// org.apache.zookeeper.server.persistence.FileTxnSnapLog#save

public void save(DataTree dataTree,

        ConcurrentHashMap<Long, Integer> sessionsWithTimeouts)

    throws IOException {

    long lastZxid = dataTree.lastProcessedZxid;

    // 快照文件名为 snapshot.lastZxid

    File snapshotFile = new File(snapDir, Util.makeSnapshotName(lastZxid));

    LOG.info("Snapshotting: 0x{} to {}", Long.toHexString(lastZxid),

            snapshotFile);

    snapLog.serialize(dataTree, sessionsWithTimeouts, snapshotFile);

}

// org.apache.zookeeper.server.persistence.FileTxnLog#append

public synchronized boolean append(TxnHeader hdr, Record txn)

    throws IOException

{

    if (hdr == null) {

        return false;

    }

    if (hdr.getZxid() <= lastZxidSeen) {

        LOG.warn("Current zxid " + hdr.getZxid()

                + " is <= " + lastZxidSeen + " for "

                + hdr.getType());

    } else {

        lastZxidSeen = hdr.getZxid();

    }

    if (logStream==null) {

       if(LOG.isInfoEnabled()){

            LOG.info("Creating new log file: " + Util.makeLogName(hdr.getZxid()));

       }

       // 创建一个新的事务日志文件 log.zxid，这里的 zxid 是第一条事务日志的 zxid

       logFileWrite = new File(logDir, Util.makeLogName(hdr.getZxid()));

       fos = new FileOutputStream(logFileWrite);

       logStream=new BufferedOutputStream(fos);

       oa = BinaryOutputArchive.getArchive(logStream);

       FileHeader fhdr = new FileHeader(TXNLOG_MAGIC,VERSION, dbId);

       fhdr.serialize(oa, "fileheader");

       // Make sure that the magic number is written before padding.

       logStream.flush();

       filePadding.setCurrentSize(fos.getChannel().position());

       streamsToFlush.add(fos);

    }

    filePadding.padFile(fos.getChannel());

    byte[] buf = Util.marshallTxnEntry(hdr, txn);

    if (buf == null || buf.length == 0) {

        throw new IOException("Faulty serialization for header " +

                "and txn");

    }

    Checksum crc = makeChecksumAlgorithm();

    crc.update(buf, 0, buf.length);

    oa.writeLong(crc.getValue(), "txnEntryCRC");

    Util.writeTxnBytes(oa, buf);

    return true;

}

// org.apache.zookeeper.server.persistence.FileTxnSnapLog#restore

public long restore(DataTree dt, Map<Long, Integer> sessions,

        PlayBackListener listener) throws IOException {

    snapLog.deserialize(dt, sessions);

    return fastForwardFromEdits(dt, sessions, listener);

}

public long deserialize(DataTree dt, Map<Long, Integer> sessions)

        throws IOException {

    // we run through 100 snapshots (not all of them)

    // if we cannot get it running within 100 snapshots

    // we should  give up

    List<File> snapList = findNValidSnapshots(100);

    if (snapList.size() == 0) {

        return -1L;

    }

    File snap = null;

    boolean foundValid = false;

    for (int i = 0; i < snapList.size(); i++) {

        snap = snapList.get(i);

        InputStream snapIS = null;

        CheckedInputStream crcIn = null;

        try {

            LOG.info("Reading snapshot " + snap);

            snapIS = new BufferedInputStream(new FileInputStream(snap));

            crcIn = new CheckedInputStream(snapIS, new Adler32());

            InputArchive ia = BinaryInputArchive.getArchive(crcIn);

            deserialize(dt, sessions, ia);

            long checkSum = crcIn.getChecksum().getValue();

            long val = ia.readLong("val");

            if (val != checkSum) {

                throw new IOException("CRC corruption in snapshot :  " + snap);

            }

            foundValid = true;

            break;

        } catch(IOException e) {

            LOG.warn("problem reading snap file " + snap, e);

        } finally {

            if (snapIS != null)

                snapIS.close();

            if (crcIn != null)

                crcIn.close();

        }

    }

    if (!foundValid) {

        throw new IOException("Not able to find valid snapshots in " + snapDir);

    }

    dt.lastProcessedZxid = Util.getZxidFromName(snap.getName(), SNAPSHOT_FILE_PREFIX);

    return dt.lastProcessedZxid;

}

// org.apache.zookeeper.server.persistence.FileTxnLog.FileTxnIterator#init

void init() throws IOException {

    storedFiles = new ArrayList<File>();

    List<File> files = Util.sortDataDir(FileTxnLog.getLogFiles(logDir.listFiles(), 0), LOG_FILE_PREFIX, false);

    for (File f: files) {

        if (Util.getZxidFromName(f.getName(), LOG_FILE_PREFIX) >= zxid) {

            storedFiles.add(f);

        }

        // add the last logfile that is less than the zxid

        else if (Util.getZxidFromName(f.getName(), LOG_FILE_PREFIX) < zxid) {

            storedFiles.add(f);

            break;

        }

    }

    goToNextLog();

    if (!next())

        return;

    while (hdr.getZxid() < zxid) {

        if (!next())

            return;

    }

}