# Should I use ElasticSearch across two datacenters for convenient data replication? **URL:** \ **Category:** Elasticsearch\ **Created:** [September 17, 2011, 9:31am UTC](https://discuss.elastic.co/t/should-i-use-elasticsearch-across-two-datacenters-for-convenient-data-replication/5399 "2011-09-17T09:31:00Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![AEvar\_Arnfjord\_Bjarm](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/aevar_arnfjord_bjarm/32/2746_2.png) [@AEvar\_Arnfjord\_Bjarm](https://discuss.elastic.co/u/AEvar_Arnfjord_Bjarm)\ **Post date:** [September 17, 2011, 9:31am UTC](https://discuss.elastic.co/t/should-i-use-elasticsearch-across-two-datacenters-for-convenient-data-replication/5399/1 "2011-09-17T09:31:00Z")
I currently have 3x ElasticSearch nodes in a single cluster in a single datacenter. They have 1x active index at any time (there's an alias that I move around) which is miniscule by most standards, it has around 4 million documents and is ~1GB on disk and is updated daily. There's 1 shard for the index and the replica configuration is such that there's always a complete copy of the index on every machine, and when I make queries I use "preference: local" to ask ElasticSearch not to forward the query to other machines. I'm about to add 3x more machines in a second datacenter, and I'm wondering whether to set them all up in one cluster or have two clusters. The reasons I'd set it up in one cluster: - Easy to monitor the entire installation with one instance of ElasticSearch Head / other monitoring tools. - Easier to maintain it, I just have to run one cron script per day that talks to one cluster, although running it against N clusters would also be trivial. - I don't really care about the slowness of the inter-DC link. It doesn't really matter if it takes 10 minutes or 1 hour to populate the index. While it's being populated I have yesterday's index around serving requests. Reasons not to do it: - Perhaps even with my setup of having a complete copy of the index on each machine and using "preference: local" there will be some situations (e.g. a resource exhaustion on one node) that'll cause an ElasticSearch node in one datacenter to forward requests it can't handle to a node in another datacenter, at which point the inter-DC link _would_ become a very painful bottleneck. - If I ever start using indexes that need to be more synchronous I'd have to change the configuration to use two clusters. I'll probably just set it up as two clusters because it's easy to run two cronjobs & to have the chance to start using more synchronous indexes. But I thought I'd post this here in case it generates some interesting discussion, in particular I'd be interested in more details about when ES nodes decide to forward requests to other nodes. ---
**Author:** ![ppearcy](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/ppearcy/32/980_2.png) [@ppearcy](https://discuss.elastic.co/u/ppearcy)\ **Post date:** [September 20, 2011, 10:05pm UTC](https://discuss.elastic.co/t/should-i-use-elasticsearch-across-two-datacenters-for-convenient-data-replication/5399/2 "2011-09-20T22:05:55Z")
Hey, From my experience, I'd strongly discourage this. The biggest pain point I've had with ES is network partitions within a datacenter causing things to blow up. The 0.16 release has improved that greatly, but I've still had some issues (I'm hoping 0.17 addresses these). Across DCs, you're much more susceptible to partitions. There are some newer features that might mitigate most of the issues, eg minimum number of masters, the quorum support, but I haven't started using these. The approach I'd recommend for multiple data centers is to have elasticsearch sit on top of a cross-DC data store (eg, couchdb) and use the datastore for cross DC replication. Hope this helps. Best Regards, Paul On Sep 17, 3:31 am, Ævar Arnfjörð Bjarmason [ava...@gmail.com](mailto:ava...@gmail.com) wrote: > I currently have 3x Elasticsearch nodes in a single cluster in a > single datacenter. > > They have 1x active index at any time (there's an alias that I move > around) which is miniscule by most standards, it has around 4 million > documents and is ~1GB on disk and is updated daily. > > There's 1 shard for the index and the replica configuration is such > that there's always a complete copy of the index on every machine, and > when I make queries I use "preference: local" to ask Elasticsearch not > to forward the query to other machines. > > I'm about to add 3x more machines in a second datacenter, and I'm > wondering whether to set them all up in one cluster or havetwo > clusters. > > The reasons I'd set it up in one cluster: > > - Easy to monitor the entire installation with one instance of > Elasticsearch Head / other monitoring tools. > > - Easier to maintain it, I just have to run one cron script per day > that talks to one cluster, although running it against N clusters > would also be trivial. > > - I don't really care about the slowness of the inter-DC link. It > doesn't really matter if it takes 10 minutes or 1 hour to populate > the index. While it's being populated I have yesterday's index > around serving requests. > > Reasons not to do it: > > - Perhaps even with my setup of having a complete copy of the index > on each machine and using "preference: local" there will be some > situations (e.g. a resource exhaustion on one node) that'll cause > an Elasticsearch node in one datacenter to forward requests it > can't handle to a node in another datacenter, at which point the > inter-DC link _would_ become a very painful bottleneck. > > - If I ever start using indexes that need to be more synchronous I'd > have to change the configuration to usetwoclusters. > > I'll probably just set it up astwoclusters because it's easy to runtwocronjobs & to have the chance to start using more synchronous > indexes. > > But I thought I'd post this here in case it generates some interesting > discussion, in particular I'd be interested in more details about when > ES nodes decide to forward requests to other nodes. ---
**Author:** ![system](https://us1.discourse-cdn.com/elastic/original/3X/1/a/1ac57faf039f6b580b3f104ef42a2a89e41014de.png) [@system](https://discuss.elastic.co/u/system)\ **Post date:** [July 6, 2017, 3:54am UTC](https://discuss.elastic.co/t/should-i-use-elasticsearch-across-two-datacenters-for-convenient-data-replication/5399/3 "2017-07-06T03:54:00Z")