RE: ZooKeeper ensemble. Size and Impact ?
ZooKeeper 3.5.1-alpha version will be released soon. Hopefully by this(July) month, release candidate vote is progressing. -Rakesh -Original Message- From: Srinivasan Veerapandian [mailto:srinivasan.veerapand...@ericsson.com] Sent: 14 July 2015 10:15 To: Rakesh; user@zookeeper.apache.org Cc: Alexander Shraer Subject: RE: ZooKeeper ensemble. Size and Impact ? Thanks to all of you guys. I will try it out. By any chance does anyone know when would the final/stable version of 3.5.0 be available. Because I can see it's in 3.5.0-alpha. Thanks, Srini -Original Message- From: Rakesh [mailto:rakeshr.apa...@gmail.com] Sent: Tuesday, July 14, 2015 10:12 AM To: user@zookeeper.apache.org; Srinivasan Veerapandian Cc: Alexander Shraer Subject: Re: ZooKeeper ensemble. Size and Impact ? Thank you Alex for the info. Hi Srini, I think Observer would be fine for your case. Probably you can try it out. -Rakesh On 13 Jul 2015 21:41, Alexander Shraer shra...@gmail.com wrote: In 3.4 releases you can't connect an observer to a standalone zookeeper server, but in 3.5.0 if you set standaloneEnabled=false your server will run in a distributed mode even if its the only one and you'll be able to have observers or reconfigure adding more servers later if needed. On Mon, Jul 13, 2015 at 5:34 AM, Rakesh R rake...@huawei.com wrote: Is it so that only ensemble would be down but other functions would be up and running like data-sync ... ? Say, if a ZooKeeper server lost connection with the quorum. It will shutdown all the services and try to join the quorum by starting internal election algo. There is a special type of read-only server, on connection lost, it will automatically transition to r-o mode and serve only the requests from r-o client. Please visit http://zookeeper.apache.org/doc/trunk/zookeeperAdmin.html for more details about r-o feature. My need to run only 2 ZKS as I'm ok with have +1 copy of the data. Is there a way to run a dummy ZKS in any of the instance ? There is a typical 'Observer' server mode which will act as an observer and only syncup data with the Leader server, but I'm not really sure whether it will work along with Standalone server. I haven't tried yet, probably you can do a try http://zookeeper.apache.org/doc/r3.5.0-alpha/zookeeperObservers.html To begin with, you can run both as Participant and later if you want to change servers you can use reconfig feature, http://zookeeper.apache.org/doc/trunk/zookeeperReconfig.html In 1+1 deployment, tolerated failure is 0 and you should ensure both servers are up running for the availability of ZooKeeper service. I could see one advantage of this approach is, you have a backup 'dataDir'. Administrator can use this if one is lost. -Rakesh -Original Message- From: Srinivasan Veerapandian [mailto:srinivasan.veerapand...@ericsson.com ] Sent: 13 July 2015 15:01 To: user@zookeeper.apache.org Subject: RE: ZooKeeper ensemble. Size and Impact ? Rakesh Garry, Thanks for the information and details. From both of your responses I can see that, more failures will cause drop of quorum automatically. Is it so that only ensemble would be down but other functions would be up and running like data-sync ... ? Sorry If this is very basic question. I see a note below note, does this means we can form ensemble with leaderServes turned ON. Turning on leader selection is highly recommended when you have more than three ZooKeeper servers in an ensemble. http://zookeeper.apache.org/doc/r3.3.2/zookeeperAdmin.html My need to run only 2 ZKS as I'm ok with have +1 copy of the data. Is there a way to run a dummy ZKS in any of the instance ? Thanks, Srini -Original Message- From: Rakesh R [mailto:rake...@huawei.com] Sent: Monday, July 13, 2015 1:43 PM To: user@zookeeper.apache.org; Srinivasan Veerapandian Subject: RE: ZooKeeper ensemble. Size and Impact ? Hi Srini, ZooKeeper service will be available if 'quorum' number of servers are running(simple majority voting factors). I could see, one of the reason to get a majority vote is to avoid split-brain problem. In a network failure we don't want the two parts of the system to continue as usual. We need only one part to continue and the other to understand that it is out of the cluster and keep quiet. The main reason for suggesting odd number is, with even there won't get much benefit to the tolerated failures in terms of majority. With 3 and 4 servers, we could see the majority is 2 and 3. But in both the cases, the tolerated number of failure is 1. Quorum = Leader + Followers, (2n+1) nodes can tolerate failure of 'n' nodes. For example, n=0, (2*0+1) - 1 server = standalone. Here there is no quorum majority
RE: ZooKeeper ensemble. Size and Impact ?
