Zookeeper是如何保证创建的节点是唯一的？
典型回答 Zookeeper通过两个手段来保证节点创建的唯一性： 1、所有的写请求都会由Leader进行，即使是请求到Follower节点，也会被转发到Leader节点上执行。 2、在Leader上写入数据的时候，通过加锁(synchronized)和CAS（ConcurrentHashMap）操作，保证了并发情况下只有一个线程可以添加成功。 第一点我们就不展开讲了，这个是ZK中的角色天然具备的特性，我们重点展开说说第二点原因。我们来看下ZK的源码，看看到底是如何创建节点的。 以下代码在github中可以看到，在DataTree的createNode方法就是进行节点创建的： 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 public void createNode(final String path, byte[] data, List<ACL> acl, long ephemeralOwner, int parentCVersion, long zxid, long time, Stat outputStat) throws NoNodeException, NodeExistsException { int lastSlash = path.lastIndexOf('/'); String parentName = path.substring(0, lastSlash); String childName = path.substring(lastSlash + 1); StatPersisted stat = createStat(zxid, time, ephemeralOwner); DataNode parent = nodes.get(parentName); if (parent == null) { throw new NoNodeException(); } synchronized (parent) { // Add the ACL to ACL cache first, to avoid the ACL not being // created race condition during fuzzy snapshot sync. // // This is the simplest fix, which may add ACL reference count // again if it's already counted in the ACL map of fuzzy // snapshot, which might also happen for deleteNode txn, but // at least it won't cause the ACL not exist issue. // // Later we can audit and delete all non-referenced ACLs from // ACL map when loading the snapshot/txns from disk, like what // we did for the global sessions. Long acls = aclCache.convertAcls(acl); Set<String> children = parent.getChildren(); if (children.contains(childName)) { throw new NodeExistsException(); } nodes.preChange(parentName, parent); if (parentCVersion == -1) { parentCVersion = parent.stat.getCversion(); parentCVersion++; } // There is possibility that we'll replay txns for a node which // was created and then deleted in the fuzzy range, and it's not // exist in the snapshot, so replay the creation might revert the // cversion and pzxid, need to check and only update when it's // larger. if (parentCVersion > parent.stat.getCversion()) { parent.stat.setCversion(parentCVersion); parent.stat.setPzxid(zxid); } DataNode child = new DataNode(data, acls, stat); parent.addChild(childName); nodes.postChange(parentName, parent); nodeDataSize.addAndGet(getNodeSize(path, child.data)); nodes.put(path, child); EphemeralType ephemeralType = EphemeralType.get(ephemeralOwner); if (ephemeralType == EphemeralType.CONTAINER) { containers.add(path); } else if (ephemeralType == EphemeralType.TTL) { ttls.add(path); } else if (ephemeralOwner != 0) { HashSet<String> list = ephemerals.computeIfAbsent(ephemeralOwner, k -> new HashSet<>()); synchronized (list) { list.add(path); } } if (outputStat != null) { child.copyStat(outputStat); } } // now check if its one of the zookeeper node child if (parentName.startsWith(quotaZookeeper)) { // now check if it's the limit node if (Quotas.limitNode.equals(childName)) { // this is the limit node // get the parent and add it to the trie pTrie.addPath(Quotas.trimQuotaPath(parentName)); } if (Quotas.statNode.equals(childName)) { updateQuotaForPath(Quotas.trimQuotaPath(parentName)); } } String lastPrefix = getMaxPrefixWithQuota(path); long bytes = data == null ? 0 : data.length; // also check to update the quotas for this node if (lastPrefix != null) { // ok we have some match and need to update updateQuotaStat(lastPrefix, bytes, 1); } updateWriteStat(path, bytes); dataWatches.triggerWatch(path, Event.EventType.NodeCreated, zxid); childWatches.triggerWatch(parentName.equals("") ? "/" : parentName, Event.EventType.NodeChildrenChanged, zxid); } 以上代码中，第10 行，针对当前要创建的节点的父节点添加了互斥锁，主要是用来避免并发，这样就可以在同一时刻，只有一个线程可以在父节点下写内容。 ...