datanode启动源码分析

hdfs/dataNode启动过程.md

@@ -126,6 +126,7 @@ private void doRefreshNamenodes(
     // Step 1. For each of the new nameservices, figure out whether
     // it's an update of the set of NNs for an existing NS,
     // or an entirely new nameservice.
+    // 判断当前DN中的nameservice是否已经存在：如果已经存在则需要刷新；否则需要添加。
     for (String nameserviceId : addrMap.keySet()) {
       if (bpByNameserviceId.containsKey(nameserviceId)) {
         toRefresh.add(nameserviceId);
@@ -136,20 +137,12 @@ private void doRefreshNamenodes(
     
     // Step 2. Any nameservices we currently have but are no longer present
     // need to be removed.
+    // 计算当前DN需要删除的nameservice
     toRemove = Sets.newHashSet(Sets.difference(
         bpByNameserviceId.keySet(), addrMap.keySet()));
     
-    assert toRefresh.size() + toAdd.size() ==
-      addrMap.size() :
-        "toAdd: " + Joiner.on(",").useForNull("<default>").join(toAdd) +
-        "  toRemove: " + Joiner.on(",").useForNull("<default>").join(toRemove) +
-        "  toRefresh: " + Joiner.on(",").useForNull("<default>").join(toRefresh);
-
-    
-    // Step 3. Start new nameservices
+    // Step 3. 启动新的nameservice
     if (!toAdd.isEmpty()) {
-      LOG.info("Starting BPOfferServices for nameservices: " +
-          Joiner.on(",").useForNull("<default>").join(toAdd));
     
       for (String nsToAdd : toAdd) {
         Map<String, InetSocketAddress> nnIdToAddr = addrMap.get(nsToAdd);
@@ -176,12 +169,10 @@ private void doRefreshNamenodes(
     startAll();
   }
 
-  // Step 4. Shut down old nameservices. This happens outside
+  // Step 4. 停止掉老的nameservice. This happens outside
   // of the synchronized(this) lock since they need to call
   // back to .remove() from another thread
   if (!toRemove.isEmpty()) {
-    LOG.info("Stopping BPOfferServices for nameservices: " +
-        Joiner.on(",").useForNull("<default>").join(toRemove));
     
     for (String nsToRemove : toRemove) {
       BPOfferService bpos = bpByNameserviceId.get(nsToRemove);
@@ -192,9 +183,8 @@ private void doRefreshNamenodes(
   }
   
   // Step 5. Update nameservices whose NN list has changed 
+  // 更新nameservices
   if (!toRefresh.isEmpty()) {
-    LOG.info("Refreshing list of NNs for nameservices: " +
-        Joiner.on(",").useForNull("<default>").join(toRefresh));
     
     for (String nsToRefresh : toRefresh) {
       BPOfferService bpos = bpByNameserviceId.get(nsToRefresh);
@@ -218,6 +208,7 @@ private void doRefreshNamenodes(
             .doAs(new PrivilegedExceptionAction<Object>() {
               @Override
               public Object run() throws Exception {
+                // 刷新NNList 核心实现.
                 bpos.refreshNNList(nsToRefresh, nnIds, addrs, lifelineAddrs);
                 return null;
               }