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Charging Engine
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| /* eslint-disable @typescript-eslint/no-floating-promises */ | |
| /* eslint-disable @typescript-eslint/strict-boolean-expressions */ | |
| /* eslint-disable @typescript-eslint/restrict-template-expressions */ | |
| /* eslint-disable @typescript-eslint/no-misused-promises */ | |
| import express from "express"; | |
| import { json } from "body-parser"; | |
| import { createClient } from "redis"; | |
| const DEFAULT_BALANCE = 100; | |
| interface ChargeResult { | |
| isAuthorized: boolean; | |
| remainingBalance: number; | |
| charges: number; | |
| } | |
| interface EvalOptions { | |
| keys?: string[]; | |
| arguments?: string[]; | |
| } | |
| interface LockResult { | |
| success: number; | |
| balance: number; | |
| } | |
| async function connect(): Promise<ReturnType<typeof createClient>> { | |
| const url = `redis://${process.env.REDIS_HOST ?? "localhost"}:${process.env.REDIS_PORT ?? "6379"}`; | |
| const client = createClient({ url }); | |
| await client.connect(); | |
| return client; | |
| } | |
| async function reset(account: string): Promise<void> { | |
| const balanceKey = `${account}/balance`; | |
| const client = await connect(); | |
| try { | |
| await client.set(balanceKey, DEFAULT_BALANCE); | |
| } finally { | |
| await client.disconnect(); | |
| } | |
| } | |
| // Added function to set a lock and retrieve balance in a single transaction | |
| async function setLock(client: any, account: string): Promise<LockResult> { | |
| const balanceVersionKey = `${account}/balance/version`; | |
| const balanceVersionValue = generateRandomString(10); | |
| const balanceKey = `${account}/balance`; | |
| // We need the key to self destruct after 30 milliseconds | |
| // to remove the need of manually having to delete it | |
| const ttlMilliseconds = 30; | |
| // Use a Lua script to set the key if it doesn't exist and set TTL | |
| // And get the account balance | |
| const luaScript = ` | |
| local success | |
| local balance = redis.call("GET", KEYS[1]) | |
| if redis.call("EXISTS", KEYS[2]) == 0 then | |
| redis.call("SET", KEYS[2], ARGV[1]) | |
| redis.call("PEXPIRE", KEYS[2], ARGV[2]) | |
| success = 1 | |
| else | |
| success = 0 | |
| end | |
| return {success, balance} | |
| `; | |
| // Define EvalOptions | |
| const evalOptions: EvalOptions = { | |
| keys: [balanceKey, balanceVersionKey], | |
| arguments: [balanceVersionValue, ttlMilliseconds.toString()], | |
| }; | |
| const result = await client.eval(luaScript, evalOptions); | |
| const lockResult: LockResult = { | |
| success: result[0], | |
| balance: parseInt(result[1]), | |
| }; | |
| return lockResult; | |
| } | |
| // updated charge function to use setLock function | |
| async function charge(account: string, charges: number): Promise<ChargeResult> { | |
| const client = await connect(); | |
| const balanceKey = `${account}/balance`; | |
| try { | |
| const lockResult = await setLock(client, account); | |
| const balance = lockResult.balance; | |
| if (lockResult.success === 0 || charges > balance) { | |
| return { isAuthorized: false, remainingBalance: balance, charges: 0 }; | |
| } | |
| const newBalance = balance - charges; | |
| await client.set(balanceKey, newBalance); | |
| return { isAuthorized: true, remainingBalance: newBalance, charges }; | |
| } finally { | |
| await client.disconnect(); | |
| } | |
| } | |
| // Added function to generate random string | |
| const generateRandomString = (length: number): string => { | |
| let result = ""; | |
| const characters = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789"; | |
| const charactersLength = characters.length; | |
| for (let i = 0; i < length; i++) { | |
| result += characters.charAt(Math.floor(Math.random() * charactersLength)); | |
| } | |
| return result; | |
| }; | |
| export function buildApp(): express.Application { | |
| const app = express(); | |
| app.use(json()); | |
| app.post("/reset", async (req, res) => { | |
| try { | |
| const account = req.body.account ?? "account"; | |
| await reset(account); | |
| console.log(`Successfully reset account ${account}`); | |
| res.sendStatus(204); | |
| } catch (e) { | |
| console.error("Error while resetting account", e); | |
| res.status(500).json({ error: String(e) }); | |
| } | |
| }); | |
| app.post("/charge", async (req, res) => { | |
| try { | |
| const account = req.body.account ?? "account"; | |
| const result = await charge(account, req.body.charges ?? 10); | |
| // Changed what is logged to make it easier to observe operation success and account balance | |
| console.log(result); | |
| res.status(200).json(result); | |
| } catch (e) { | |
| console.error("Error while charging account", e); | |
| res.status(500).json({ error: String(e) }); | |
| } | |
| }); | |
| return app; | |
| } |
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