ZooKeeper and the Zab Protocol

Coordination as a file tree

Apache ZooKeeper is a coordination service used by systems such as older Kafka versions, HBase and many Hadoop-era tools. It stores small data nodes called znodes in a tree like a file system. A znode can be ephemeral (deleted when the client's session ends) and/or sequential (ZooKeeper appends a monotonically increasing counter to its name). Clients can set watches to be notified of changes; classic watches fire once and must be re-registered (persistent watches were added in ZooKeeper 3.6). ZooKeeper replicates with Zab (ZooKeeper Atomic Broadcast), a leader-based protocol in the same family as Raft and Multi-Paxos: a leader orders all writes, identified by a zxid that combines an epoch (like a Raft term) and a counter, and commits them once a quorum acknowledges. Its guarantees are linearizable writes and FIFO order per client; ordinary reads are served by whichever server the client is connected to and may be slightly stale, unless the client first issues a sync.

Leader election recipe with ephemeral sequential znodes

Each candidate creates an ephemeral sequential child; the lowest sequence number is the leader. Each other candidate watches only the node just before its own to avoid a 'herd' of notifications.

# In zkCli.sh
create /election ""
create -e -s /election/n_ "node1"   # -> /election/n_0000000000
create -e -s /election/n_ "node2"   # -> /election/n_0000000001
ls /election
stat -w /election/n_0000000000     # node2 sets a watch on its predecessor
# node2 now waits; when node1's session dies, that
# ephemeral znode is deleted and node2 becomes the lowest, so it leads.

The queue at a ticket counter

Everyone takes a numbered slip (sequential znode). The lowest number is served (leader). You only watch the person directly ahead of you; when they leave the queue (session ends, ephemeral znode deleted), you know it is your turn.

Quick check: What happens to an ephemeral znode when the client session that created it expires?

• It is deleted automatically
• It becomes persistent
• It is moved to the end of the sequence
• It is kept but marked read-only