对namenode启动时的相关操作及相关类有一个大体了解，后续深入研究时，再对本文进行补充
>实现类
HDFS启动脚本为$HADOOP_HOME/sbin/start-dfs.sh，查看start-dfs.sh可以看出，namenode是通过bin/hdfs命令来启动
$ vi start-dfs.sh
# namenodes
NAMENODES=$($HADOOP_PREFIX/bin/hdfs getconf -namenodes)
echo "Starting namenodes on [$NAMENODES]"
"$HADOOP_PREFIX/sbin/hadoop-daemons.sh" \
--config "$HADOOP_CONF_DIR" \
--hostnames "$NAMENODES" \
--script "$bin/hdfs" start namenode $nameStartOpt
#---------------------------------------------------------
查看$HADOOP_HOME/bin/hdfs，可以找到namenode启动所调用的java类。
$ vi bin/hdfs:
if [ "$COMMAND" = "namenode" ] ; then
CLASS='org.apache.hadoop.hdfs.server.namenode.NameNode'
HADOOP_OPTS="$HADOOP_OPTS $HADOOP_NAMENODE_OPTS"
>源码查看
按照前文hadoop2.5.2学习及实践笔记（二）—— 编译源代码及导入源码至eclipse步骤，源码已经导入到eclipse中，快捷键ctrl+shift+R搜索并打开NameNode.java查看源码
NameNode类中有一个静态代码块，表示在加载器加载NameNode类过程中的准备阶段，就会执行代码块中的代码。HdfsConfiguration的init()方法的方法体是空的，这里的作用是触发对HdfsConfiguration的主动调用，从而保证在执行NameNode类相关调用时，如果HdfsConfiguration类没有被加载和初始化，先触发HdfsConfiguration的初始化过程。
//org.apache.hadoop.hdfs.server.namenode.NameNode.java
static{
//HdfsConfiguration类init()方法：public static void init() {}
HdfsConfiguration.init();
}
查看其main方法，可以看出namenode相关操作的主要入口方法是createNameNode(String argv[], Configuration conf)方法。
//org.apache.hadoop.hdfs.server.namenode.NameNode.java
public static void main(String argv[]) throws Exception {
if (DFSUtil.parseHelpArgument(argv, NameNode.USAGE, System.out, true)) {
System.exit(0);
}
try {
//打印namenode启动或关闭日志信息
StringUtils.startupShutdownMessage(NameNode.class, argv, LOG);
//namenode相关主要操作
NameNode namenode = createNameNode(argv, null);
if (namenode != null) {
//向客户端和datanode提供RPC服务，直到RPC服务器结束运行
namenode.join();
}
} catch (Throwable e) {
LOG.fatal("Exception in namenode join", e);
terminate(1, e);
}
}
createNameNode方法中通过一个switch语句对不同的命令执行不同的操作。比如搭建环境时格式化文件系统时的操作，可以查看FORMAT分支。
//org.apache.hadoop.hdfs.server.namenode.NameNode.java
public static NameNode createNameNode(String argv[], Configuration conf)
throws IOException {
LOG.info("createNameNode " + Arrays.asList(argv));
if (conf == null)
conf = new HdfsConfiguration();
//参数为空时默认： -regular
StartupOption startOpt = parseArguments(argv);
if (startOpt == null) {
printUsage(System.err);
return null;
}
setStartupOption(conf, startOpt);
switch (startOpt) {
case FORMAT: {//格式化文件系统，伪分布式环境搭建时调用过namenode -format命令
boolean aborted = format(conf, startOpt.getForceFormat(),
startOpt.getInteractiveFormat());
terminate(aborted ? 1 : 0);
return null; // avoid javac warning
}
case GENCLUSTERID: {
System.err.println("Generating new cluster id:");
System.out.println(NNStorage.newClusterID());
terminate(0);
return null;
}
case FINALIZE: {
System.err.println("Use of the argument '" + StartupOption.FINALIZE +
"' is no longer supported. To finalize an upgrade, start the NN " +
" and then run `hdfs dfsadmin -finalizeUpgrade'");
terminate(1);
return null; // avoid javac warning
}
case ROLLBACK: {
boolean aborted = doRollback(conf, true);
terminate(aborted ? 1 : 0);
return null; // avoid warning
}
case BOOTSTRAPSTANDBY: {
String toolArgs[] = Arrays.copyOfRange(argv, 1, argv.length);
