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According to the e-commerce written on the resume, how does redis realize inventory deduction and prevent oversold?

2022-07-23 19:36:00 【Ah size】

E-commerce project experience has been very common , Whether you are packaged or real , At least be able to explain the common problems in e-commerce , For example, how to prevent the inventory operation from being oversold

Solution
analysis
- Based on database single inventory
- Multi inventory based on Database
- be based on redis
be based on redis Realize the specific realization of inventory deduction
- Initialize inventory callback function （IStockCallback）
- Deduct inventory service （StockService）
- call

In daily development, there are many similar operations of inventory deduction , For example, commodity inventory in e-commerce system , Prize inventory in the lottery system .

Solution

Use mysql database , Use a field to store inventory , Update this field every time you deduct inventory .
Or use a database , But the inventory is layered and stored in multiple records , When deducting inventory, route , This increases the amount of concurrency , But still can not avoid a large number of access to the database to update the inventory .
Put stock in redis Use redis Of incrby Features to reduce inventory .

analysis

The first and second methods above are based on data to deduct inventory .

Based on database single inventory

The first way is to wait for the lock here for all requests , Acquire lock to deduct inventory . It can be used when the concurrency is not high , But once the concurrency is large, there will be a lot of requests blocking here , Causes the request to time out , And then the whole system avalanches ; And will visit the database frequently , Take up a lot of database resources , So in the case of high concurrency, this method is not applicable .

Multi inventory based on Database

The second way is actually the optimized version of the first way , To some extent, it improves the concurrency , However, there will still be a large number of database update operations taking up a large number of database resources .

There are still some problems in reducing inventory based on database ：

How to deduct inventory with database , The inventory deduction must be performed in one statement , Not first selec stay update, In this way, there will be a situation of over deduction under the concurrency . Such as ：

update  number  set x=x -1  where x >  0

MySQL There will be problems with its own processing performance for high concurrency , Generally speaking ,MySQL The processing performance of will follow the concurrency thread Rise and rise , But after a certain degree of concurrency, there will be obvious inflection points , Then all the way down , In the end, it's even better than Dan thread The performance is even worse .
When inventory reduction and high concurrency come together , Because the number of stocks in operation is on the same line , There will be a fight InnoDB The problem of line lock , Lead to waiting for each other or even deadlock , Thus greatly reducing MySQL Processing performance of , Finally, a timeout exception occurs on the front-end page .

be based on redis

To solve the above problems, we have a third plan , Put inventory in cache , utilize redis Of incrby Features to reduce inventory , Solve the problem of over buckle and performance . But once the cache is lost, we need to consider the recovery plan . For example, when the lottery system buckles the prize stock , Initial stock = Total stock - The number of awards that have been awarded , But if it's asynchronous Awards , Need to wait until MQ Message consumption is over before restart redis Initialize inventory , Otherwise, there is the problem of inconsistent inventory . Project practice （ Click to download ）：SpringBoot+SpringCloud+Mybatis+Vue E-commerce project actual battle

be based on redis Realize the specific realization of inventory deduction

We use redis Of lua Script to reduce inventory
In the distributed environment, a distributed lock is needed to control that only one service can initialize the inventory
You need to provide a callback function , When initializing the inventory, call this function to get the initialization inventory

Initialize inventory callback function (IStockCallback )

/**
 *  Get inventory callback
 * @author yuhao.wang
 */
public  interface IStockCallback {

  /**
  *  Get inventory
  * @return
  */
  int getStock();
}

Deduct inventory service （StockService）

/**
 *  Inventory deduction
 *
 * @author yuhao.wang
 */
@Service
public  class StockService {
    Logger logger = LoggerFactory.getLogger(StockService .class);

     /**
     *  No stock limit
     */
     public  static  final  long UNINITIALIZED_STOCK = - 3L;

     /**
     * Redis  client
     */
     @Autowired
     private RedisTemplate<String, Object> redisTemplate;

     /**
     *  Execute stock deduction script
     */
     public  static  final String STOCK_LUA;

     static {
         /**
         *
         * @desc  Deducting the inventory Lua Script
         *  stock （stock）-1： Means unlimited stock
         *  stock （stock）0： Means there is no stock
         *  stock （stock） Greater than 0： Represents the remaining stock
         *
         * @params  stock key
         * @return
         *   -3: Inventory not initialized
         *   -2: Insufficient inventory
         *   -1: No stock limit
         *    Greater than or equal to 0: Surplus stock （ The remaining stock after deduction ）
         *      redis Cached inventory (value) yes -1 Means unlimited stock , Go straight back to 1
         */
        StringBuilder sb =  new StringBuilder();
        sb.append( "if (redis.call('exists', KEYS[1]) == 1) then");
        sb.append( "    local stock = tonumber(redis.call('get', KEYS[1]));");
        sb.append( "    local num = tonumber(ARGV[1]);");
        sb.append( "    if (stock == -1) then");
        sb.append( "        return -1;");
        sb.append( "    end;");
        sb.append( "    if (stock >= num) then");
        sb.append( "        return redis.call('incrby', KEYS[1], 0 - num);");
        sb.append( "    end;");
        sb.append( "    return -2;");
        sb.append( "end;");
        sb.append( "return -3;");
        STOCK_LUA = sb.toString();
    }

