Merge pull request #36 from sharelatex/bg-limit-resync-requests-to-web

limit resync requests to web (connects to overleaf/sharelatex#775)

services/document-updater/app/coffee/DocumentManager.coffee

@@ -181,14 +181,19 @@ module.exports = DocumentManager =
 				callback(null, lines, version)
 
 	resyncDocContents: (project_id, doc_id, callback) ->
+		logger.log {project_id: project_id, doc_id: doc_id}, "start resyncing doc contents"
 		RedisManager.getDoc project_id, doc_id, (error, lines, version, ranges, pathname, projectHistoryId) ->
 			return callback(error) if error?
 
 			if !lines? or !version?
+				logger.log {project_id: project_id, doc_id: doc_id}, "resyncing doc contents - not found in redis - retrieving from web"
 				PersistenceManager.getDoc project_id, doc_id, (error, lines, version, ranges, pathname, projectHistoryId) ->
-					return callback(error) if error?
+					if error?
+						logger.error {project_id: project_id, doc_id: doc_id, getDocError: error}, "resyncing doc contents - error retrieving from web"
+						return callback(error)
 					ProjectHistoryRedisManager.queueResyncDocContent project_id, projectHistoryId, doc_id, lines, version, pathname, callback
 			else
+				logger.log {project_id: project_id, doc_id: doc_id}, "resyncing doc contents - doc in redis - will queue in redis"
 				ProjectHistoryRedisManager.queueResyncDocContent project_id, projectHistoryId, doc_id, lines, version, pathname, callback
 
 	getDocWithLock: (project_id, doc_id, callback = (error, lines, version) ->) ->

services/document-updater/app/coffee/HistoryManager.coffee

@@ -65,10 +65,12 @@ module.exports = HistoryManager =
 		newBlock  = Math.floor(length / threshold)
 		return newBlock != prevBlock
 
+	MAX_PARALLEL_REQUESTS: 4
+
 	resyncProjectHistory: (project_id, projectHistoryId, docs, files, callback) ->
 		ProjectHistoryRedisManager.queueResyncProjectStructure project_id, projectHistoryId, docs, files, (error) ->
 			return callback(error) if error?
 			DocumentManager = require "./DocumentManager"
 			resyncDoc = (doc, cb) ->
 				DocumentManager.resyncDocContentsWithLock project_id, doc.doc, cb
-			async.each docs, resyncDoc, callback
+			async.eachLimit docs, HistoryManager.MAX_PARALLEL_REQUESTS, resyncDoc, callback