Distributed Coordination With ZooKeeper Part 1: Introduction

Posted on June 25, 2013 by Scott Leberknight

This is the first in a series of blogs that introduce Apache ZooKeeper. This blog provides an introduction to ZooKeeper and its core concepts and use cases. In later blogs you will test drive ZooKeeper, see some examples of the Java API, learn about its architecture, build a distributed data structure which can be used across independent processes and machines, and finally get a brief introduction to a higher-level API on top of ZooKeeper.

Consider a distributed system with multiple servers, each of which is responsible for holding data and performing operations on that data. This could be a distributed search engine, a distributed build system, or even something like Hadoop which has both a distributed file system and a Map/Reduce data processing framework that operates on the data in the file system. How would you determine which servers are alive and operating at any given moment in time? Or, how would you determine which servers are available to process a build in a distributed build system? Or for a distributed search system how would you know which servers are available to hold data and handle search requests? Most importantly, how would you do these things reliably in the face of the difficulties of distributed computing such as network failures, bandwidth limitations, variable latency connections, security concerns, and anything else that can go wrong in a networked environment, perhaps even across multiple data centers?

These and similar questions are the focus of Apache ZooKeeper, which is a fast, highly available, fault tolerant, distributed coordination service. Using ZooKeeper you can build reliable, distributed data structures for group membership, leader election, coordinated workflow, and configuration services, as well as generalized distributed data structures like locks, queues, barriers, and latches.

Many well-known and successful projects already rely on ZooKeeper. Just a few of them include HBase, Hadoop 2.0, Solr Cloud, Neo4J, Apache Blur (incubating), and Accumulo.

Core Concepts

ZooKeeper is a distributed, hierarchical file system that facilitates loose coupling between clients and provides an eventually consistent view of its znodes, which are like files and directories in a traditional file system. It provides basic operations such as creating, deleting, and checking existence of znodes. It provides an event-driven model in which clients can watch for changes to specific znodes, for example if a new child is added to an existing znode. ZooKeeper achieves high availability by running multiple ZooKeeper servers, called an ensemble, with each server holding an in-memory copy of the distributed file system to service client read requests. Each server also holds a persistent copy on disk.

One of the servers is elected as the leader, and all other servers are followers. The leader is responsible for all writes and for broadcasting changes to to followers. Assuming a majority of followers commit a change successfully, the write succeeds and the data is then durable even if the leader then fails. This means ZooKeeper is an eventually consistent system, because the followers may lag the leader by some small amount of time, hence clients might not always see the most up-to-date information. Importantly, the leader is not a master as in a master/slave architecture and thus is not a single point of failure; rather, if the leader dies, then the remaining followers hold an election for a new leader, and the new leader takes over where the old one left off.

Each client connects to ZooKeeper, passing in the list of servers in the ensemble. The client connects to one of the servers in the ensemble at random until a connection is established. Once connected, ZooKeeper creates a session with the client-specified timeout period. The ZooKeeper client automatically sends periodic heartbeats to keep the session alive if no operations are performed for a while, and automatically handles failover. If the ZooKeeper server a client is connected to fails, the client automatically detects this and tries to reconnect to a different server in the ensemble. The nice thing is that the same client session is retained during this failover event; however during failover it is possible that client operations could fail and, as with almost all ZooKeeper operations, client code must be vigilant and detect errors and deal with them as necessary.

Partial Failure

One of the fallacies of distributed computing is that the network is reliable. Having worked on a project for the past few years with multiple Hadoop, Apache Blur, and ZooKeeper clusters including hundreds of servers, I can definitely say from experience that the network is not reliable. Simply put, things break and you cannot assume the network is 100% reliable all the time. When designing distributed systems, you must keep this in mind and handle things you ordinarily would not even consider when building software for a single server. For example, assume a client sends an update to a server, but before the response is received the network connection is lost for a brief period. You need to ask several questions in this case. Did the message get through to the server? If it did, then did the operation actually complete successfully? Is it safe to retry an operation for which you don't even know whether it reached the server or if it failed at the server, in other words is the operation idempotent? You need to consider questions like these when building distributed systems. ZooKeeper cannot help with network problems or partial failures, but once you are aware of the kinds of problems which can arise, you are much better prepared to deal with problems when (not if) they occur. ZooKeeper provides certain guarantees regarding data consistency and atomicity that can aid you when building systems, as you will see later.

Conclusion to Part 1

In this blog we've learned that ZooKeeper is a distributed coordination service that facilitates loose coupling between distributed components. It is implemented as a distributed, hierarchical file system and you can use it to build distributed data structures such as locks, queues, and so on. In the next blog, we'll take a test drive of ZooKeeper using its command line shell.


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