Is it so that only ensemble would be down but other functions would be up and running like data-sync ... ? Say, if a ZooKeeper server lost connection with the quorum. It will shutdown all the services and try to join the quorum by starting internal election algo. There is a special type of read-only server, on connection lost, it will automatically transition to r-o mode and serve only the requests from r-o client. Please visit http://zookeeper.apache.org/doc/trunk/zookeeperAdmin.html for more details about r-o feature. My need to run only 2 ZKS as I'm ok with have +1 copy of the data. Is there a way to run a dummy ZKS in any of the instance ? There is a typical 'Observer' server mode which will act as an observer and only syncup data with the Leader server, but I'm not really sure whether it will work along with Standalone server. I haven't tried yet, probably you can do a try http://zookeeper.apache.org/doc/r3.5.0-alpha/zookeeperObservers.html To begin with, you can run both as Participant and later if you want to change servers you can use reconfig feature, http://zookeeper.apache.org/doc/trunk/zookeeperReconfig.html In 1+1 deployment, tolerated failure is 0 and you should ensure both servers are up running for the availability of ZooKeeper service. I could see one advantage of this approach is, you have a backup 'dataDir'. Administrator can use this if one is lost. -Rakesh -Original Message- From: Srinivasan Veerapandian [mailto:srinivasan.veerapand...@ericsson.com] Sent: 13 July 2015 15:01 To: user@zookeeper.apache.org Subject: RE: ZooKeeper ensemble. Size and Impact ? Rakesh Garry, Thanks for the information and details. From both of your responses I can see that, more failures will cause drop of quorum automatically. Is it so that only ensemble would be down but other functions would be up and running like data-sync ... ? Sorry If this is very basic question. I see a note below note, does this means we can form ensemble with leaderServes turned ON. Turning on leader selection is highly recommended when you have more than three ZooKeeper servers in an ensemble. http://zookeeper.apache.org/doc/r3.3.2/zookeeperAdmin.html My need to run only 2 ZKS as I'm ok with have +1 copy of the data. Is there a way to run a dummy ZKS in any of the instance ? Thanks, Srini -Original Message- From: Rakesh R [mailto:rake...@huawei.com] Sent: Monday, July 13, 2015 1:43 PM To: user@zookeeper.apache.org; Srinivasan Veerapandian Subject: RE: ZooKeeper ensemble. Size and Impact ? Hi Srini, ZooKeeper service will be available if 'quorum' number of servers are running(simple majority voting factors). I could see, one of the reason to get a majority vote is to avoid split-brain problem. In a network failure we don't want the two parts of the system to continue as usual. We need only one part to continue and the other to understand that it is out of the cluster and keep quiet. The main reason for suggesting odd number is, with even there won't get much benefit to the tolerated failures in terms of majority. With 3 and 4 servers, we could see the majority is 2 and 3. But in both the cases, the tolerated number of failure is 1. Quorum = Leader + Followers, (2n+1) nodes can tolerate failure of 'n' nodes. For example, n=0, (2*0+1) - 1 server = standalone. Here there is no quorum majority. - 2 servers = majority is 2. So it needs min 2 servers to form quorum. Tolerated failure is 0, if 0 failure will drop quorum automatically. n=1, (2*1+1) - 3 servers = majority is 2. So it needs min 2 servers to form quorum. Tolerated failure is 1, if 1 failures will drop quorum automatically. - 4 servers = majority is 3. So it needs min 3 servers to form quorum. Tolerated failure is 1, if 1 failures will drop quorum automatically. n=2, (2*2+1) - 5 servers = majority is 3. So it needs min 3 servers to form quorum. Tolerated failure is 2, if 2 failures will drop quorum automatically. - 6 servers = majority is 4. So it needs min 4 servers to form quorum. Tolerated failure is 2, if 2 failures will drop quorum automatically. n=3, (2*3+1) - 7 servers = majority is 4. So it needs min 4 servers to form quorum. Tolerated failure is 3, if 3 failures will drop quorum automatically. - 8 servers = majority is 5. So it needs min 5 servers to form quorum. Tolerated failure is 3, if 3 failures will drop quorum automatically. -Rakesh -Original Message- From: Srinivasan Veerapandian [mailto:srinivasan.veerapand...@ericsson.com] Sent: 13 July 2015 11:48 To: user@zookeeper.apache.orgmailto:user@zookeeper.apache.org Subject: ZooKeeper ensemble. Size and Impact ? Hi, We know ZK demands odd number of servers to provide reliability. My requirement on having zookeeper in my application is to know the application status from all the clients(Max 100). And today my application can
RE: ZooKeeper ensemble. Size and Impact ?
