# High Availability Across AZs Configuration questions **URL:** \ **Category:** Dgraph\ **Tags:** kind:question\ **Created:** [November 30, 2021, 5:05am UTC](https://discuss.dgraph.io/t/high-availability-across-azs-configuration-questions/16187 "2021-11-30T05:05:29Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![jrhayward](https://yyz1.discourse-cdn.com/flex007/user_avatar/discuss.dgraph.io/jrhayward/32/9103_2.png) [@jrhayward](https://discuss.dgraph.io/u/jrhayward)\ **Post date:** [November 30, 2021, 5:05am UTC](https://discuss.dgraph.io/t/high-availability-across-azs-configuration-questions/16187/1 "2021-11-30T05:05:30Z")
First let me apologize for having posted this twice but, I could not figure out how to tag the first one as a question. Continuing the discussion from [Replicas across availability zones](http://discuss.hypermode.com/t/replicas-across-availability-zones/13995): I am planning to implement the same thing, replication across AZs, and looking to prevent having the entire cluster go down when an AZ goes down. I am a little lost in the terminology, most specifically the seemingly interchangeable use of servers, nodes, alphas and zeros. In the second diagram from @nickpoorman’s post if he lost say, AZ-1, would the cluster go down because there is not a majority of alphas to run Group1? Also, when it says the number of replicas is the number of nodes for a group I assume that refers to the number of Alphas and that the number of zeros does not come into play for a quorum? I also assume that the number of Alpha nodes must be a multiple of the number of replicas specified for the failover to work properly? In the raft=‘group=…’ I am assuming that refers to the raft id provided in the zero command so given a zero command of… dgraph zero --my=zero0:5080 --replicas 3 --raft=“idx=0” then the alpha command would be dgraph alpha --my=alpha1:7080 --zero=zero0:5080 --raft=‘group=0’ From which I infer that I need to have at least one zero for each group. May I have 2 zeros per group that I could distribute across AZs equally? as in… AZ-1 dgraph zero --my=zero0:5080 --replicas 3 --raft=“idx=0” AZ-2 dgraph zero --my=zero0:5080 --replicas 3 --raft=“idx=0” So, to keep the cluster up when an AZ goes down I would need replicas=5 and cross at least 3 AZs (ignoring latency issues for now) AZ-1 Alphas Group-1, Group-1, Group-2 Zero0 AZ-2 Alphas Group-1, Group-1, Group-2, Group-2 Zero1 AZ-3 Alphas Group-2, Group-2, Group-1 Zero2 And in that way losing one AZ would never take out a majority of the Alphas in a quorum. Is that correct? And how many zeros should I have and how should I distribute them? ---
**Author:** ![dmai](https://yyz1.discourse-cdn.com/flex007/user_avatar/discuss.dgraph.io/dmai/32/1254_2.png) [@dmai](https://discuss.dgraph.io/u/dmai)\ **Post date:** [November 30, 2021, 5:42am UTC](https://discuss.dgraph.io/t/high-availability-across-azs-configuration-questions/16187/2 "2021-11-30T05:42:33Z")
> [@jrhayward](#): > > In the second diagram from @nickpoorman’s post if he lost say, AZ-1, would the cluster go down because there is not a majority of alphas to run Group1? If the configuration in AZ-1 is Group1,Group1,Group2 and AZ-2 is Group1,Group2,Group2, then AZ-1 has two out of the three nodes in Group 1 (aka the majority) and if AZ-1 went down then group 1 would not be available for linearizable reads and writes (it would still be available for best-effort reads). > [@jrhayward](#): > > Also, when it says the number of replicas is the number of nodes for a group I assume that refers to the number of Alphas and that the number of zeros does not come into play for a quorum? I also assume that the number of Alpha nodes must be a multiple of the number of replicas specified for the failover to work properly? The `--replicas` configuration determines the max replica size per group for Alphas. Zeros are also part of a quorum (e.g., you can run three Zeros in a quorum and the Zero group is available when the majority aka two are up). If `--replicas=3`, then you’d ideally want 3 Alphas per group to maintain a high availability configuration. > [@jrhayward](#): > > So, to keep the cluster up when an AZ goes down I would need replicas=5 and cross at least 3 AZs (ignoring latency issues for now) If you have 3 AZs, then there at least needs to be one replica per AZ to maintain availability in a single AZ going down. So, you only need replicas=3 at the minimum. > [@jrhayward](#): > > Is that correct? And how many zeros should I have and how should I distribute them? Three Zeros for a high availability quorum. One per AZ. > [@jrhayward](#): > > In the raft=‘group=…’ I am assuming that refers to the raft id provided in the zero command so given a zero command of… > > dgraph zero --my=zero0:5080 --replicas 3 --raft=“idx=0” > > then the alpha command would be > > dgraph alpha --my=alpha1:7080 --zero=zero0:5080 --raft=‘group=0’ Zero comprises group 0 (hence the name). Alphas are part of group 1, 2, and so on. So, When configuring the Alpha and you want to specify the group (it’s optional—when a new Alpha connects to Zero it will get auto-assigned a group), you’d specify group 1 or higher (not group=0, that’s not valid). ---
**Author:** ![jrhayward](https://yyz1.discourse-cdn.com/flex007/user_avatar/discuss.dgraph.io/jrhayward/32/9103_2.png) [@jrhayward](https://discuss.dgraph.io/u/jrhayward)\ **Post date:** [November 30, 2021, 5:45am UTC](https://discuss.dgraph.io/t/high-availability-across-azs-configuration-questions/16187/3 "2021-11-30T05:45:41Z")
Thank you