 		rclient.smembers Keys.docsWithHistoryOps({project_id}), callback
 
 	# iterate over keys asynchronously using redis scan (non-blocking)
+	# handle all the cluster nodes or single redis server
 	_getKeys: (pattern, callback) ->
+		nodes = rclient.nodes?('master') || [ rclient ];
+		doKeyLookupForNode = (node, cb) ->
+			RedisManager._getKeysFromNode node, pattern, cb
+		async.concatSeries nodes, doKeyLookupForNode, callback
+
+	_getKeysFromNode: (node, pattern, callback) ->
 		cursor = 0  # redis iterator
 		keySet = {} # use hash to avoid duplicate results
 		# scan over all keys looking for pattern
 		doIteration = (cb) ->
-			rclient.scan cursor, "MATCH", pattern, "COUNT", 1000, (error, reply) ->
+			node.scan cursor, "MATCH", pattern, "COUNT", 1000, (error, reply) ->
 				return callback(error) if error?
 				[cursor, keys] = reply
 				for key in keys

redis = require("redis-sharelatex")
rclient = redis.createClient(Settings.redis.history)
Keys = Settings.redis.history.key_schema

module.exports = RedisManager =


		doIteration()

	# extract ids from keys like DocsWithHistoryOps:57fd0b1f53a8396d22b2c24b
	# or DocsWithHistoryOps:{57fd0b1f53a8396d22b2c24b} (for redis cluster)
	_extractIds: (keyList) ->
		ids = for key in keyList
			m = key.match(/:\{?([0-9a-f]{24})\}?/) # extract object id
			m[1]
		return ids

	getProjectIdsWithHistoryOps: (callback = (error, project_ids) ->) ->
		RedisManager._getKeys Keys.docsWithHistoryOps({project_id:"*"}), (error, project_keys) ->
			return callback(error) if error?
			project_ids = RedisManager._extractIds project_keys
			callback(error, project_ids)

	getAllDocIdsWithHistoryOps: (callback = (error, doc_ids) ->) ->
		# return all the docids, to find dangling history entries after
		# everything is flushed.
		RedisManager._getKeys Keys.uncompressedHistoryOps({doc_id:"*"}), (error, doc_keys) ->
