LocalState
A simple, durable, atomic K/V database. *Very inefficient*, should only be used for occasional reads/writes. Every read/write hits disk.
基于map实现, 每次读写都需要从磁盘上将数据读出, 并反序列化成map, 这个过程称为snapshot. 所以说是比较简单和低效的, 只能用于读取配置或参数, 这种偶尔读取的场景.
public synchronized Map<Object, Object> snapshot() throws IOException {
int attempts = 0;
while(true) {
String latestPath = _vs.mostRecentVersionPath();
if(latestPath==null) return new HashMap<Object, Object>();
try {
return (Map<Object, Object>) Utils.deserialize(FileUtils.readFileToByteArray(new File(latestPath)));
} catch(IOException e) {
attempts++;
if(attempts >= 10) {
throw e;
}
}
}
读写操作都是基于map的操作, get和put, 但是put需要做persist操作.
这里使用synchronized来做对象的线程间同步, 对于一个LocalState对象, 所有synchronized标有的函数只能被串行操作.
public Object get(Object key) throws IOException {
return snapshot().get(key);
}
public synchronized void put(Object key, Object val, boolean cleanup) throws IOException {
Map<Object, Object> curr = snapshot();
curr.put(key, val);
persist(curr, cleanup);
}
当然不止这么简单, 为了达到atomic, 还使用了VersionedStore, 参考下一章
persist不会去update现有的文件, 而是不断的产生递增version的文件, 故每一批更新都会产生一个新的文件
把需要写入的数据序列化
创建新的versionfile的path
把数据写入versionfile
调用succeedVersion, 创建tokenfile以标志versionfile的写入完成
清除旧版本, 只保留4个版本
private void persist(Map<Object, Object> val, boolean cleanup) throws IOException {
byte[] toWrite = Utils.serialize(val);
String newPath = _vs.createVersion();
FileUtils.writeByteArrayToFile(new File(newPath), toWrite);
_vs.succeedVersion(newPath);
if(cleanup) _vs.cleanup(4);
}
VersionedStore
public VersionedStore(String path) throws IOException {
_root = path;
mkdirs(_root);
}
这个store, 其实就是_root目录下的一堆文件
文件分两种,
VersionFile, _root + version, 真正的数据存储文件
TokenFile, _root + version + “.version”, 标志位文件, 标志version文件是否完成写操作, 以避免读到正在更新的文件
getAllVersions就是读出所有_root目录下的所有完成写操作的文件, 读出version, 并做从大到小的排序
public List<Long> getAllVersions() throws IOException {
List<Long> ret = new ArrayList<Long>();
for(String s: listDir(_root)) {
if(s.endsWith(FINISHED_VERSION_SUFFIX)) {
ret.add(validateAndGetVersion(s));
}
}
Collections.sort(ret);
Collections.reverse(ret);
return ret;
}
找到最新的版本文件
public Long mostRecentVersion() throws IOException {
List<Long> all = getAllVersions();
if(all.size()==0) return null;
return all.get(0);
创建新版本号, 用当前时间作为version
public String createVersion() throws IOException {
Long mostRecent = mostRecentVersion();
long version = Time.currentTimeMillis();
if(mostRecent!=null && version <= mostRecent) {
version = mostRecent + 1;
}
return createVersion(version);
}
public String createVersion(long version) throws IOException {
String ret = versionPath(version);
if(getAllVersions().contains(version))
throw new RuntimeException("Version already exists or data already exists");
else
return ret;
}
创建tokenfile, 以标记versionfile写完成
public void succeedVersion(String path) throws IOException {
long version = validateAndGetVersion(path);
// should rewrite this to do a file move
createNewFile(tokenPath(version));
}
清除旧的版本, 只保留versionsToKeep个, 清除操作就是删除versionfile和tokenfile
public void cleanup(int versionsToKeep) throws IOException {
List<Long> versions = getAllVersions();
if(versionsToKeep >= 0) {
versions = versions.subList(0, Math.min(versions.size(), versionsToKeep));
}
HashSet<Long> keepers = new HashSet<Long>(versions);
for(String p: listDir(_root)) {
Long v = parseVersion(p);
if(v!=null && !keepers.contains(v)) {
deleteVersion(v);
}
}
}
本文章摘自博客园，原文发布日期：2013-07-11
