There are some situations (esp in virtualized/cloud environments) where "hardwiring" the IP into ZooKeeper can create reachability issues and an FQDN (or, better/also, the verbatim string/line from the concerned config file) would fix this problem.
For example, hostname node1.company.com specified in configuration files resolves to an Amazon EC2 internal IP of 10.2.3.4 (internal on virtualized network). Externally (e.g. from your dev machine, your offsite/non-VPN/non-VPCed data center, other client machines on different networks/clouds), node1.company.com will resolve to a public IP (e.g. Amazon Elastic IP, etc) of something more routeable, like 22.214.171.124.
Accumulo currently stores 10.2.3.4 in ZooKeeper based on this resolution, but, if you try to connect to Accumulo from outside these machines/machines in the same cloud/vitualized network/non routeable network, and the same FQDN (node1.company.com) resolves to the public address now (126.96.36.199), you will not be able to connect, because the Accumulo client will have pulled the resolved, and from here, unreachable, IP of 10.2.3.4.
Using the FQDN (or in some other way allowing for client-side name resolution/address translation, although this seems kludgy) would fix this issue in a relatively standard way. Ideally, this would not incur a performance issue beyond the first resolution assuming the TCP/IP stack is doing its job and caching stuff effectively (I assume).
This doesn't really hurt/break things if you give an option in some config, and, really, taking the literal from the file allows you to use whatever you want, the ultimate in flexibility.
See discussion http://mail-archives.apache.org/mod_mbox/accumulo-user/201307.mbox/%3CCAGFNOZTMVz0R2e0meDj%3DKqPPPJP6f5baaMqh8%3D07V7NZ8vToJg%40mail.gmail.com%3E for more details and others having the same issue.
I will look into creating a patch for this as soon as I have some time to find/look at relevant code portions (I need to find where accumulo is making these writes to ZK and if the read FQDNs would need any resolution/their use further down the line expects strictly IP or is in host or IP safe API calls, etc). Any suggestions on where I can begin this are always appreciated. Otherwise, I'll try and submit a patch when I can.
Figure I'd open this issue to at least provide a discussion on what more experienced Accumulo devs and users think and what a solution based on the style/patterns accepted for Accumulo development/configuration are. I can read the guidelines myself, of course, and will, but someone suggested opening an issue, so I am...