The Fundamentals of Apache ZooKeeper for Data Engineering

Apache ZooKeeper is an open-source distributed coordination service that enables distributed systems to work in a coordinated manner. It essentially solves the problem of race conditions that occur when distributed systems over multiple nodes try to work with the same resource. ZooKeeper lays the foundation for building stable and dependable distributed systems.

In this article, we will cover the fundamentals of Apache ZooKeeper, from its architecture to the use cases where it can be applied.

ZooKeeper Architecture

The ZooKeeper architecture follows a client-server model in which clients interact with ZooKeeper servers. The ZooKeeper service runs on a cluster, with each server in the cluster holding an in-memory data representation of the service.

The ZooKeeper server follows a hierarchical tree-like structure. Each node in the tree structure, also known as a znode, can hold data and children znodes. The data of each znode is limited to 1MB. These znodes are defined in a path-like naming system where each level of the path is separated by a forward slash (/).

Image Source: ZooKeeper Architecture (opens in a new tab)

In the ZooKeeper architecture, clients don't write data to disk. Instead, they communicate with the ZooKeeper servers to create, read, update, and delete znodes. In case a client fails or gets disconnected, ZooKeeper will continue to serve them by storing all the necessary data in memory.

Finally, the ZooKeeper servers follow the concept of quorums to maintain consistency and availability, with each server requiring a majority vote to operate. This concept ensures that even if some servers in the cluster fail, the system can still operate.

Use Cases of ZooKeeper

The applications of ZooKeeper are widespread, as it can be used in various scenarios that require distributed coordination. Here are some of the popular use cases of ZooKeeper:

Service Discovery

In a distributed system, services can be added or removed based on the load or the requirement of the system. ZooKeeper makes service discovery easier by maintaining a registry of available services in the system. The clients can query this registry to find the available services and how to connect with them.

Configuration Management

ZooKeeper can also be used as a configuration management system for distributed applications. In this case, the configuration data is stored in the znodes, and clients can read the data based on their requirements.

Leader Election

In a distributed system, it is essential to have a leader to ensure consistency and avoid conflicts. ZooKeeper can be used to implement leader election by having each node attempting to become a leader by creating an ephemeral znode in a specific directory. The one that successfully creates a znode becomes the leader, while the rest become followers.

Locking

Locking is a fundamental feature in distributed systems that prevents multiple nodes from accessing the same resource at the same time. ZooKeeper provides the ability to create locking mechanisms using znodes. A client can create a znode, which acts as a lock, and the other clients trying to access the same resource must wait until the lock is removed.

Conclusion

In conclusion, Apache ZooKeeper plays a critical role in building dependable and stable distributed systems. Its client-server architecture, hierarchical data model, and quorum-based system ensure data consistency, availability, and fault tolerance. Moreover, its multiple use cases like service discovery, configuration management, leader election, and locking make it a versatile tool for data engineers working with distributed systems.

Category: Distributed System