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分布式鎖場景
在分布式環(huán)境下多個操作需要以原子的方式執(zhí)行
首先啟一個springboot項(xiàng)目,再引入redis依賴包:
org.springframework.boot
spring-boot-starter-data-redis
2.2.2.RELEASE
以下是一個扣減庫存的接口作為例子:
@RestController
public class IndexController {
@Autowired
private StringRedisTemplate stringRedisTemplate;
@RequestMapping("/deduct_stock")
public Stirng deductStock() {
int stock = Integer.parseInt(stringRedisTemplate.opsForValue().get("stock"));//jedis.get(key)
if (stock > 0) {
int realStock = stock - 1;
stringRedisTemplate.opsForValue.set("stock",realStock+"");//jedis.set(key,value)
System.out.println(扣減成功,剩余庫存:" + realStock + "");
} else {
System.out.println(扣減失敗,庫存不足!" );
}
return "end";
}
}
1.單實(shí)例應(yīng)用場景
以上代碼使用JMeter壓測工具進(jìn)行調(diào)用,設(shè)置參數(shù)為:
Number Of Threads[users]:100
Ramp Up Period[in seconds]:0
Loop Count:2
用單個web調(diào)用,結(jié)果出現(xiàn)并發(fā)問題:
解決方案:加入同步鎖(synchronized)
@RestController
public class IndexController {
@Autowired
private StringRedisTemplate stringRedisTemplate;
@RequestMapping("/deduct_stock")
public Stirng deductStock() {
synchronized(this) {
int stock = Integer.parseInt(stringRedisTemplate.opsForValue().get("stock"));//jedis.get(key)
if (stock > 0) {
int realStock = stock - 1;
stringRedisTemplate.opsForValue.set("stock",realStock+"");//jedis.set(key,value)
System.out.println(扣減成功,剩余庫存:" + realStock + "");
} else {
System.out.println(扣減失敗,庫存不足!" );
}
return "end";
}
}
}
2.多實(shí)例分布式場景
以上代碼,比如有多個應(yīng)用程序,用nginx做負(fù)載均衡,進(jìn)行同時調(diào)用壓測
兩個程序存在同樣的扣減,出現(xiàn)并發(fā)現(xiàn)象。
第一個應(yīng)用扣減結(jié)果顯示:
第二個應(yīng)用扣減結(jié)果顯示:
解決方案:redis的setnx方法(可參考SETNX的api)
多個線程setnx調(diào)用時,有且僅有一個線程會拿到這把鎖,所以拿到鎖的執(zhí)行業(yè)務(wù)代碼,最后釋放掉鎖,代碼如下:
@RestController
public class IndexController {
@Autowired
private StringRedisTemplate stringRedisTemplate;
@RequestMapping("/deduct_stock")
public Stirng deductStock() {
String lockkey = "lockkey";
Boolean result = stringRedisTemplate.opsForValue.setIfAbsent(lockkey,"lockvalue");//jedis.setnx
if(!result) {
return "";
}
int stock = Integer.parseInt(stringRedisTemplate.opsForValue().get("stock"));//jedis.get(key)
if (stock > 0) {
int realStock = stock - 1;
stringRedisTemplate.opsForValue.set("stock",realStock+"");//jedis.set(key,value)
System.out.println(扣減成功,剩余庫存:" + realStock + "");
} else {
System.out.println(扣減失敗,庫存不足!" );
}
springRedisTemplate.delete(lockkey);
return "end";
}
}
調(diào)用200次,壓測結(jié)果顯示還是有問題,只減掉了一部分:
這時,加大壓測次數(shù),結(jié)果正常了:
第一個應(yīng)用扣減結(jié)果顯示:
第二個應(yīng)用扣減結(jié)果顯示:
這個只是因?yàn)榧哟罅苏{(diào)用次數(shù),執(zhí)行業(yè)務(wù)代碼需要一點(diǎn)時間,這段時間拒絕了很多等待獲取鎖的請求。但是,還是有問題,假如redis服務(wù)掛掉了,拋出異常了,這時鎖不會被釋放掉,出現(xiàn)死鎖問題,可以添加try catch處理,代碼如下:
@RestController
public class IndexController {
@Autowired
private StringRedisTemplate stringRedisTemplate;
@RequestMapping("/deduct_stock")
public Stirng deductStock() {
String lockkey = "lockkey";
try{鄭州專業(yè)婦科醫(yī)院 http://fk.zyfuke.com/
Boolean result = stringRedisTemplate.opsForValue.setIfAbsent(lockkey,"lockvalue");//jedis.setnx
if(!result) {
return "";
}
int stock = Integer.parseInt(stringRedisTemplate.opsForValue().get("stock"));//jedis.get(key)
if (stock > 0) {
int realStock = stock - 1;
stringRedisTemplate.opsForValue.set("stock",realStock+"");//jedis.set(key,value)
System.out.println(扣減成功,剩余庫存:" + realStock + "");
} else {
System.out.println(扣減失敗,庫存不足!" );
}
}finally{
springRedisTemplate.delete(lockkey);
}
return "end";
}
}
這時,Redis服務(wù)掛掉導(dǎo)致死鎖的問題解決了,但是,如果服務(wù)器果宕機(jī)了,又會導(dǎo)致鎖不能被釋放的現(xiàn)象,所以可以設(shè)置超時時間為10s,代碼如下:
@RestController
public class IndexController {
