Storm中的LocalState 代码解析

/**
 * A simple, durable, atomic K/V database. *Very inefficient*, should only be
 * used for occasional reads/writes. Every read/write hits disk.
 */

public LocalState(String backingDir) throws IOException {
        _vs = new VersionedStore(backingDir);
    }

    public synchronized Map<Object, Object> snapshot() throws IOException {
        int attempts = 0;
        while (true) {
            String latestPath = _vs.mostRecentVersionPath();   //获取最近的版本
            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;
                }
            }
        }
    }

    public Object get(Object key) throws IOException {
        return snapshot().get(key);   
    }

    public synchronized void put(Object key, Object val) throws IOException {
        put(key, val, true);
    }

    public synchronized void put(Object key, Object val, boolean cleanup)
            throws IOException {
        Map<Object, Object> curr = snapshot();
        curr.put(key, val);
        persist(curr, cleanup);    //persist会将其写入到磁盘中
    }

    public synchronized void remove(Object key) throws IOException {
        remove(key, true);   
    }

    public synchronized void remove(Object key, boolean cleanup)
            throws IOException {
        Map<Object, Object> curr = snapshot();
        curr.remove(key);
        persist(curr, cleanup);
    }

    public synchronized void cleanup(int keepVersions) throws IOException {
        _vs.cleanup(keepVersions);
    }

    private void persist(Map<Object, Object> val, boolean cleanup)
            throws IOException {
        byte[] toWrite = Utils.serialize(val);
        String newPath = _vs.createVersion();    //创建一个新的版本号
        FileUtils.writeByteArrayToFile(new File(newPath), toWrite);
        _vs.succeedVersion(newPath);    //如果写入成功，那么会生成 id.version 文件来声明该文件写入成功
        if (cleanup)
            _vs.cleanup(4);  //默认保留4个版本
    }

    public void succeedVersion(String path) throws IOException {
        long version = validateAndGetVersion(path);   //验证一下这个文件是否存在
        // should rewrite this to do a file move
        createNewFile(tokenPath(version));   //创建对应的 id.version 文件说明写入成功
    }

    private long validateAndGetVersion(String path) {
        Long v = parseVersion(path);
        if (v == null)
            throw new RuntimeException(path + " is not a valid version");
        return v;
    }

    private Long parseVersion(String path) {
        String name = new File(path).getName();
        if (name.endsWith(FINISHED_VERSION_SUFFIX)) {
            name = name.substring(0,
                    name.length() - FINISHED_VERSION_SUFFIX.length());
        }
        try {
            return Long.parseLong(name);
        } catch (NumberFormatException e) {
            return null;
        }
    }

    private String tokenPath(long version) {
        return new File(_root, "" + version + FINISHED_VERSION_SUFFIX)
                .getAbsolutePath();
    }

    private void createNewFile(String path) throws IOException {
        new File(path).createNewFile();
    }

    public void cleanup(int versionsToKeep) throws IOException {
        List<Long> versions = getAllVersions();   //获取所有的版本，这个返回的是以倒序排列的，最新的版本在最前面
        if (versionsToKeep >= 0) {
            versions = versions.subList(0,
                    Math.min(versions.size(), versionsToKeep));   //所以可以用subList来得到需要的版本
        }
        HashSet<Long> keepers = new HashSet<Long>(versions);   //存在HashSet中方便快速存取

        for (String p : listDir(_root)) {
            Long v = parseVersion(p);
            if (v != null && !keepers.contains(v)) {
                deleteVersion(v);    //删除其他的版本
            }
        }
    }

    /**
     * Sorted from most recent to oldest
     */
    public List<Long> getAllVersions() throws IOException {
        List<Long> ret = new ArrayList<Long>();
        for (String s : listDir(_root)) {   //获取该目录下的所有文件
            if (s.endsWith(FINISHED_VERSION_SUFFIX)) {   
                ret.add(validateAndGetVersion(s));   //验证该文件是否存在
            }
        }
        Collections.sort(ret);
        Collections.reverse(ret);  //逆序排列
        return ret;
    }

    public void deleteVersion(long version) throws IOException {
        File versionFile = new File(versionPath(version));
        File tokenFile = new File(tokenPath(version));

        if (versionFile.exists()) {
            FileUtils.forceDelete(versionFile);
        }
        if (tokenFile.exists()) {
            FileUtils.forceDelete(tokenFile);
        }
    }

    public String mostRecentVersionPath() throws IOException {
        Long v = mostRecentVersion();
        if (v == null)
            return null;
        return versionPath(v);
    }

    public Long mostRecentVersion() throws IOException {
        List<Long> all = getAllVersions();
        if (all.size() == 0)
            return null;
        return all.get(0);
    }

    public String mostRecentVersionPath(long maxVersion) throws IOException {
        Long v = mostRecentVersion(maxVersion);
        if (v == null)
            return null;
        return versionPath(v);
    }

    public Long mostRecentVersion(long maxVersion) throws IOException {
        List<Long> all = getAllVersions();
        for (Long v : all) {
            if (v <= maxVersion)   //取出比maxVersion小的最大version
                return v;
        }
        return null;
    }