Zookeeper ZNodes – Characteristics & Example

In our last ZooKeeper tutorial, we learned about ZooKeeper barriers. Today, we will discuss ZooKeeper ZNodes. Basically, every node in a ZooKeeper tree is a ZNode.

Though there is much more to learn about ZooKeeper ZNodes. So, here we will learn the whole concept of Zookeeper ZNodes. Also, there are various characteristics (watches, Ephemeral Nodes, etc.) of ZNodes in ZooKeeper.

So, let’s start ZooKeeper ZNodes.

What is Zookeeper ZNodes?

The term ZNode is referred to every node in a ZooKeeper tree. The main purpose of the Znode is to maintain a stat structure. However, stat structure includes version numbers for data changes and ACL changes. Also, a stat structure includes timestamps in it.

Hence, timestamps and the version number together permits ZooKeeper to validate the cache as well as to coordinate updates. Although make sure, the version number increases, each time a ZNode’s data changes.

For instance, a Client also receives the version of the data, whenever it retrieves data. Though, a client must supply the version of the data of the ZNode it is changing, when a client performs an update or a delete. Because the update will fail if the version it supplies doesn’t match the actual version of the data.

In addition, note that the word node can refer to many in a distributed application engineering. for example, a generic host machine, a server, a member of an ensemble, a client process, etc. Basically, ZNodes refer to the data nodes, in the ZooKeeper documentation.

Along with it, the Servers to refer to machines which make up the ZooKeeper service, similarly, quorum peers refer to the servers which make up an ensemble; in the same way, a client refers to any host or process that uses a ZooKeeper service.

Although, we can say, the main entity that a programmer access are ZNodes.

Characteristics of Zookeeper ZNodes

Further, there are several characteristics of Zookeeper Znodes, such as:

a. Watches

It is possible for clients to set watches on ZNodes. So, when we make changes to that ZNode, as a result, it triggers the watch and then further clear the watch. Also, ZooKeeper sends the client a notification, when a watch triggers.

b. Data Access

At each ZNode, the data which is stored in a namespace is read and written atomically. Basically, Read process get all the data bytes which are associated with a ZNode.

Whereas, the writing process replaces all the data. In addition, there is an Access Control List (ACL) of each node, so that restricts the function of all.

c. Ephemeral Nodes

Well, we can say, there is a slight notion of ephemeral nodes in ZooKeeper. As long as the session which creates the ZNode is active, until that time these ZNodes exists.

Similarly, the ZNode is deleted, when the session ends. Thus, ephemeral ZNodes are not allowed to have children, because of this behavior only.

d. Sequence Nodes – Unique Naming

We can also request that ZooKeeper append a monotonically increasing counter to the end of the path while creating a ZNode. Well, it is a unique counter to the parent ZNode.

The format of the counter is %010d — basically, it is 10 digits with 0 (zero) padding. However, to simplify sorting, the counter is formatted in this way, like “<path>0000000001”

Also, note that maintained by the parent node, the counter used to store the next sequence number is a signed int (4bytes), and also the counter will overflow while it is incremented beyond 2147483647.
So, this was all about ZooKeeper ZNodes. Hope you like our explanation.

Conclusion

Hence, we have learned the whole concept of Zookeeper ZNodes along with its characteristics in detail. So, if any doubt occurs regarding Znodes in ZooKeeper, feel free to ask in the comment section. We are happy to help!