Dubbo提供者本地缓存配置源码解析-gongstring(野渔)的博客-技术人有正能量

##	问题

![dubbo架构图](/img/1586050015286_cstk7.png "dubbo架构图")

根据官方图，dubbo调用者需要通过注册中心（例如：ZK）注册信息，获取提供者，但是如果频繁往ZK获取信息，肯定会存在单点故障问题，所以dubbo提供了将提供者信息缓存在本地的方法。

Dubbo调用者在连接ZK后，提供者列表是如何进行本地缓存的？

##	源码

com.alibaba.dubbo.registry.support.AbstractRegistry类

读取配置：
```
public AbstractRegistry(URL url) {
    setUrl(url);
    // Start file save timer
    //<------------------------此处用于配置是否将提供者缓存到本地，如果不配置，则默认为false
    syncSaveFile = url.getParameter(Constants.REGISTRY_FILESAVE_SYNC_KEY, false);
    String filename = url.getParameter(Constants.FILE_KEY, System.getProperty("user.home") + "/.dubbo/dubbo-registry-" + url.getParameter(Constants.APPLICATION_KEY) + "-" + url.getAddress() + ".cache");
    File file = null;
    if (ConfigUtils.isNotEmpty(filename)) {
        file = new File(filename);
        if (!file.exists() && file.getParentFile() != null && !file.getParentFile().exists()) {
            if (!file.getParentFile().mkdirs()) {
                throw new IllegalArgumentException("Invalid registry store file " + file + ", cause: Failed to create directory " + file.getParentFile() + "!");
            }
        }
    }
    this.file = file;
    loadProperties();
    notify(url.getBackupUrls());
}

.......省略很多代码

private void saveProperties(URL url) {
    if (file == null) {
        return;
    }

try {
        StringBuilder buf = new StringBuilder();
        Map<String, List<URL>> categoryNotified = notified.get(url);
        if (categoryNotified != null) {
            for (List<URL> us : categoryNotified.values()) {
                for (URL u : us) {
                    if (buf.length() > 0) {
                        buf.append(URL_SEPARATOR);
                    }
                    buf.append(u.toFullString());
                }
            }
        }
        properties.setProperty(url.getServiceKey(), buf.toString());
        long version = lastCacheChanged.incrementAndGet();
        //<-----------如果开启了缓存，将会调用保存方法
        if (syncSaveFile) {
            doSaveProperties(version);
        } else {
            registryCacheExecutor.execute(new SaveProperties(version));
        }
    } catch (Throwable t) {
        logger.warn(t.getMessage(), t);
    }
}

public void doSaveProperties(long version) {
    if (version < lastCacheChanged.get()) {
        return;
    }
    if (file == null) {
        return;
    }
    // Save
    try {
        File lockfile = new File(file.getAbsolutePath() + ".lock");
        if (!lockfile.exists()) {
            lockfile.createNewFile();
        }
        RandomAccessFile raf = new RandomAccessFile(lockfile, "rw");
        try {
            FileChannel channel = raf.getChannel();
            try {
                FileLock lock = channel.tryLock();
                if (lock == null) {
                    throw new IOException("Can not lock the registry cache file " + file.getAbsolutePath() + ", ignore and retry later, maybe multi java process use the file, please config: dubbo.registry.file=xxx.properties");
                }
                // Save
                try {
                    if (!file.exists()) {
                        file.createNewFile();
                    }
                    FileOutputStream outputFile = new FileOutputStream(file);
                    try {
                        properties.store(outputFile, "Dubbo Registry Cache");
                    } finally {
                        outputFile.close();
                    }
                } finally {
                    lock.release();
                }
            } finally {
                channel.close();
            }
        } finally {
            raf.close();
        }
    } catch (Throwable e) {
        if (version < lastCacheChanged.get()) {
            return;
        } else {
            registryCacheExecutor.execute(new SaveProperties(lastCacheChanged.incrementAndGet()));
        }
        logger.warn("Failed to save registry store file, cause: " + e.getMessage(), e);
    }
}
```
Constants常量类中，定义了很多dubbo中配置需要的参数名信息。

Constants.REGISTRY_FILESAVE_SYNC_KEY 源码
```
 public static final String REGISTRY_FILESAVE_SYNC_KEY = "save.file";
```
默认配置文件本地缓存路径，代码如下：
```
String filename = url.getParameter(Constants.FILE_KEY, System.getProperty("user.home") + "/.dubbo/dubbo-registry-" + url.getParameter(Constants.APPLICATION_KEY) + "-" + url.getAddress() + ".cache");
    File file = null;
    if (ConfigUtils.isNotEmpty(filename)) {
        file = new File(filename);
        if (!file.exists() && file.getParentFile() != null && !file.getParentFile().exists()) {
            if (!file.getParentFile().mkdirs()) {
                throw new IllegalArgumentException("Invalid registry store file " + file + ", cause: Failed to create directory " + file.getParentFile() + "!");
            }
        }
    }
    this.file = file;
```
上面配置文件读取的常量Key：

Constants.FILE_KEY = "file";

doSaveProperties 方法中通过在本地利用文件锁实现配置信息占用的问题，所以如果本地有多个dubbo实例，使用默认值会出现冲突的情况。此时需要在dubbo配置中添加不同的dubbo.registry.file进行区分；否则会有如下日志提示：
```
Can not lock the registry cache file " + file.getAbsolutePath() + ", ignore and retry later, maybe multi java process use the file, please config: dubbo.registry.file=xxx.properties
```