Rakesh Garry, Thanks for the information and details. From both of your responses I can see that, more failures will cause drop of quorum automatically. Is it so that only ensemble would be down but other functions would be up and running like data-sync ... ? Sorry If this is very basic question. I see a note below note, does this means we can form ensemble with leaderServes turned ON. Turning on leader selection is highly recommended when you have more than three ZooKeeper servers in an ensemble. http://zookeeper.apache.org/doc/r3.3.2/zookeeperAdmin.html My need to run only 2 ZKS as I'm ok with have +1 copy of the data. Is there a way to run a dummy ZKS in any of the instance ? Thanks, Srini -Original Message- From: Rakesh R [mailto:rake...@huawei.com] Sent: Monday, July 13, 2015 1:43 PM To: user@zookeeper.apache.org; Srinivasan Veerapandian Subject: RE: ZooKeeper ensemble. Size and Impact ? Hi Srini, ZooKeeper service will be available if 'quorum' number of servers are running(simple majority voting factors). I could see, one of the reason to get a majority vote is to avoid split-brain problem. In a network failure we don't want the two parts of the system to continue as usual. We need only one part to continue and the other to understand that it is out of the cluster and keep quiet. The main reason for suggesting odd number is, with even there won't get much benefit to the tolerated failures in terms of majority. With 3 and 4 servers, we could see the majority is 2 and 3. But in both the cases, the tolerated number of failure is 1. Quorum = Leader + Followers, (2n+1) nodes can tolerate failure of 'n' nodes. For example, n=0, (2*0+1) - 1 server = standalone. Here there is no quorum majority. - 2 servers = majority is 2. So it needs min 2 servers to form quorum. Tolerated failure is 0, if 0 failure will drop quorum automatically. n=1, (2*1+1) - 3 servers = majority is 2. So it needs min 2 servers to form quorum. Tolerated failure is 1, if 1 failures will drop quorum automatically. - 4 servers = majority is 3. So it needs min 3 servers to form quorum. Tolerated failure is 1, if 1 failures will drop quorum automatically. n=2, (2*2+1) - 5 servers = majority is 3. So it needs min 3 servers to form quorum. Tolerated failure is 2, if 2 failures will drop quorum automatically. - 6 servers = majority is 4. So it needs min 4 servers to form quorum. Tolerated failure is 2, if 2 failures will drop quorum automatically. n=3, (2*3+1) - 7 servers = majority is 4. So it needs min 4 servers to form quorum. Tolerated failure is 3, if 3 failures will drop quorum automatically. - 8 servers = majority is 5. So it needs min 5 servers to form quorum. Tolerated failure is 3, if 3 failures will drop quorum automatically. -Rakesh -Original Message- From: Srinivasan Veerapandian [mailto:srinivasan.veerapand...@ericsson.com] Sent: 13 July 2015 11:48 To: user@zookeeper.apache.orgmailto:user@zookeeper.apache.org Subject: ZooKeeper ensemble. Size and Impact ? Hi, We know ZK demands odd number of servers to provide reliability. My requirement on having zookeeper in my application is to know the application status from all the clients(Max 100). And today my application can support deployment 1+1(=2) to N+1(=100) Given this I would like to go with 2 ZK servers in two different instances because adding one more server for this purpose would be demand one more instance itself in my 1+1 deployment model. Questions: 1. What would happen to ensemble formed ? Would the service goes down automatically ? 2. What would be the impact if number ZK server instances are even (E.g. 2) How do I size a Zoo Keeper ensemble (cluster)? https://cwiki.apache.org/confluence/display/ZOOKEEPER/FAQ Designing a Zoo Keeper Deployment http://zookeeper.apache.org/doc/r3.1.2/zookeeperAdmin.html Thanks, Srini
Re: ZooKeeper ensemble. Size and Impact ?