     /**
     * @param key            stock key
     * @param expire         Stock effective time , Unit second
     * @param num            Quantity deducted
     * @param stockCallback  Initialize inventory callback function
     * @return -2: Insufficient inventory ; -1: No stock limit ;  Greater than or equal to 0: Remaining stock after deducting stock
     */
     public long stock(String key, long expire, int num, IStockCallback stockCallback) {
         long stock = stock(key, num);
         //  Initialize inventory
         if (stock == UNINITIALIZED_STOCK) {
            RedisLock redisLock =  new RedisLock(redisTemplate, key);
             try {
                 //  Get the lock
                 if (redisLock.tryLock()) {
                     //  Double verification , Avoid returning to the source to the database repeatedly in case of concurrency
                    stock = stock(key, num);
                     if (stock == UNINITIALIZED_STOCK) {
                         //  Get initialization stock
                         final  int initStock = stockCallback.getStock();
                         //  Set inventory to redis
                        redisTemplate.opsForValue().set(key, initStock, expire, TimeUnit.SECONDS);
                         //  Adjust the operation of stock deduction once
                        stock = stock(key, num);
                    }
                }
            }  catch (Exception e) {
                logger.error(e.getMessage(), e);
            }  finally {
                redisLock.unlock();
            }

        }
         return stock;
    }

     /**
     *  Add inventory ( Restore inventory )
     *
     * @param key     stock key
     * @param num     Inventory quantity
     * @return
     */
     public long addStock(String key, int num) {

         return addStock(key,  null, num);
    }

     /**
     *  Add inventory
     *
     * @param key     stock key
     * @param expire  Expiration time （ second ）
     * @param num     Inventory quantity
     * @return
     */
     public long addStock(String key, Long expire, int num) {
         boolean hasKey = redisTemplate.hasKey(key);
         //  Judge key Whether there is , Live and update
         if (hasKey) {
             return redisTemplate.opsForValue().increment(key, num);
        }

        Assert.notNull(expire, " Failed to initialize inventory , Stock expiration time cannot be null");
        RedisLock redisLock =  new RedisLock(redisTemplate, key);
         try {
             if (redisLock.tryLock()) {
                 //  After obtaining the lock, judge whether there is key
                hasKey = redisTemplate.hasKey(key);
                 if (!hasKey) {
                     //  Initialize inventory
                    redisTemplate.opsForValue().set(key, num, expire, TimeUnit.SECONDS);
                }
            }
        }  catch (Exception e) {
            logger.error(e.getMessage(), e);
        }  finally {
            redisLock.unlock();
        }

         return num;
    }

     /**
     *  Get inventory
     *
     * @param key  stock key
     * @return -1: No stock limit ;  Greater than or equal to 0: Surplus stock
     */
     public int getStock(String key) {
        Integer stock = (Integer) redisTemplate.opsForValue().get(key);
         return stock ==  null ? - 1 : stock;
    }

     /**
     *  Inventory deduction
     *
     * @param key  stock key
     * @param num  Deduct stock quantity
     * @return  The remaining stock after deduction 【-3: Inventory not initialized ; -2: Insufficient inventory ; -1: No stock limit ;  Greater than or equal to 0: Remaining stock after deducting stock 】
     */
     private Long stock(String key, int num) {
         //  In the script KEYS Parameters
        List<String> keys =  new ArrayList<>();
        keys.add(key);
         //  In the script ARGV Parameters
        List<String> args =  new ArrayList<>();
        args.add(Integer.toString(num));

         long result = redisTemplate.execute( new RedisCallback<Long>() {
             @Override
             public Long doInRedis(RedisConnection connection) throws DataAccessException {
                Object nativeConnection = connection.getNativeConnection();
                 //  The cluster mode and the stand-alone mode execute scripts in the same way , But there is no common interface , So it can only be carried out separately
                 //  Cluster pattern
                 if (nativeConnection  instanceof JedisCluster) {
                     return (Long) ((JedisCluster) nativeConnection).eval(STOCK_LUA, keys, args);
                }

                 //  standalone mode
                 else  if (nativeConnection  instanceof Jedis) {
                     return (Long) ((Jedis) nativeConnection).eval(STOCK_LUA, keys, args);
                }
                 return UNINITIALIZED_STOCK;
            }
        });
         return result;
    }

}

call

/**
 * @author yuhao.wang
 */
@RestController
public  class StockController {

     @Autowired
     private StockService stockService;

     @RequestMapping(value =  "stock", produces = MediaType.APPLICATION_JSON_UTF8_VALUE)
     public Object stock() {
         //  goods ID
         long commodityId =  1;
         //  stock ID
        String redisKey =  "redis_key:stock:" + commodityId;
         long stock = stockService.stock(redisKey,  60 *  60,  2, () -> initStock(commodityId));
         return stock >=  0;
    }

     /**
     *  Get the initial stock
     *
     * @return
     */
     private int initStock(long commodityId) {
         // TODO  Here are some operations to initialize the inventory
         return  1000;
    }

     @RequestMapping(value =  "getStock", produces = MediaType.APPLICATION_JSON_UTF8_VALUE)
     public Object getStock() {
         //  goods ID
         long commodityId =  1;
         //  stock ID
        String redisKey =  "redis_key:stock:" + commodityId;

         return stockService.getStock(redisKey);
    }

     @RequestMapping(value =  "addStock", produces = MediaType.APPLICATION_JSON_UTF8_VALUE)
     public Object addStock() {
         //  goods ID
         long commodityId =  2;
         //  stock ID
        String redisKey =  "redis_key:stock:" + commodityId;

         return stockService.addStock(redisKey,  2);
    }
}

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