feat: connection pooling

connection.ts

@@ -34,10 +34,12 @@ export interface SendCommandOptions {
 }
 
 export interface Connection extends TypedEventTarget<ConnectionEventMap> {
+  /** @deprecated */
   name: string | null;
   isClosed: boolean;
   isConnected: boolean;
   close(): void;
+  [Symbol.dispose](): void;
   connect(): Promise<void>;
   reconnect(): Promise<void>;
   sendCommand(
@@ -309,6 +311,10 @@ export class RedisConnection
     this.#close(false);
   }
 
+  [Symbol.dispose](): void {
+    return this.close();
+  }
+
   #close(canReconnect = false) {
     const isClosedAlready = this._isClosed;
 

executor.ts

@@ -2,6 +2,9 @@ import type { Connection, SendCommandOptions } from "./connection.ts";
 import type { RedisReply, RedisValue } from "./protocol/shared/types.ts";
 
 export interface CommandExecutor {
+  /**
+   * @deprecated
+   */
   readonly connection: Connection;
   /**
    * @deprecated

pool/default_pool.ts

@@ -0,0 +1,95 @@
+import type { Pool } from "./pool.ts";
+
+const kDefaultTimeout = 5_000;
+class DefaultPool<T extends Disposable> implements Pool<T> {
+  readonly #idle: Array<T> = [];
+  readonly #connections: Array<T> = [];
+  #connectionCount: number = 0;
+  readonly #deferredQueue: Array<PromiseWithResolvers<T>> = [];
+  readonly #options: Required<PoolOptions<T>>;
+
+  constructor(
+    {
+      maxConnections = 8,
+      acquire,
+    }: PoolOptions<T>,
+  ) {
+    this.#options = {
+      acquire,
+      maxConnections,
+    };
+  }
+
+  async acquire(signal?: AbortSignal): Promise<T> {
+    signal ||= AbortSignal.timeout(kDefaultTimeout);
+    signal.throwIfAborted();
+    if (this.#idle.length > 0) {
+      const conn = this.#idle.shift()!;
+      return Promise.resolve(conn);
+    }
+
+    if (this.#connectionCount < this.#options.maxConnections) {
+      this.#connectionCount++;
+      try {
+        const connection = await this.#options.acquire();
+        this.#connections.push(connection);
+        return connection;
+      } catch (error) {
+        this.#connectionCount--;
+        throw error;
+      }
+    }
+
+    const deferred = Promise.withResolvers<T>();
+    this.#deferredQueue.push(deferred);
+    const { promise, reject } = deferred;
+    const onAbort = () => {
+      const i = this.#deferredQueue.indexOf(deferred);
+      if (i === -1) return;
+      this.#deferredQueue.splice(i, 1);
+      reject(signal.reason);
+    };
+    signal.addEventListener("abort", onAbort, { once: true });
+    return promise;
+  }
+
+  release(conn: T): void {
+    if (!this.#connections.includes(conn)) {
+      throw new Error(
+        "This connection has already been removed from the pool.",
+      );
+    } else if (this.#deferredQueue.length > 0) {
+      const i = this.#deferredQueue.shift()!;
+      i.resolve(conn);
+    } else {
+      this.#idle.push(conn);
+    }
+  }
+
+  close() {
+    const errors: Array<unknown> = [];
+    for (const x of this.#connections) {
+      try {
+        x[Symbol.dispose]();
+      } catch (error) {
+        errors.push(error);
+      }
+    }
+    this.#connections.length = 0;
+    this.#idle.length = 0;
+    if (errors.length > 0) {
+      throw new AggregateError(errors);
+    }
+  }
+}
+
+export interface PoolOptions<T extends Disposable> {
+  maxConnections?: number;
+  acquire(): Promise<T>;
+}
+
+export function createDefaultPool<T extends Disposable>(
+  options: PoolOptions<T>,
+): Pool<T> {
+  return new DefaultPool<T>(options);
+}

pool/default_pool_test.ts

@@ -0,0 +1,81 @@
+import { assert, assertEquals, assertRejects } from "../deps/std/assert.ts";
+import { createDefaultPool } from "./default_pool.ts";
+
+class FakeConnection implements Disposable {
+  #isClosed = false;
+  isClosed() {
+    return this.#isClosed;
+  }
+  [Symbol.dispose]() {
+    if (this.#isClosed) {
+      throw new Error("Already closed");
+    }
+    this.#isClosed = true;
+  }
+}
+
+Deno.test("DefaultPool", async () => {
+  const openConnections: Array<FakeConnection> = [];
+  const pool = createDefaultPool({
+    acquire: () => {
+      const connection = new FakeConnection();
+      openConnections.push(connection);
+      return Promise.resolve(connection);
+    },
+    maxConnections: 2,
+  });
+  assertEquals(openConnections, []);
+
+  const signal = AbortSignal.timeout(200);
+
+  const conn1 = await pool.acquire(signal);
+  assertEquals(openConnections, [conn1]);
+  assert(openConnections.every((x) => !x.isClosed()));
+  assert(!signal.aborted);
+
+  const conn2 = await pool.acquire(signal);
+  assertEquals(openConnections, [conn1, conn2]);
+  assert(!conn2.isClosed());
+  assert(openConnections.every((x) => !x.isClosed()));
+  assert(!signal.aborted);
+
+  {
+    // Tests timeout handling
+    await assertRejects(
+      () => pool.acquire(signal),
+      "Intentionally aborted",
+    );
+    assert(signal.aborted);
+    assertEquals(openConnections, [conn1, conn2]);
+    assert(openConnections.every((x) => !x.isClosed()));
+  }
+
+  {
+    // Tests `release()`
+    pool.release(conn2);
+    assertEquals(openConnections, [conn1, conn2]);
+
+    const conn = await pool.acquire(new AbortController().signal);
+    assert(conn === conn2, "A new connection should not be created");
+    assertEquals(openConnections, [conn1, conn2]);
+  }
+
+  {
+    // `Pool#acquire` should wait for an active connection to be released.
+    const signal = AbortSignal.timeout(3_000);
+    const promise = pool.acquire(signal);
+    setTimeout(() => {
+      pool.release(conn1);
+    }, 50);
+    const conn = await promise;
+    assert(conn === conn1, "A new connection should not be created");
+    assertEquals(openConnections, [conn1, conn2]);
+    assert(!signal.aborted);
+  }
+
+  {
+    // `Pool#close` closes all connections
+    pool.close();
+    assert(openConnections.every((x) => x.isClosed()));
+  }
+});

pool/pool.ts

@@ -0,0 +1,5 @@
+export interface Pool<T extends Disposable> {
+  acquire(signal?: AbortSignal): Promise<T>;
+  release(conn: T): void;
+  close(): void;
+}