Jzab : A Standalone Zab Library

1. Jzab : A Standalone Zab Library
Yisheng Liao Mentor : Michi Mutsuzaki
with great help from
Flavio Junqueira, Hongchao Deng, Alexander Shraer

2. Background
● Zab is an atomic broadcast protocol designed for replicating state
of ZooKeeper servers.
● Zab has similar goals with Paxos and Raft, but it has its own
properties (multiple outstanding transactions while guaranteeing
primary order, allows for fuzzy snapshot, etc.).
● ZooKeeper implementation of Zab is entangled with ZooKeeper so
it’s hard for other applications to use it.
● We implemented Jzab as a standalone Java library to enable users
to build their applications on top of Zab easily.

3. Features
● Authentication and encryption (SSL)
● Dynamic reconfiguration (add/remove server one at a
time)
● Snapshot
● Fast leader election
○ Best effort to elect a leader with the “best” history
● Exponential backoff for timeout(initLimit) (related to
ZOOKEEPER-1977)
○ Problem : How to decide the appropriate timeout value?
○ Proposals :
■ Proportional to the log + snapshot size
■ exponential backoff

4. States of Jzab from user’s perspective
From user’s perspective, Zab can be only in one of the three
states (Recovering, Leading, Following)

5. Workflow from user’s perspective

6. Benchmark for log with different batch size
We did all the benchmarks on 3 machines each with Intel
Xeon X3430 2.40GHz 4 cores, 7200 rpm hard drive, 16G
memory.
Batch size Machine1 Machine2 Machine3
1 105 txns/second 108 txns/second 106 txns/second
100 2779 txns/second 3361 txns/second 2787 txns/second
1000 26122 txns/second 26381 txns/second 25732 txns/second

7. Benchmark for Jzab cluster
3 server cluster, 1GB in-memory state, 128 bytes per transaction and maximum
batch size is 1000 txns.

8. Benchmark (different snapshot strategies)
Same setup with previous benchmark, but with different strategies for taking
snapshot.

9. Benchmark (different garbage collectors)
Same setup with previous benchmark except on just one server cluster and no
snapshot.

10. Jepsen
Jzab passed Jepsen test!

11. Conclusion
● Taking snapshot with just one fsync still blocks the
service even ZooKeeper allows fuzzy snapshot.
● Garbage collectors affect the throughput.
o If you do care the performance, do not use default GC, try
different GCs and find the one works best for you.
● Checkout Jzab on Github github.com/zk1931/jzab
Thanks!