Thank you Alex for the info. Hi Srini, I think Observer would be fine for your case. Probably you can try it out. -Rakesh On 13 Jul 2015 21:41, Alexander Shraer shra...@gmail.com wrote: In 3.4 releases you can't connect an observer to a standalone zookeeper server, but in 3.5.0 if you set standaloneEnabled=false your server will run in a distributed mode even if its the only one and you'll be able to have observers or reconfigure adding more servers later if needed. On Mon, Jul 13, 2015 at 5:34 AM, Rakesh R rake...@huawei.com wrote: Is it so that only ensemble would be down but other functions would be up and running like data-sync ... ? Say, if a ZooKeeper server lost connection with the quorum. It will shutdown all the services and try to join the quorum by starting internal election algo. There is a special type of read-only server, on connection lost, it will automatically transition to r-o mode and serve only the requests from r-o client. Please visit http://zookeeper.apache.org/doc/trunk/zookeeperAdmin.html for more details about r-o feature. My need to run only 2 ZKS as I'm ok with have +1 copy of the data. Is there a way to run a dummy ZKS in any of the instance ? There is a typical 'Observer' server mode which will act as an observer and only syncup data with the Leader server, but I'm not really sure whether it will work along with Standalone server. I haven't tried yet, probably you can do a try http://zookeeper.apache.org/doc/r3.5.0-alpha/zookeeperObservers.html To begin with, you can run both as Participant and later if you want to change servers you can use reconfig feature, http://zookeeper.apache.org/doc/trunk/zookeeperReconfig.html In 1+1 deployment, tolerated failure is 0 and you should ensure both servers are up running for the availability of ZooKeeper service. I could see one advantage of this approach is, you have a backup 'dataDir'. Administrator can use this if one is lost. -Rakesh -Original Message- From: Srinivasan Veerapandian [mailto:srinivasan.veerapand...@ericsson.com ] Sent: 13 July 2015 15:01 To: user@zookeeper.apache.org Subject: RE: ZooKeeper ensemble. Size and Impact ? Rakesh Garry, Thanks for the information and details. From both of your responses I can see that, more failures will cause drop of quorum automatically. Is it so that only ensemble would be down but other functions would be up and running like data-sync ... ? Sorry If this is very basic question. I see a note below note, does this means we can form ensemble with leaderServes turned ON. Turning on leader selection is highly recommended when you have more than three ZooKeeper servers in an ensemble. http://zookeeper.apache.org/doc/r3.3.2/zookeeperAdmin.html My need to run only 2 ZKS as I'm ok with have +1 copy of the data. Is there a way to run a dummy ZKS in any of the instance ? Thanks, Srini -Original Message- From: Rakesh R [mailto:rake...@huawei.com] Sent: Monday, July 13, 2015 1:43 PM To: user@zookeeper.apache.org; Srinivasan Veerapandian Subject: RE: ZooKeeper ensemble. Size and Impact ? Hi Srini, ZooKeeper service will be available if 'quorum' number of servers are running(simple majority voting factors). I could see, one of the reason to get a majority vote is to avoid split-brain problem. In a network failure we don't want the two parts of the system to continue as usual. We need only one part to continue and the other to understand that it is out of the cluster and keep quiet. The main reason for suggesting odd number is, with even there won't get much benefit to the tolerated failures in terms of majority. With 3 and 4 servers, we could see the majority is 2 and 3. But in both the cases, the tolerated number of failure is 1. Quorum = Leader + Followers, (2n+1) nodes