feat: add rate limiter class

dev/src/rate-limiter.ts

@@ -0,0 +1,153 @@
+/*!
+ * Copyright 2020 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+import * as assert from 'assert';
+
+import {Timestamp} from './timestamp';
+
+/**
+ * A helper that uses the Token Bucket algorithm to rate limit the number of
+ * operations that can be made in a second.
+ *
+ * Before a given request containing a number of operations can proceed,
+ * RateLimiter determines doing so stays under the provided rate limits. It can
+ * also determine how much time is required before a request can be made.
+ *
+ * RateLimiter can also implement a gradually increasing rate limit. This is
+ * used to enforce the 500/50/5 rule
+ * (https://cloud.google.com/datastore/docs/best-practices#ramping_up_traffic).
+ *
+ * @private
+ */
+export class RateLimiter {
+  // Number of tokens available. Each operation consumes one token.
+  availableTokens: number;
+
+  // When the token bucket was last refilled.
+  lastRefillTimeMillis: number;
+
+  /**
+   * @param initialCapacity Initial maximum number of operations per second.
+   * @param multiplier Rate by which to increase the capacity.
+   * @param multiplierMillis How often the capacity should increase in
+   * milliseconds.
+   * @param startTimeMillis The starting time in epoch milliseconds that the
+   * rate limit is based on. Used for testing the limiter.
+   */
+  constructor(
+    private readonly initialCapacity: number,
+    private readonly multiplier: number,
+    private readonly multiplierMillis: number,
+    private readonly startTimeMillis = Date.now()
+  ) {
+    this.availableTokens = initialCapacity;
+    this.lastRefillTimeMillis = startTimeMillis;
+  }
+
+  /**
+   * Tries to make the number of operations. Returns true if the request
+   * succeeded and false otherwise.
+   *
+   * @param requestTimeMillis The date used to calculate the number of available
+   * tokens. Used for testing the limiter.
+   * @private
+   */
+  tryMakeRequest(
+    numOperations: number,
+    requestTimeMillis = Date.now()
+  ): boolean {
+    this.refillTokens(requestTimeMillis);
+    if (numOperations <= this.availableTokens) {
+      this.availableTokens -= numOperations;
+      return true;
+    }
+    return false;
+  }
+
+  /**
+   * Returns the number of ms needed to make a request with the provided number
+   * of operations. Returns 0 if the request can be made with the existing
+   * capacity. Returns -1 if the request is not possible with the current
+   * capacity.
+   *
+   * @param requestTimeMillis The date used to calculate the number of available
+   * tokens. Used for testing the limiter.
+   * @private
+   */
+  getNextRequestDelayMs(
+    numOperations: number,
+    requestTimeMillis = Date.now()
+  ): number {
+    if (numOperations < this.availableTokens) {
+      return 0;
+    }
+
+    const capacity = this.calculateCapacity(requestTimeMillis);
+    if (capacity < numOperations) {
+      return -1;
+    }
+
+    const requiredTokens = numOperations - this.availableTokens;
+    return Math.ceil((requiredTokens * 1000) / capacity);
+  }
+
+  /**
+   * Refills the number of available tokens based on how much time has elapsed
+   * since the last time the tokens were refilled.
+   *
+   * @param requestTimeMillis The date used to calculate the number of available
+   * tokens. Used for testing the limiter.
+   * @private
+   */
+  private refillTokens(requestTimeMillis = Date.now()): void {
+    if (requestTimeMillis >= this.lastRefillTimeMillis) {
+      const elapsedTime = requestTimeMillis - this.lastRefillTimeMillis;
+      const capacity = this.calculateCapacity(requestTimeMillis);
+      const tokensToAdd = Math.floor((elapsedTime * capacity) / 1000);
+      if (tokensToAdd > 0) {
+        this.availableTokens = Math.min(
+          capacity,
+          this.availableTokens + tokensToAdd
+        );
+        this.lastRefillTimeMillis = requestTimeMillis;
+      }
+    } else {
+      throw new Error(
+        'Request time should not be before the last token refill time.'
+      );
+    }
+  }
+
+  /**
+   * Calculates the maximum capacity based on the provided date.
+   *
+   * @private
+   */
+  // Visible for testing.
+  calculateCapacity(requestTimeMillis: number): number {
+    assert(
+      requestTimeMillis >= this.startTimeMillis,
+      'startTime cannot be after currentTime'
+    );
+    const millisElapsed = requestTimeMillis - this.startTimeMillis;
+    const operationsPerSecond = Math.floor(
+      Math.pow(
+        this.multiplier,
+        Math.floor(millisElapsed / this.multiplierMillis)
+      ) * this.initialCapacity
+    );
+    return operationsPerSecond;
+  }
+}