@Autowired
private StringRedisTemplate stringRedisTemplate;
@RequestMapping("/deduct_stock")
public Stirng deductStock() {
String lockkey = "lockkey";
try{
Boolean result = stringRedisTemplate.opsForValue.setIfAbsent(lockkey,"lockvalue",10,TimeUnit.SECONDS);//jedis.setnx
//Boolean result = stringRedisTemplate.opsForValue.setIfAbsent(lockkey,"lockvalue");//jedis.setnx
//stringRedisTemplate.expire(lockkey,10,TimeUnit.SECONDS);
if(!result) {
return "";
}
int stock = Integer.parseInt(stringRedisTemplate.opsForValue().get("stock"));//jedis.get(key)
if (stock > 0) {
int realStock = stock - 1;
stringRedisTemplate.opsForValue.set("stock",realStock+"");//jedis.set(key,value)
System.out.println(扣減成功,剩余庫存:" + realStock + "");
} else {
System.out.println(扣減失敗,庫存不足!" );
}
}finally{
springRedisTemplate.delete(lockkey);
}
return "end";
}
}
這時,如果有一個線程執(zhí)行需要15s,當(dāng)執(zhí)行到10s時第二個線程進(jìn)來拿到這把鎖,會出現(xiàn)多個線程拿到同一把鎖執(zhí)行,在第一個線程執(zhí)行完時會釋放掉第二個線程的鎖,以此類推…就會導(dǎo)致鎖的永久失效。所以,只能自己釋放自己的鎖,可以給當(dāng)前線程取一個名字,代碼如下:
@RestController
public class IndexController {
@Autowired
private StringRedisTemplate stringRedisTemplate;
@RequestMapping("/deduct_stock")
public Stirng deductStock() {
String lockkey = "lockkey";
String clientId = UUID.randomUUID().toString();
try{
Boolean result = stringRedisTemplate.opsForValue.setIfAbsent(lockkey,clientId ,10,TimeUnit.SECONDS);//jedis.setnx
//Boolean result = stringRedisTemplate.opsForValue.setIfAbsent(lockkey,"lockvalue");//jedis.setnx
//stringRedisTemplate.expire(lockkey,10,TimeUnit.SECONDS);
if(!result) {
return "";
}
int stock = Integer.parseInt(stringRedisTemplate.opsForValue().get("stock"));//jedis.get(key)
if (stock > 0) {
int realStock = stock - 1;
stringRedisTemplate.opsForValue.set("stock",realStock+"");//jedis.set(key,value)
System.out.println(扣減成功,剩余庫存:" + realStock + "");
} else {
System.out.println(扣減失敗,庫存不足!" );
}
}finally{
springRedisTemplate.delete(lockkey);
}
return "end";
}
}
永久失效的問題解決了,但是,如果第一個線程執(zhí)行15s,還是會存在多個線程擁有同一把鎖的現(xiàn)象。所以,需要續(xù)期超時時間,當(dāng)一個線程執(zhí)行5s后對超時時間進(jìn)行續(xù)期都10s,就可以解決了,續(xù)期設(shè)置可以借助redission工具。
Redission使用
Redission分布式鎖實(shí)現(xiàn)原理
pom.xml
org.redisson
redisson
3.6.5
Application.java啟動類
@bean
public Redission redission {
//此為單機(jī)模式
Config config = new Config();
config.useSingleServer().setAddress("redis://120.0.0.1:6379").setDatabase(0);
return (Redission)Redission.creat(config);
}
最終解決以上所有問題的代碼如下:
@RestController
public class IndexController {
@Autowired
private StringRedisTemplate stringRedisTemplate;
@Autowired
private Redissionredission;
@RequestMapping("/deduct_stock")
public Stirng deductStock() {
String lockkey = "lockkey";
//String clientId = UUID.randomUUID().toString();
RLock lock = redission.getLock();
try{
//Boolean result = stringRedisTemplate.opsForValue.setIfAbsent(lockkey,clientId ,10,TimeUnit.SECONDS);//jedis.setnx
//Boolean result = stringRedisTemplate.opsForValue.setIfAbsent(lockkey,"lockvalue");//jedis.setnx
//stringRedisTemplate.expire(lockkey,10,TimeUnit.SECONDS);
//加鎖:redission默認(rèn)超時時間為30s,每10s續(xù)期一次,也可以自己設(shè)置時間
lock.lock(60,TimeUnit.SECONDS);
int stock = Integer.parseInt(stringRedisTemplate.opsForValue().get("stock"));//jedis.get(key)
if (stock > 0) {
int realStock = stock - 1;
stringRedisTemplate.opsForValue.set("stock",realStock+"");//jedis.set(key,value)
System.out.println(扣減成功,剩余庫存:" + realStock + "");
} else {
System.out.println(扣減失敗,庫存不足!" );
}
}finally{
lock.unlock();
//springRedisTemplate.delete(lockkey);
}
return "end";
}
}
高并發(fā)分布式鎖的問題得到解決。
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