Merge pull request #72 from oslabs-beta/jd/slide

SlidingWindowCounter spec completed, some functionality copied over from token bucket

Author: jondeweydev

src/@types/rateLimit.d.ts

@@ -23,6 +23,12 @@ export interface RedisBucket {
     timestamp: number;
 }
 
+export interface RedisWindow {
+    currentTokens: number;
+    previousTokens: number;
+    fixedWindowStart: number;
+}
+
 export type RateLimiterSelection =
     | 'TOKEN_BUCKET'
     | 'LEAKY_BUCKET'

src/rateLimiters/slidingWindowCounter.ts

@@ -0,0 +1,95 @@
+import Redis from 'ioredis';
+import { RateLimiter, RateLimiterResponse, RedisWindow } from '../@types/rateLimit';
+
+/**
+ * The SlidingWindowCounter instance of a RateLimiter limits requests based on a unique user ID.
+ * This algorithm improves upon the FixedWindowCounter because this algorithm prevents fixed window's
+ * flaw of allowing doubled capacity requests when hugging the window's borders with a rolling window,
+ * allowing us to average the requests between both windows proportionately with the rolling window's
+ * takeup in each.
+ *
+ * Whenever a user makes a request the following steps are performed:
+ *  1. Fixed minute windows are defined along with redis caches if previously undefined.
+ *  2. Rolling minute windows are defined or updated based on the timestamp of the new request.
+ *  3. Counter of the current fixed window is updated with the new request's token usage.
+ *  4. If a new minute interval is reached, the averaging formula is run to prevent fixed window's flaw
+ *     of flooded requests around window borders
+ *    (ex. 10 token capacity: 1m59s 10 reqs 2m2s 10 reqs)
+ */
+class SlidingWindowCounter implements RateLimiter {
+    private windowSize: number;
+
+    private capacity: number;
+
+    private client: Redis;
+
+    /**
+     * Create a new instance of a TokenBucket rate limiter that can be connected to any database store
+     * @param windowSize size of each window in milliseconds (fixed and rolling)
+     * @param capacity max capacity of tokens allowed per fixed window
+     * @param client redis client where rate limiter will cache information
+     */
+    constructor(windowSize: number, capacity: number, client: Redis) {
+        this.windowSize = windowSize;
+        this.capacity = capacity;
+        this.client = client;
+        if (windowSize <= 0 || capacity <= 0)
+            throw SyntaxError('SlidingWindowCounter windowSize and capacity must be positive');
+    }
+
+    /**
+     *
+     *
+     * @param {string} uuid - unique identifer used to throttle requests
+     * @param {number} timestamp - time the request was recieved
+     * @param {number} [tokens=1]  - complexity of the query for throttling requests
+     * @return {*}  {Promise<RateLimiterResponse>}
+     * @memberof SlidingWindowCounter
+     */
+    async processRequest(
+        uuid: string,
+        timestamp: number,
+        tokens = 1
+    ): Promise<RateLimiterResponse> {
+        // set the expiry of key-value pairs in the cache to 24 hours
+        const keyExpiry = 86400000;
+
+        // attempt to get the value for the uuid from the redis cache
+        const windowJSON = await this.client.get(uuid);
+
+        // // if the response is null, we need to create a window for the user
+        // if (windowJSON === null) {
+        //     // rolling window is 1 minute long
+        //     const rollingWindowEnd = timestamp + 60000;
+
+        //     // grabs the actual minute from the timestamp to create fixed window
+        //     const fixedWindowStart = timestamp - (timestamp % 10000);
+        //     const fixedWindowEnd = fixedWindowStart + 60000;
+
+        //     const newUserWindow: RedisWindow = {
+        //         // conditionally set tokens depending on how many are requested compared to the capacity
+        //         tokens: tokens > this.capacity ? this.capacity : this.capacity - tokens,
+        //         timestamp,
+        //     };
+
+        //     // reject the request, not enough tokens could even be in the bucket
+        //     if (tokens > this.capacity) {
+        //         await this.client.setex(uuid, keyExpiry, JSON.stringify(newUserWindow));
+        //         return { success: false, tokens: this.capacity };
+        //     }
+        //     await this.client.setex(uuid, keyExpiry, JSON.stringify(newUserWindow));
+        //     return { success: true, tokens: newUserWindow.tokens };
+        // }
+
+        return { success: true, tokens: 0 };
+    }
+
+    /**
+     * Resets the rate limiter to the intial state by clearing the redis store.
+     */
+    public reset(): void {
+        this.client.flushall();
+    }
+}
+
+export default SlidingWindowCounter;