Writing Complex Scripts in HBase Shell

The Story

HBase installations include a shell for running arbitrary commands.
For instance, if you want to view all of your snapshots, you can do something like:

[me@server ~]$ hbase shell
HBase Shell; enter 'help<RETURN>' for list of supported commands.
Type "exit<RETURN>" to leave the HBase Shell
Version 0.94.22, rb3b224ddca3530139c0685fd5c52e37368b428df, Fri Aug  1 20:11:17 UTC 2014

hbase(main):001:0> list_snapshots
SNAPSHOT                                             TABLE + CREATION TIME
 snapname                                            tablename (Sun Oct 18 04:31:03 +0000 2015)
 ...
35 row(s) in 0.9470 seconds

However, these functions are implemented in such a way that they print their output to STDOUT directly. If you wanted to create a complex script, like deleting all snapshots taken yesterday, you’re out of luck:

[me@server ~]$ hbase shell
HBase Shell; enter 'help<RETURN>' for list of supported commands.
Type "exit<RETURN>" to leave the HBase Shell
Version 0.94.22, rb3b224ddca3530139c0685fd5c52e37368b428df, Fri Aug  1 20:11:17 UTC 2014

hbase(main):001:0> a = list_snapshots
...
hbase(main):002:0> a.nil?
=> true

Until now, I accomplished things by wrapping them around in bash scripts, but this got annoying fast.

The Solution

I sat around to messing with the HBase source code, and found the way these functions are defined.
I’m referencing v0.94.22 because that’s what I’m running, but the functions are similarly defined at the master branch too.
This is the interesting bit of list_snapshots:

list = admin.list_snapshot.select {|s| regex.match(s.getName)}
list.each do |snapshot|
  creation_time = Time.at(snapshot.getCreationTime() / 1000).to_s
  formatter.row([ snapshot.getName, snapshot.getTable + " (" + creation_time + ")" ])
end

Problem is, this admin variable isn’t accessible from the main scope:

hbase(main):010:0> admin
NameError: undefined local variable or method `admin' for #<Object:0x3faea1b0>

After much digging around, I found out that admin is accessible via @shell.hbase_admin, so I can do something like @shell.hbase_admin.list_snapshot.
To remove a snapshot, we’ll copy from delete_snapshot, and now I have my cleanup script:

snaps = @shell.hbase_admin.list_snapshot
snaps = snaps.select{|s| Time.now - (60*60*24) > Time.at(s.getCreationTime/1000)}
snaps.each{|s|@shell.hbase_admin.delete_snapshot(s)}

I can then run it using something like

/path/to/hbase shell ~/scripts/cleanup.rb

And all is well.