Dubbo提供者本地缓存配置源码解析

问题

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