This python script is used to increase the number of replicas in an ES cluster and force those new replicas to be allocated to a new host to enable a filesystem backup of the ES data directory.

es-backup-control.py

from requests import get, post, put
from json import dumps, loads
from sys import argv
from math import ceil

commands = []

def addShard(idx, shard):
    commands.append({
        "allocate" : {
            "index" : idx,
            "shard" : int(shard),
            "allow_primary" : False,
            "node" : backup_node
        }
    })

state = loads(get('http://localhost:9200/_cluster/state?filter_metadata=true').content)
data_nodes = []
for node in state['nodes']:
    if "data" in state['nodes'][node]['attributes'] and state['nodes'][node]['attributes']['data'] == "false":
        continue
    data_nodes.append({ 'node': node, 'address': state['nodes'][node]['transport_address']})

replica_count = int(ceil(len(data_nodes)/2.0))

if len(argv) != 2:
    print "You must specify which node is the backup node or the 'reset' command.  Available nodes right now are:"
    for node in data_nodes:
        print "\t- %s (%s)" % (node['node'], node['address'])
    quit()

if argv[1] == "reset":
    put('http://localhost:9200/_settings', data='{ "index.routing.allocation.disable_allocation": false }')
    put('http://localhost:9200/_settings', data='{ "index" : { "number_of_replicas" : %d } }' % replica_count)
    quit()

backup_node = argv[1]

if backup_node not in state['nodes']:
    print "Invalid data node"
    quit()

put('http://localhost:9200/_settings', data='{ "index.routing.allocation.disable_allocation": true }')
put('http://localhost:9200/_settings', data='{ "index" : { "number_of_replicas" : %d } }' % (replica_count+1))

for idx in state['routing_table']['indices']:
    for shard in state['routing_table']['indices'][idx]['shards']:
        addShard(idx, shard)

# this is a lot of output, TODO: consolidate this into just what's useful
print dumps(loads(post('http://localhost:9200/_cluster/reroute?pretty', data=dumps({ "commands": commands })).content), indent=4)