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Assignment 2
Description
You will implement a replicated key-value data store maintained by N servers. Each server will
maintain a copy of the data store and expose two functions
Read(key): will read the value associated with the key.
Add_Update(key, value): will add/update the value associated with the key.
A client may contact any of the server to read/add/update the data store. All Add/Update(key,
value) requests will be routed to the leader server, who will be responsible for any write
requests to the key-value. The leader will also be responsible for propagating the updates to other
replicas. You will use ZooKeeper (a coordination service) to elect the leader (see more
information below on how to use ZooKeeper for leader lection).
All Read(key) request will be served as follows. If the key exists in the local data store, it will
send the value even if it is “stale”. Else, it will return empty.
Implementation
You will create a server program (e.g., server.py) that spawns the server process and connects to
the ZooKeeper service.
python server.py –host –port -zookeeper -zookeeper_port

This will spawn a server that will connect to the zookeeper service, create a /election/ znode
and register itself in the /election/ znode. Servers under the /election/ znode will
participate in the election, and a server will be identified as a leader.
Use Znode type SEQUENTIAL, so when the server registers under /election/ it is assigned a
value “server_”. e.g., /election/server_000000001.
For leader election, query all the children nodes under election and select the znode with the
smallest sequence number.
You may use REST protocols or RPCs for implementing the add/update and read functions.
Testing
You will provide a driver-test program that will spawn the servers and connect to the zookeeper
service. The test driver program will also be responsible for sending / receiving key value store
updates to one of the servers. Use the Docker Zookeeper service script to spawn zookeeper
servers and test the following scenarios:
- Add and Read: Start 3 servers. The servers elect a leader and all add requests are routed
to the leader. Subsequent Read requests of the key can be fetched from any of the server.
- Leader election: Same as above but kill the leader node. A new leader is elected and all
subsequent requests is routed to the new leader.
- Stale Read: The killed leader is back online but is not the leader and may have stale data
(if main data store was updated). Since the key value is in memory, it will return empty
until the key was updated. Once key value is updated, the server will output the updated
value.
Please put appropriate print statements.
Extra Credit (10%)
Start the Zookeeper service on Google’s cloud platform and compare the leader election latency
with a local zookeeper service. Report the latency over multiple runs.
Submission
Please upload your code and your report on Gradescope. Your submission should contain all the
code including the test cases and log files of your execution.
Grading Criteria
Component %
Implementation 60
Testing 30
Code documentation 10
Extra credit 10
Zookeeper Documentation
Leader selection via Zookeeper
ZooKeeper is a distributed, open-source coordination service for distributed applications. It
exposes a simple set of primitives that distributed applications can build upon to implement
higher level services for synchronization, configuration maintenance, and groups and naming. It
is designed to be easy to program to, and uses a data model styled after the familiar directory tree
structure of file systems.
Installation:
We are going to use docker to install Zookeeper.
Go get docker on: https://docs.docker.com/desktop/
Once the docker is successfully installed, use your cmd/terminal to check via command:
$ docker --version
Get Zookeeper from docker on: https://hub.docker.com/_/zookeeper
Then we can use docker to get ZooKeeper Service setup on our machine via command:
$ docker pull zookeeper
Check your installation with
$ docker image ls
To lunch a Zookeeper server,
$ docker run --name my-zookeeper --restart always -d zookeeper
Where the –name sets the name of the container you started and -d sets the image the container
utilized. You can check running containers with
$ docker container ls
By default, your zookeeper will use following ports
Client port::2181
Follower port::2888
Election port::3888
AdminServer port::8080
When a server is on, you can connect via commands,
$ docker run -it --rm --link my-zookeeper:zookeeper zookeeper zkCli.sh -server
zookeeper
Where use your container’s name to replace my-zookeeper
Programming in ZooKeeper
Online resources: https://www.tutorialspoint.com/zookeeper/index.htm)
Example Python implementation : KAZOO
If you are using python. Kazoo is a good choice as an API for zookeeper.
Documentation: https://kazoo.readthedocs.io/en/latest/basic_usage.html
Usage:
from kazoo.client import KazooClient
zk = KazooClient(hosts='localhost:2181')
zk.start()
It supports all UI required for leader election such as zk.create(), zk.set(), zk.delete(). Etc.
You can check znode under current znode with: zk.get_children(dir). The returned object will be
a list of children’s names.
Docker script to run your Zookeeper server on replicate mode
To build a distributed file system we need at least 3 zookeeper servers, this requires us to run 3
zookeeper servers as a service in docker. To do that:
1. Initialize your docker to swarm mode to simulate distributed environment: docker
swarm init
2. Use docker stack deploy to launch 3 servers (in following example we use zookeeper as
our service name, you can replace that with any name you what when practicing):
a. Use a config file which specifies server configs in a *.yml file (such as
“zookeeper.yml”): an example on Appendix
b. Then you can deploy stack via: docker stack deploy --compose-file
zookeeper.yml zookeeper
c. You can check the running service by: docker stack services zookeeper
3. To gracefully exit (in following example we use zookeeper as our service name, you can
replace that with any name you what when practicing):
a. Bring stack down via: docker stack rm zookeeper
b. (optional)Bring service down via: docker service rm zookeeper
c. Quit swam mode by: docker swarm leave –force
To run your leader election, first make sure you have a znode created for such purpose
(such as “/leader”, “/election”)
Appendix
version: '3.1'
services:
zoo1:
image: zookeeper
restart: always
hostname: zoo1
ports:
- 2181:2181
environment:
ZOO_MY_ID: 1
ZOO_SERVERS: server.1=0.0.0.0:2888:3888;2181
server.2=zoo2:2888:3888;2181 server.3=zoo3:2888:3888;2181
zoo2:
image: zookeeper
restart: always
hostname: zoo2
ports:
- 2182:2181
environment:
ZOO_MY_ID: 2
ZOO_SERVERS: server.1=zoo1:2888:3888;2181
server.2=0.0.0.0:2888:3888;2181 server.3=zoo3:2888:3888;2181
zoo3:
image: zookeeper
restart: always
hostname: zoo3
ports:
- 2183:2181
environment:
ZOO_MY_ID: 3
ZOO_SERVERS: server.1=zoo1:2888:3888;2181 server.2=zoo2:2888:3888;2181
server.3=0.0.0.0:2888:3888;2181

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