To add a ZooKeeper instance to the ZooKeeper ensemble, you need to add the following parameter to the file "zoo.cfg":
server.<id> = <zk_host_address>:<zk_port_1>:<zk_port_2>[:<zk_role>];[<client_port_address>:]<client_port>

• <id>: the ZooKeeper instance id
  
  
• <zk_host_address>: the host name or ip address where the ZooKeeper instance is running
  
  
• <zk_port_1>: is used by peer ZooKeeper instances to communicate with each other
  
  
• <zk_port_2>: is used for the ZooKeeper leader election.
  
  
• <zk_role>: ZooKeeper instances roles ("participant", "observer"). The role is optional, and if not specified it defaults to "participant".
  
  
• <client_port_address>: The client port address is optional, and if not specified it defaults to "0.0.0.0".
  
  
• <client_port>: A client port of a ZooKeeper instance is the port on which the ZooKeeper instance accepts client connection requests.
  

To configure ZooKeeper instances that are parts of a ZooKeeper ensemble you can configure the file "zoo.cfg" of each instance to list all instances of the ZooKeeper ensemble.

For example, I have 3 instances of ZooKeeper installed on 3 hosts: "zkhost1", "zkhost2", and "zkhost3".
For each instance, I had to modify the file "zoo.cfg" to add the following:
server.1=zkhost1:2888:3888:participant;0.0.0.0:2181 server.2=zkhost2:2888:3888:participant;0.0.0.0:2181 server.3=zkhost3:2888:3888:participant;0.0.0.0:2181
Example: ZooKeeper configuration file "zoo.cfg" of the host "zkhost1"
# The number of milliseconds of each tick. tickTime=2000 # The number of ticks that the initial synchronization phase can take. initLimit=10 # The number of ticks that can pass between sending a request and getting an acknowledgement. syncLimit=5 # the port at which the clients will connect: 2181 # the port address at which the clients will connect: 0.0.0.0 server.1=zkhost1:2888:3888:participant;0.0.0.0:2181 server.2=zkhost2:2888:3888:participant;0.0.0.0:2181 server.3=zkhost3:2888:3888:participant;0.0.0.0:2181 # The directory where the snapshot data is stored. dataDir=/opt/zookeeper/data/ # The directory the transaction log of updates is stored. dataLogDir=/opt/zookeeper/data-log/
Please note that in previous versions of ZooKeeper, when you start ZooKeeper, the static configuration of the zookeeper ensemble will be copied to a new file and a new parameter ("dynamicConfigFile") will be added to the file "zoo.cfg" to reference this new file:
dynamicConfigFile=/opt/zookeeper/conf/zoo.cfg.dynamic.100000000

You can configure the file "zoo.cfg" to add a dynamic configuration of the ZooKeeper instances which are parts of the ZooKeeper ensemble.

For example, I have 3 instances of ZooKeeper installed on 3 hosts: "zkhost1", "zkhost2", and "zkhost3".
For each instance, I had to modify the file "zoo.cfg" to add the parameter "dynamicConfigFile" as follow:
dynamicConfigFile=/opt/zookeeper/conf/zoo.cfg.dynamic
Example: ZooKeeper configuration file "zoo.cfg" of the host "zkhost1"
# The number of milliseconds of each tick. tickTime=2000 # The number of ticks that the initial synchronization phase can take. initLimit=10 # The number of ticks that can pass between sending a request and getting an acknowledgement. syncLimit=5 # ZooKeeper ensemble: Dynamic configuration file dynamicConfigFile=/opt/zookeeper/conf/zoo.cfg.dynamic # The directory where the snapshot data is stored. dataDir=/opt/zookeeper/data/ # The directory the transaction log of updates is stored. dataLogDir=/opt/zookeeper/data-log/
The "zoo.cfg.dynamic" file will contain the dynamic configuration of the ZooKeeper ensemble:
# the port at which the clients will connect: 2181 # the port address at which the clients will connect: 0.0.0.0 server.1=zkhost1:2888:3888:participant;0.0.0.0:2181 server.2=zkhost2:2888:3888:participant;0.0.0.0:2181 server.3=zkhost3:2888:3888:participant;0.0.0.0:2181