dev/test/rate-limiter.ts

@@ -0,0 +1,109 @@
+// Copyright 2020 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+import {expect} from 'chai';
+
+import {RateLimiter} from '../src/rate-limiter';
+
+describe('RateLimiter', () => {
+  let limiter: RateLimiter;
+
+  beforeEach(() => {
+    limiter = new RateLimiter(
+      /* initialCapacity= */ 500,
+      /* multiplier= */ 1.5,
+      /* multiplierMillis= */ 5 * 60 * 1000,
+      /* startTime= */ new Date(0).getTime()
+    );
+  });
+
+  it('accepts and rejects requests based on capacity', () => {
+    expect(limiter.tryMakeRequest(250, new Date(0).getTime())).to.be.true;
+    expect(limiter.tryMakeRequest(250, new Date(0).getTime())).to.be.true;
+
+    // Once tokens have been used, further requests should fail.
+    expect(limiter.tryMakeRequest(1, new Date(0).getTime())).to.be.false;
+
+    // Tokens will only refill up to max capacity.
+    expect(limiter.tryMakeRequest(501, new Date(1 * 1000).getTime())).to.be
+      .false;
+    expect(limiter.tryMakeRequest(500, new Date(1 * 1000).getTime())).to.be
+      .true;
+
+    // Tokens will refill incrementally based on the number of ms elapsed.
+    expect(limiter.tryMakeRequest(250, new Date(1 * 1000 + 499).getTime())).to
+      .be.false;
+    expect(limiter.tryMakeRequest(249, new Date(1 * 1000 + 500).getTime())).to
+      .be.true;
+
+    // Scales with multiplier.
+    expect(limiter.tryMakeRequest(751, new Date((5 * 60 - 1) * 1000).getTime()))
+      .to.be.false;
+    expect(limiter.tryMakeRequest(751, new Date(5 * 60 * 1000).getTime())).to.be
+      .false;
+    expect(limiter.tryMakeRequest(750, new Date(5 * 60 * 1000).getTime())).to.be
+      .true;
+
+    // Tokens will never exceed capacity.
+    expect(limiter.tryMakeRequest(751, new Date((5 * 60 + 3) * 1000).getTime()))
+      .to.be.false;
+
+    // Rejects requests made before lastRefillTime
+    expect(() =>
+      limiter.tryMakeRequest(751, new Date((5 * 60 + 2) * 1000).getTime())
+    ).to.throw('Request time should not be before the last token refill time.');
+  });
+
+  it('calculates the number of ms needed to place the next request', () => {
+    // Should return 0 if there are enough tokens for the request to be made.
+    let timestamp = new Date(0).getTime();
+    expect(limiter.getNextRequestDelayMs(500, timestamp)).to.equal(0);
+
+    // Should factor in remaining tokens when calculating the time.
+    expect(limiter.tryMakeRequest(250, timestamp));
+    expect(limiter.getNextRequestDelayMs(500, timestamp)).to.equal(500);
+
+    // Once tokens have been used, should calculate time before next request.
+    timestamp = new Date(1 * 1000).getTime();
+    expect(limiter.tryMakeRequest(500, timestamp)).to.be.true;
+    expect(limiter.getNextRequestDelayMs(100, timestamp)).to.equal(200);
+    expect(limiter.getNextRequestDelayMs(250, timestamp)).to.equal(500);
+    expect(limiter.getNextRequestDelayMs(500, timestamp)).to.equal(1000);
+    expect(limiter.getNextRequestDelayMs(501, timestamp)).to.equal(-1);
+
+    // Scales with multiplier.
+    timestamp = new Date(5 * 60 * 1000).getTime();
+    expect(limiter.tryMakeRequest(750, timestamp)).to.be.true;
+    expect(limiter.getNextRequestDelayMs(250, timestamp)).to.equal(334);
+    expect(limiter.getNextRequestDelayMs(500, timestamp)).to.equal(667);
+    expect(limiter.getNextRequestDelayMs(750, timestamp)).to.equal(1000);
+    expect(limiter.getNextRequestDelayMs(751, timestamp)).to.equal(-1);
+  });
+
+  it('calculates the maximum number of operations correctly', async () => {
+    expect(limiter.calculateCapacity(new Date(0).getTime())).to.equal(500);
+    expect(
+      limiter.calculateCapacity(new Date(5 * 60 * 1000).getTime())
+    ).to.equal(750);
+    expect(
+      limiter.calculateCapacity(new Date(10 * 60 * 1000).getTime())
+    ).to.equal(1125);
+    expect(
+      limiter.calculateCapacity(new Date(15 * 60 * 1000).getTime())
+    ).to.equal(1687);
+    expect(
+      limiter.calculateCapacity(new Date(90 * 60 * 1000).getTime())
+    ).to.equal(738945);
+  });
+});