feat(smithy-client): add handler cache

.changeset/tall-cameras-reply.md

@@ -0,0 +1,5 @@
+---
+"@smithy/smithy-client": minor
+---
+
+add client handler caching

packages/smithy-client/src/client.spec.ts

@@ -9,7 +9,7 @@ describe("SmithyClient", () => {
   const getCommandWithOutput = (output: string) => ({
     resolveMiddleware: mockResolveMiddleware,
   });
-  const client = new Client({} as any);
+  const client = new Client({ cacheMiddleware: true } as any);
 
   beforeEach(() => {
     jest.clearAllMocks();
@@ -50,4 +50,28 @@ describe("SmithyClient", () => {
     };
     client.send(getCommandWithOutput("foo") as any, options, callback);
   });
+
+  describe("handler caching", () => {
+    beforeEach(() => {
+      delete (client as any).handlers;
+    });
+
+    const privateAccess = () => (client as any).handlers;
+
+    it("should cache the resolved handler", async () => {
+      await expect(client.send(getCommandWithOutput("foo") as any)).resolves.toEqual("foo");
+      expect(privateAccess().get({}.constructor)).toBeDefined();
+    });
+
+    it("should not cache the resolved handler if called with request options", async () => {
+      await expect(client.send(getCommandWithOutput("foo") as any, {})).resolves.toEqual("foo");
+      expect(privateAccess()).toBeUndefined();
+    });
+
+    it("unsets the cache if client.destroy() is called.", async () => {
+      await expect(client.send(getCommandWithOutput("foo") as any)).resolves.toEqual("foo");
+      client.destroy();
+      expect(privateAccess()).toBeUndefined();
+    });
+  });
 });

packages/smithy-client/src/client.ts

@@ -3,6 +3,7 @@ import {
   Client as IClient,
   Command,
   FetchHttpHandlerOptions,
+  Handler,
   MetadataBearer,
   MiddlewareStack,
   NodeHttpHandlerOptions,
@@ -24,6 +25,22 @@ export interface SmithyConfiguration<HandlerOptions> {
    * @internal
    */
   readonly apiVersion: string;
+  /**
+   * @public
+   *
+   * Default false.
+   *
+   * When true, the client will only resolve the middleware stack once per
+   * Command class. This means modifying the middlewareStack of the
+   * command or client after requests have been made will not be
+   * recognized.
+   *
+   * Calling client.destroy() also clears this cache.
+   *
+   * Enable this only if needing the additional time saved (0-1ms per request)
+   * and not needing middleware modifications between requests.
+   */
+  cacheMiddleware?: boolean;
 }
 
 /**
@@ -32,6 +49,7 @@ export interface SmithyConfiguration<HandlerOptions> {
 export type SmithyResolvedConfiguration<HandlerOptions> = {
   requestHandler: RequestHandler<any, any, HandlerOptions>;
   readonly apiVersion: string;
+  cacheMiddleware?: boolean;
 };
 
 /**
@@ -45,10 +63,13 @@ export class Client<
 > implements IClient<ClientInput, ClientOutput, ResolvedClientConfiguration>
 {
   public middlewareStack: MiddlewareStack<ClientInput, ClientOutput> = constructStack<ClientInput, ClientOutput>();
-  readonly config: ResolvedClientConfiguration;
-  constructor(config: ResolvedClientConfiguration) {
-    this.config = config;
-  }
+  /**
+   * May be used to cache the resolved handler function for a Command class.
+   */
+  private handlers?: WeakMap<Function, Handler<any, any>> | undefined;
+
+  constructor(public readonly config: ResolvedClientConfiguration) {}
+
   send<InputType extends ClientInput, OutputType extends ClientOutput>(
     command: Command<ClientInput, InputType, ClientOutput, OutputType, SmithyResolvedConfiguration<HandlerOptions>>,
     options?: HandlerOptions
@@ -69,7 +90,28 @@ export class Client<
   ): Promise<OutputType> | void {
     const options = typeof optionsOrCb !== "function" ? optionsOrCb : undefined;
     const callback = typeof optionsOrCb === "function" ? (optionsOrCb as (err: any, data?: OutputType) => void) : cb;
-    const handler = command.resolveMiddleware(this.middlewareStack as any, this.config, options);
+
+    const useHandlerCache = options === undefined && this.config.cacheMiddleware === true;
+
+    let handler: Handler<any, any>;
+
+    if (useHandlerCache) {
+      if (!this.handlers) {
+        this.handlers = new WeakMap();
+      }
+      const handlers = this.handlers!;
+
+      if (handlers.has(command.constructor)) {
+        handler = handlers.get(command.constructor)!;
+      } else {
+        handler = command.resolveMiddleware(this.middlewareStack as any, this.config, options);
+        handlers.set(command.constructor, handler);
+      }
+    } else {
+      delete this.handlers;
+      handler = command.resolveMiddleware(this.middlewareStack as any, this.config, options);
+    }
+
     if (callback) {
       handler(command)
         .then(
@@ -87,6 +129,7 @@ export class Client<
   }
 
   destroy() {
-    if (this.config.requestHandler.destroy) this.config.requestHandler.destroy();
+    this.config?.requestHandler?.destroy?.();
+    delete this.handlers;
   }
 }