int rc = BootstrapStandby.run(toolArgs, conf);
terminate(rc);
return null; // avoid warning
}
case INITIALIZESHAREDEDITS: {
boolean aborted = initializeSharedEdits(conf,
startOpt.getForceFormat(),
startOpt.getInteractiveFormat());
terminate(aborted ? 1 : 0);
return null; // avoid warning
}
case BACKUP:
case CHECKPOINT: {//backupnode和checkpointnode启动
NamenodeRole role = startOpt.toNodeRole();
DefaultMetricsSystem.initialize(role.toString().replace(" ", ""));
//backupnode继承NameNode类，代码最终执行的还是NameNode的构造方法
return new BackupNode(conf, role);
}
case RECOVER: {
NameNode.doRecovery(startOpt, conf);
return null;
}
case METADATAVERSION: {
printMetadataVersion(conf);
terminate(0);
return null; // avoid javac warning
}
default: {
DefaultMetricsSystem.initialize("NameNode");
//启动时startOpt=“-regular”，代码执行default分支，通过构造函数返回一个namenode实例
return new NameNode(conf);
}
}
}
namenode的构造方法
//org.apache.hadoop.hdfs.server.namenode.NameNode.java
public NameNode(Configuration conf) throws IOException {
this(conf, NamenodeRole.NAMENODE);
}
protected NameNode(Configuration conf, NamenodeRole role)
throws IOException {
this.conf = conf;
this.role = role;
//获取fs.defaultFS，设置namenode地址
setClientNamenodeAddress(conf);
String nsId = getNameServiceId(conf);
String namenodeId = HAUtil.getNameNodeId(conf, nsId);
//是否启用HA
this.haEnabled = HAUtil.isHAEnabled(conf, nsId);
//HA状态：启用/备用
state = createHAState(getStartupOption(conf));
//读取dfs.ha.allow.stale.reads，设置namenode在备用状态时是否允许读操作,默认为false
this.allowStaleStandbyReads = HAUtil.shouldAllowStandbyReads(conf);
this.haContext = createHAContext();
try {
//联邦环境下，使用该方法配置一系列使用一个逻辑上的nsId组合在一起的namenode
initializeGenericKeys(conf, nsId, namenodeId);
//namenode初始化
initialize(conf);
try {
haContext.writeLock();
state.prepareToEnterState(haContext);
//namenode进入相应状态：active state/backup state/standby state
state.enterState(haContext);
} finally {
haContext.writeUnlock();
}
} catch (IOException e) {
this.stop();
throw e;
} catch (HadoopIllegalArgumentException e) {
this.stop();
throw e;
}
}
namenode初始化方法代码
//org.apache.hadoop.hdfs.server.namenode.NameNode.java
protected void initialize(Configuration conf) throws IOException {
if (conf.get(HADOOP_USER_GROUP_METRICS_PERCENTILES_INTERVALS) == null) {
String intervals = conf.get(DFS_METRICS_PERCENTILES_INTERVALS_KEY);
if (intervals != null) {
conf.set(HADOOP_USER_GROUP_METRICS_PERCENTILES_INTERVALS,
intervals);
}
}
//设置权限，根据hadoop.security.authentication获取认证方式及规则
UserGroupInformation.setConfiguration(conf);
//登录：如果认证方式为simple则退出该方法
//否则调用UserGroupInformation.loginUserFromKeytab进行登陆，登陆使用dfs.namenode.kerberos.principal作为用户名
loginAsNameNodeUser(conf);
//初始化度量系统，用于度量namenode服务状态
NameNode.initMetrics(conf, this.getRole());
StartupProgressMetrics.register(startupProgress);
if (NamenodeRole.NAMENODE == role) {
//启动http服务器
startHttpServer(conf);
}
//根据命令对命名空间进行操作，如：前文所述启动时加载本地命名空间镜像和应用编辑日志，在内存中建立命名空间的映像
loadNamesystem(conf);
//创建RPC服务器
rpcServer = createRpcServer(conf);
if (clientNamenodeAddress == null) {
// This is expected for MiniDFSCluster. Set it now using
// the RPC server's bind address.
