FileSnap实现了SnapShot接口，主要用作存储、序列化、反序列化、访问相应snapshot文件。
1. deserialize函数
代码语言：javascript复制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;
long snapZxid = -1;
boolean foundValid = false;
for (int i = 0, snapListSize = snapList.size(); i < snapListSize; i ) {
snap = snapList.get(i);
LOG.info("Reading snapshot {}", snap);
snapZxid = Util.getZxidFromName(snap.getName(), SNAPSHOT_FILE_PREFIX);
try (CheckedInputStream snapIS = SnapStream.getInputStream(snap)) {
InputArchive ia = BinaryInputArchive.getArchive(snapIS);
deserialize(dt, sessions, ia);
SnapStream.checkSealIntegrity(snapIS, ia);
// Digest feature was added after the CRC to make it backward
// compatible, the older code can still read snapshots which
// includes digest.
//
// To check the intact, after adding digest we added another
// CRC check.
if (dt.deserializeZxidDigest(ia, snapZxid)) {
SnapStream.checkSealIntegrity(snapIS, ia);
}
foundValid = true;
break;
} catch (IOException e) {
LOG.warn("problem reading snap file {}", snap, e);
}
}
if (!foundValid) {
throw new IOException("Not able to find valid snapshots in " snapDir);
}
dt.lastProcessedZxid = snapZxid;
lastSnapshotInfo = new SnapshotInfo(dt.lastProcessedZxid, snap.lastModified() / 1000);
// compare the digest if this is not a fuzzy snapshot, we want to compare
// and find inconsistent asap.
if (dt.getDigestFromLoadedSnapshot() != null) {
dt.compareSnapshotDigests(dt.lastProcessedZxid);
}
return dt.lastProcessedZxid;
}说明：deserialize主要用作反序列化，并将反序列化结果保存至dt和sessions中。其大致步骤如下
- 获取100个合法的snapshot文件，并且snapshot文件已经通过zxid进行降序排序
- 遍历100个snapshot文件，从zxid最大的开始，读取该文件，并创建相应的InputArchive
- 调用deserialize(dt,sessions, ia)函数完成反序列化操作
- 验证从文件中读取的Checksum是否与新生的Checksum相等，若不等，则抛出异常
- 跳出循环并关闭相应的输入流，并从文件名中解析出相应的zxid返回。
- 在遍历100个snapshot文件后仍然无法找到通过验证的文件，则抛出异常。
在deserialize函数中，会调用findNValidSnapshots以及同名的deserialize(dt,sessions, ia)函数。findNValidSnapshots函数主要是查找N个合法的snapshot文件并进行降序排序后返回。deserialize(dt,sessions, ia)函数主要作用反序列化，并将反序列化结果保存至header和sessions中。其中会验证header的魔数是否相等。
2. serialize函数
代码语言：javascript复制public synchronized void serialize(
DataTree dt,
Map<Long, Integer> sessions,
File snapShot,
boolean fsync) throws IOException {
if (!close) {
try (CheckedOutputStream snapOS = SnapStream.getOutputStream(snapShot, fsync)) {
OutputArchive oa = BinaryOutputArchive.getArchive(snapOS);
FileHeader header = new FileHeader(SNAP_MAGIC, VERSION, dbId);
serialize(dt, sessions, oa, header);
SnapStream.sealStream(snapOS, oa);
// Digest feature was added after the CRC to make it backward
// compatible, the older code cal still read snapshots which
// includes digest.
//
// To check the intact, after adding digest we added another
// CRC check.
if (dt.serializeZxidDigest(oa)) {
SnapStream.sealStream(snapOS, oa);
}
lastSnapshotInfo = new SnapshotInfo(
Util.getZxidFromName(snapShot.getName(), SNAPSHOT_FILE_PREFIX),
snapShot.lastModified() / 1000);
}
}
}该函数用于将header、sessions、dt序列化至本地snapshot文件中，并且在最后会写入"/"字符。该方法是同步的，即是线程安全的。 FileHeader header = new FileHeader(SNAP_MAGIC, VERSION, dbId);可以看出写入了magic、version、dbid，也就是在之前的文章中介绍的，snapshot文件解析之后的第一行。另外还会序列号摘要
