Getting Started with Apache Zookeeper

Explore Zookeeper basics and how its system-like api replaces explicit primitives with z nodes, enabling master worker coordination through workers, tasks, and assign z nodes.

Explore the Zookeeper api overview. Learn znode operations—create, delete, exist, set data, get data, and get children—and how data is stored as a byte array with serialization options.

Explore the four znode modes in ZooKeeper: persistent, ephemeral, sequential, and ephemeral sequential, and how they affect creation, deletion, and session-based behavior in master and worker examples.

Learn how watches and notifications replace polling by letting clients register for changes to z nodes, using one-shot watches, and receiving ordered updates for data or children.

Explore how ZooKeeper increments version numbers on data changes and uses them to guard concurrent updates, with set data and delete succeeding only when the client version matches the server.

Explore how Zookeeper architecture enables client interactions through the client library and servers. Learn how standalone and quorum modes use a Zookeeper ensemble to replicate state and serve requests.

Learn about zookeeper quorums, where the data tree is replicated across the ensemble and a majority keeps updates durable. Prefer odd server counts to tolerate crashes and avoid split-brain.

Explore zookeeper sessions, where a client establishes a session, ephemeral nodes disappear when the session ends, and requests execute in Fifo order within that session.

Download and extract the zookeeper distribution from zookeeper.apache.org, ensure java is installed, and use the bin scripts in the distribution to start zookeeper, noting Unix and Windows script differences.

Set up a local ZooKeeper session in standalone mode, start the server, connect with the ZK CLI to localhost:2181, and create and delete a worker znode.

Explore the lifetime of a ZooKeeper session from creation to end, detailing states such as connecting, connected, and closed, and the events that trigger transitions.

Configure a three-node ZooKeeper ensemble on a single machine to form a quorum, enabling leader election and reliable coordination. Connect clients via a host:port connect string for quorum access.

Implement a log primitive in zookeeper by creating an ephemeral z node to grant access to a critical section. Watch for log deletions and retry to avoid deadlock.

Implement a master worker pattern in Apache Zookeeper by creating an ephemeral /master znode, watching for changes, and coordinating client tasks.

Create three persistent znodes /workers, /task, and /assign to track available workers and pending tasks, with the master watching the children of these znodes to trigger assignments.

Workers announce availability by creating an ephemeral znode under /workers with host name. Then the worker creates a parent znode under /assign/worker1.example.com to receive assignments and watches it for tasks.

Submit a task to the master worker system by creating a sequential z node under /tasks, then watch the status z node to confirm completion.

Explore Zookeeper API and its Java and C bindings, clients connect to an ensemble and perform create, exists, get data, set data, get children, delete with synchronous or asynchronous methods.

Set the zookeeper class path and create a zookeeper handle to establish and maintain a session across servers, with a watcher to monitor session events and reconnections.

Connect to a Zookeeper ensemble using the Zookeeper constructor with a connection string, session timeout, and a watcher, and await the connection via a countdown latch in a helper class.

Create a znode in the zookeeper ensemble using the create method with a path and data, using open ACL unsafe and persistent create mode; demonstrate with a Java example.

Discover how to check a znode's existence in Apache ZooKeeper using the exist method, retrieve its metadata, and report the node version when it exists.

Explore the zookeeper getData method to retrieve data from a specified z node with a path and watcher, including one-time notifications when data changes.

Demonstrate how the zookeeper setData method updates znode data at a path using versioning, and show a java example to perform the update and verify with the zookeeper cli.

Learn how to use the Getchildren method to fetch all subnodes of a znode with a path and watcher, and print the children in a Java example.

Learn how to delete a z node in Apache Zookeeper using the delete method, checking existence and version, with a Java example that deletes the z node and its subnodes.

Learn how Zookeeper uses watches to notify clients of state changes. Avoid polling and enable rapid failover by using an ephemeral znode and triggering a master election.

Explain one time triggers and watches in zookeeper, showing how events generate notifications and callbacks, how sessions affect watch persistence across servers, and how missing events are handled.

Set data and child watches on znodes using exist, get data, and get children, triggering events such as node created or data changed; implement a watcher to process these events.

Explore watches as a replacement for explicit cache management in zookeeper, enabling clients to cache znode data and its list of children locally while receiving asynchronous update notifications.

Explore a common code pattern in Zookeeper applications: perform an asynchronous call, implement a callback, and, if needed, implement and pass a watcher, as shown with the exists example.

Explore the master worker example in Apache Zookeeper and state changes. See how mastership changes and waits for worker lists and tasks, while clients await task execution results with code.

Explore how Zookeeper handles failures by distinguishing recoverable and unrecoverable cases, locating the failure across service, network, and application, and applying restart or exit strategies.

Explore how Zookeeper recovers from recoverable failures by reestablishing sessions after disconnections, syncing connected events, and reregistering watches to maintain consistent state across clients.

Explore how the exists watch and the disconnected event affect Zookeeper session recovery; the client reestablishes watches on reconnect, but creation events can be missed in certain corner cases.

Identify unrecoverable failures when a session expires or cannot authenticate, causing Zookeeper to discard session state and delete ephemeral znodes, requiring termination or careful reinitialization.

Explore leader election and fencing token, using z id to prevent external resource corruption when Zookeeper sessions fail from overload or clock drift.