本地缓存更新方案探索

1 背景

• 大家在工作中是否遇到过某些业务数据需要频繁使用，但是数据量不大的情况，一般就是几十条甚至几百条这种。
• 一般的解决方案就是业务维护数据，然后同步redis缓存，C端使用缓存的数据。但是这里不免会出现大key/热key的问题，另外还有缓存穿透、缓存击穿等问题。
• 那么接下来我们一起探索一下如何解决上述问题吧。

2 方案探索

• 首先我们评估数据量，发现这类数据一般只有百条左右。那么在技术选型上使用本地缓存无疑是最好的方案。现在应对C端场景基本选型的都是Caffeine。详见：https://blog.csdn.net/for62/article/details/147494533
• 我们选择了本地缓存一方面可以抗大流量，做到无状态横向扩容。另一方面可以提高服务稳定性降低tp99。
• 那么接下来我们就要设计缓存一致性的实现方案了，如何将redis中的数据近实时同步到本地缓存，C端只读本地缓存，可以降级读redis。
• 这里我们参考长轮询实现配置中心的方案：https://mp.weixin.qq.com/s/YjvL0sUTGHxR3GJFqrP8qg。客户端长轮询监听服务端数据变更，感知到数据变更后更新本地缓存数据。设计图如下：
  

2.1 初始化

• 这里我们先假设刷新本地缓存的方法为：LocalCacheRefresher.refresh();

    public void refresh() {Caffeine<String, Object> cacheInfo = getLocalCacheInstance();String redisCacheKey = getRedisCacheKey();Set<String> keys = redisCache.hKeys(redisCacheKey);for (String key : keys) {String data = redisCache.hGet(redisCacheKey, key);cacheInfo.put(key, data);}}

• 服务启动时数据加载

@Component
public class LocalCacheInitRunner implements ApplicationRunner {@Overridepublic void run(ApplicationArguments args) throws Exception {LocalCacheRefresher.refresh();}}

2.2 实时更新

2.2.1 长轮询

• 这里我们用长轮询的方案监听源数据的变更来刷新本地缓存。

2.2.1.1 client

@Slf4j
public class LongPollClient {private CloseableHttpClient httpClient;private RequestConfig requestConfig;public ConfigClient() {this.httpClient = HttpClientBuilder.create().build();// httpClient 客户端超时时间要大于长轮询约定的超时时间this.requestConfig = RequestConfig.custom().setSocketTimeout(6000).build();}public void longPolling(String url, String dataId) {String endpoint = url + "?dataId=" + dataId;HttpGet request = new HttpGet(endpoint);CloseableHttpResponse response = httpClient.execute(request);switch (response.getStatusLine().getStatusCode()) {case 200: {BufferedReader rd = new BufferedReader(new InputStreamReader(response.getEntity().getContent()));StringBuilder result = new StringBuilder();String line;while ((line = rd.readLine()) != null) {result.append(line);}response.close();String configInfo = result.toString();log.info("dataId: [{}] changed, receive configInfo: {}", dataId, configInfo);longPolling(url, dataId);break;}// ② 304 响应码标记配置未变更case 304: {log.info("longPolling dataId: [{}] once finished, configInfo is unchanged, longPolling again", dataId);longPolling(url, dataId);break;}default: {throw new RuntimeException("unExcepted HTTP status code");}}}
}

2.2.2.2 server

@RestController
@Slf4j
@SpringBootApplication
public class LongPollServer {@Dataprivate static class AsyncTask {// 长轮询请求的上下文，包含请求和响应体private AsyncContext asyncContext;// 超时标记private boolean timeout;public AsyncTask(AsyncContext asyncContext, boolean timeout) {this.asyncContext = asyncContext;this.timeout = timeout;}}// guava 提供的多值 Map，一个 key 可以对应多个 valueprivate Multimap<String, AsyncTask> dataIdContext = Multimaps.synchronizedSetMultimap(HashMultimap.create());private ThreadFactory threadFactory = new ThreadFactoryBuilder().setNameFormat("longPolling-timeout-checker-%d").build();private ScheduledExecutorService timeoutChecker = new ScheduledThreadPoolExecutor(1, threadFactory);// 配置监听接入点@RequestMapping("/listener")public void addListener(HttpServletRequest request, HttpServletResponse response) {String dataId = request.getParameter("dataId");// 开启异步AsyncContext asyncContext = request.startAsync(request, response);AsyncTask asyncTask = new AsyncTask(asyncContext, true);// 维护 dataId 和异步请求上下文的关联dataIdContext.put(dataId, asyncTask);// 启动定时器，30s 后写入 304 响应timeoutChecker.schedule(() -> {if (asyncTask.isTimeout()) {dataIdContext.remove(dataId, asyncTask);response.setStatus(HttpServletResponse.SC_NOT_MODIFIED);asyncContext.complete();}}, 3000, TimeUnit.MILLISECONDS);}// 配置发布接入点@RequestMapping("/publishConfig")@SneakyThrowspublic String publishConfig(String dataId, String configInfo) {log.info("publish configInfo dataId: [{}], configInfo: {}", dataId, configInfo);Collection<AsyncTask> asyncTasks = dataIdContext.removeAll(dataId);for (AsyncTask asyncTask : asyncTasks) {asyncTask.setTimeout(false);HttpServletResponse response = (HttpServletResponse)asyncTask.getAsyncContext().getResponse();response.setStatus(HttpServletResponse.SC_OK);response.getWriter().println(configInfo);asyncTask.getAsyncContext().complete();}return "success";}public static void main(String[] args) {SpringApplication.run(ConfigServer.class, args);}}