clientNamenodeAddress =
NetUtils.getHostPortString(rpcServer.getRpcAddress());
LOG.info("Clients are to use " + clientNamenodeAddress + " to access"
+ " this namenode/service.");
}
if (NamenodeRole.NAMENODE == role) {
httpServer.setNameNodeAddress(getNameNodeAddress());
httpServer.setFSImage(getFSImage());
}
pauseMonitor = new JvmPauseMonitor(conf);
pauseMonitor.start();
metrics.getJvmMetrics().setPauseMonitor(pauseMonitor);
//启动活动状态和备用状态的公共服务：RPC服务和namenode的插件程序启动
startCommonServices(conf);
}
loadNamesystem(Configuration conf)方法调用FSNamesystem类的loadFromDisk(Configuration conf)。前文提到的，namenode启动时从本地文件系统加载镜像并重做编辑日志，都在此方法中实现。
//org.apache.hadoop.hdfs.server.namenode.FSNamesystem.java
static FSNamesystem loadFromDisk(Configuration conf) throws IOException {
//必须的编辑日志目录检查
checkConfiguration(conf);
//设在NNStorage，并初始化编辑日志目录。NNStorage主要功能是管理namenode使用的存储目录
FSImage fsImage = new FSImage(conf,
FSNamesystem.getNamespaceDirs(conf),
FSNamesystem.getNamespaceEditsDirs(conf));
//根据指定的镜像创建FSNamesystem对象
FSNamesystem namesystem = new FSNamesystem(conf, fsImage, false);
StartupOption startOpt = NameNode.getStartupOption(conf);
if (startOpt == StartupOption.RECOVER) {
namesystem.setSafeMode(SafeModeAction.SAFEMODE_ENTER);
}
long loadStart = now();
try {
//加载镜像、重做编辑日志，并打开一个新编辑文件都在此方法中
namesystem.loadFSImage(startOpt);
} catch (IOException ioe) {
LOG.warn("Encountered exception loading fsimage", ioe);
fsImage.close();
throw ioe;
}
long timeTakenToLoadFSImage = now() - loadStart;
LOG.info("Finished loading FSImage in " + timeTakenToLoadFSImage + " msecs");
NameNodeMetrics nnMetrics = NameNode.getNameNodeMetrics();
if (nnMetrics != null) {
nnMetrics.setFsImageLoadTime((int) timeTakenToLoadFSImage);
}
return namesystem;
}
private void loadFSImage(StartupOption startOpt) throws IOException {
final FSImage fsImage = getFSImage();
// format before starting up if requested
if (startOpt == StartupOption.FORMAT) {
fsImage.format(this, fsImage.getStorage().determineClusterId());// reuse current id
startOpt = StartupOption.REGULAR;
}
boolean success = false;
writeLock();
try {
// We shouldn't be calling saveNamespace if we've come up in standby state.
MetaRecoveryContext recovery = startOpt.createRecoveryContext();
final boolean staleImage
= fsImage.recoverTransitionRead(startOpt, this, recovery);
if (RollingUpgradeStartupOption.ROLLBACK.matches(startOpt)) {
rollingUpgradeInfo = null;
}
final boolean needToSave = staleImage && !haEnabled && !isRollingUpgrade();
LOG.info("Need to save fs image? " + needToSave
+ " (staleImage=" + staleImage + ", haEnabled=" + haEnabled
+ ", isRollingUpgrade=" + isRollingUpgrade() + ")");
if (needToSave) {
fsImage.saveNamespace(this);
} else {
// No need to save, so mark the phase done.
StartupProgress prog = NameNode.getStartupProgress();
prog.beginPhase(Phase.SAVING_CHECKPOINT);
prog.endPhase(Phase.SAVING_CHECKPOINT);
}
// This will start a new log segment and write to the seen_txid file, so
// we shouldn't do it when coming up in standby state
if (!haEnabled || (haEnabled && startOpt == StartupOption.UPGRADE)) {
fsImage.openEditLogForWrite();
}
success = true;
} finally {
if (!success) {
fsImage.close();
}
writeUnlock();
}
imageLoadComplete();
}
本文章为转载内容，我们尊重原作者对文章享有的著作权。如有内容错误或侵权问题，欢迎原作者联系我们进行内容更正或删除文章。
