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What is the use of JournalNode, many novices are not very clear about this, in order to help you solve this problem, the following editor will explain for you in detail, people with this need can come to learn, I hope you can gain something.

Sharing data between NameNode (NFS, Quorum Journal Node (used more))

In order to synchronize data, two NameNode communicate with each other through a set of independent processes called JournalNodes. When there is any change in the namespace of the NameNode in the active state, most JournalNodes processes are notified. NameNode in standby state has the ability to read changes in JNs, and keeps monitoring changes in edit log, applying changes to its own namespace. Standby ensures that the namespace state is fully synchronized when the cluster goes wrong.

The above green area between Active Namenode and StandBy Namenode is JournalNode. Of course, there may not be only one, which is equivalent to the NFS shared file system. Active Namenode writes editlog data to it, and StandBy reads the data from it for synchronization.

Sharing data between NameNode (NFS, Quorum Journal Node (used more))

In order to synchronize data, two NameNode communicate with each other through a set of independent processes called JournalNodes. When there is any change in the namespace of the NameNode in the active state, most JournalNodes processes are notified. NameNode in standby state has the ability to read changes in JNs, and keeps monitoring changes in edit log, applying changes to its own namespace. Standby ensures that the namespace state is fully synchronized when the cluster goes wrong.

The NameNode in Hadoop is like the human heart. It is very important and must never stop working.

The high reliability of HDFS in hadoop2.2.0 (HA) means that two NameNode can be started at the same time. One of them is working and the other is on standby. In this way, when the server where a NameNode is located goes down, you can manually or automatically switch to another NameNode to provide services without losing data.

These NameNode share data to ensure that the state of the data is consistent. Data is shared among multiple NameNode, either through Nnetwork File System or Quorum Journal Node. The former is a file system shared through linux, which belongs to the configuration of the operating system, while the latter belongs to the hadoop itself and belongs to the configuration of the software.

Here we talk about the configuration method using Quorum Journal Node, which is manually switched.

When the cluster starts, you can start two NameNode at the same time. Only one of these NameNode is active, and the other belongs to the standby state. The active state means providing the service, and the standby state means being dormant, only data synchronization, and always ready to provide the service, as shown in figure 2.

Figure 3

To ensure fast switching, it is necessary for the NameNode in the standby state to know the location of all data blocks in the cluster. To do this, all datanodes must configure two NameNode addresses and send block location information and heartbeats to both of them.

For HA clusters, it is critical to ensure that only one NameNode is in the active state at a time. Otherwise, the data state of the two NameNode will diverge and may lose data or produce incorrect results. To ensure this, JNs must ensure that only one NameNode can write data to itself at a time.

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