.buildClient(): OkHttpClient {
         followSslRedirects(false)
 
         // see: https://github.com/ktorio/ktor/issues/1708#issuecomment-609988128
+        // TODO disable this once better transient exception handling is in place
         retryOnConnectionFailure(true)
 
         connectTimeout(config.connectTimeout.toJavaDuration())
         readTimeout(config.socketReadTimeout.toJavaDuration())
         writeTimeout(config.socketWriteTimeout.toJavaDuration())
 
-        // use our own pool configured with the settings taken from config
+        // use our own pool configured with the timeout settings taken from config
         val pool = ConnectionPool(
-            maxIdleConnections = config.maxConnections.toInt(),
+            maxIdleConnections = 5, // The default from the no-arg ConnectionPool() constructor
             keepAliveDuration = config.connectionIdleTimeout.inWholeMilliseconds,
             TimeUnit.MILLISECONDS,
         )
         connectionPool(pool)
 
+        // Configure a dispatcher that uses maxConnections as a proxy for maxRequests. Note that this isn't precisely
+        // the same since some protocols (e.g., HTTP2) may use a single connection for multiple requests.
+        val dispatcher = Dispatcher().apply {
+            maxRequests = config.maxConnections.toInt()
+            maxRequestsPerHost = config.maxConnectionsPerHost.toInt()
+        }
+        dispatcher(dispatcher)
+
         // Log events coming from okhttp. Allocate a new listener per-call to facilitate dedicated trace spans.
         eventListenerFactory { call -> HttpEngineEventListener(pool, config.hostResolver, call) }
 

runtime/protocol/http-client-engines/http-client-engine-okhttp/jvm/src/aws/smithy/kotlin/runtime/http/engine/okhttp/OkHttpEngineConfig.kt

@@ -8,6 +8,11 @@ package aws.smithy.kotlin.runtime.http.engine.okhttp
 import aws.smithy.kotlin.runtime.http.engine.HttpClientEngineConfig
 
 public class OkHttpEngineConfig private constructor(builder: Builder) : HttpClientEngineConfig(builder) {
+    /**
+     * The maximum number of connections to open to a single host.
+     */
+    public val maxConnectionsPerHost: UInt = builder.maxConnectionsPerHost ?: builder.maxConnections
+
     public companion object {
         /**
          * The default engine config. Most clients should use this.
@@ -19,6 +24,10 @@ public class OkHttpEngineConfig private constructor(builder: Builder) : HttpClie
     }
 
     public class Builder : HttpClientEngineConfig.Builder() {
+        /**
+         * The maximum number of connections to open to a single host. Defaults to [maxConnections].
+         */
+        public var maxConnectionsPerHost: UInt? = null
 
         internal fun build(): OkHttpEngineConfig = OkHttpEngineConfig(this)
     }