ZooKeeper Multi-Server Setup by Example - configuration

Question

From the ZooKeeper multi-server config docs they show the following configs that can be placed inside of zoo.cfg (ZK's config file) on each server:
tickTime=2000
dataDir=/var/zookeeper/
clientPort=2181
initLimit=5
syncLimit=2
server.1=zoo1:2888:3888
server.2=zoo2:2888:3888
server.3=zoo3:2888:3888
Furthermore, they state that you need a myid file on each ZK node whose content matches one of the server.id values above. So for example, in a 3-node "ensemble" (ZK cluster), the first node's myid file would simply contain the value 1. The second node's myid file would contain 2, and so forth.
I have a few practical questions about what this looks like in the real world:
1. Can localhost be used? If zoo.cfg has to be repeated on each node in the ensemble, is it OK to define the current server as localhost? For example, in a 3-node ensemble, would it be OK for Server #2's zoo.cfg to look like:
tickTime=2000
dataDir=/var/zookeeper/
clientPort=2181
initLimit=5
syncLimit=2
server.1=zoo1:2888:3888
server.2=localhost:2888:3888 # Afterall, we're on server #2!
server.3=zoo3:2888:3888
Or is this not advised/not possible?
2. Do they server ids have to be numerical? For instance, could I have a 5-node ensemble where each server's zoo.cfg looks like:
tickTime=2000
dataDir=/var/zookeeper/
clientPort=2181
initLimit=5
syncLimit=2
server.red=zoo1:2888:3888
server.green=zoo2:2888:3888
server.blue=zoo3:2888:3888
server.orange=zoo1:2888:3888
server.purple=zoo2:2888:3888
And, say, Server 1's myid would contain the value red inside of it (etc.)?

Answer 1

1. Can localhost be used?
This is a good question as ZooKeeper docs don't make it cristal clear whether the configuration file only accepts IP addresses. It says only hostname which could mean either an IP address, a DNS, or a name in the hosts file, such as localhost.
server.x=[hostname]:nnnnn[:nnnnn], etc
(No Java system property)
servers making up the ZooKeeper ensemble. When the server starts up, it determines which server it is by looking for the file myid in the data directory. That file contains the server number, in ASCII, and it should match x in server.x in the left hand side of this setting.
However, note that ZooKeeper recommend to use the exactly same configuration file in all hosts:
ZooKeeper's behavior is governed by the ZooKeeper configuration file. This file is designed so that the exact same file can be used by all the servers that make up a ZooKeeper server assuming the disk layouts are the same. If servers use different configuration files, care must be taken to ensure that the list of servers in all of the different configuration files match.
So simply put the machine IP address and everything should work. Also, I have personally tested using 0.0.0.0 (in a situation where the interface IP address was different from the public IP address) and it does work.
2. Do they server ids have to be numerical?
From ZooKeeper multi-server configuration docs, myid need to be a numerical value from 1 to 255:
The myid file consists of a single line containing only the text of that machine's id. So myid of server 1 would contain the text "1" and nothing else. The id must be unique within the ensemble and should have a value between 1 and 255.
Since myid must match the x in server.x parameter, we can infer that x must be a numerical value as well.