can tolerate failure of 'n' nodes. For example, n=0, (2*0+1) - 1 server = standalone. Here there is no quorum majority. - 2 servers = majority is 2. So it needs min 2 servers to form quorum. Tolerated failure is 0, if 0 failure will drop quorum automatically. n=1, (2*1+1) - 3 servers = majority is 2. So it needs min 2 servers to form quorum. Tolerated failure is 1, if 1 failures will drop quorum automatically. - 4 servers = majority is 3. So it needs min 3 servers to form quorum. Tolerated failure is 1, if 1 failures will drop quorum automatically. n=2, (2*2+1) - 5 servers = majority is 3. So it needs min 3 servers to form quorum. Tolerated failure is 2, if 2 failures will drop quorum automatically. - 6 servers = majority is 4. So it needs min 4 servers to form quorum. Tolerated
RE: ZooKeeper ensemble. Size and Impact ?
Thanks to all of you guys. I will try it out. By any chance does anyone know when would the final/stable version of 3.5.0 be available. Because I can see it's in 3.5.0-alpha. Thanks, Srini -Original Message- From: Rakesh [mailto:rakeshr.apa...@gmail.com] Sent: Tuesday, July 14, 2015 10:12 AM To: user@zookeeper.apache.org; Srinivasan Veerapandian Cc: Alexander Shraer Subject: Re: ZooKeeper ensemble. Size and Impact ? Thank you Alex for the info. Hi Srini, I think Observer would be fine for your case. Probably you can try it out. -Rakesh On 13 Jul 2015 21:41, Alexander Shraer shra...@gmail.com wrote: In 3.4 releases you can't connect an observer to a standalone zookeeper server, but in 3.5.0 if you set standaloneEnabled=false your server will run in a distributed mode even if its the only one and you'll be able to have observers or reconfigure adding more servers later if needed. On Mon, Jul 13, 2015 at 5:34 AM, Rakesh R rake...@huawei.com wrote: Is it so that only ensemble would be down but other functions would be up and running like data-sync ... ? Say, if a ZooKeeper server lost connection with the quorum. It will shutdown all the services and try to join the quorum by starting internal election algo. There is a special type of read-only server, on connection lost, it will automatically transition to r-o mode and serve only the requests from r-o client. Please visit http://zookeeper.apache.org/doc/trunk/zookeeperAdmin.html for more details about r-o feature. My need to run only 2 ZKS as I'm ok with have +1 copy of the data. Is there a way to run a dummy ZKS in any of the instance ? There is a typical 'Observer' server mode which will act as an observer and only syncup data with the Leader server, but I'm not really sure whether it will work along with Standalone server. I haven't tried yet, probably you can do a try http://zookeeper.apache.org/doc/r3.5.0-alpha/zookeeperObservers.html To begin with, you can run both as Participant and later if you want to change servers you can use reconfig feature, http://zookeeper.apache.org/doc/trunk/zookeeperReconfig.html In 1+1 deployment, tolerated failure is 0 and you should ensure both servers are up running for the availability of ZooKeeper service. I could see one advantage of this approach is, you have a backup 'dataDir'. Administrator can use this if one is lost. -Rakesh -Original Message- From: Srinivasan Veerapandian [mailto:srinivasan.veerapand...@ericsson.com ] Sent: 13 July 2015 15:01 To: user@zookeeper.apache.org Subject: RE: ZooKeeper ensemble. Size and Impact ? Rakesh Garry, Thanks for the information and details. From both of your responses I can see that, more failures will cause drop of quorum automatically. Is it so that only ensemble would be down but other functions would be up and running like data-sync ... ? Sorry If this is very basic question. I see a note below note, does this means we can form ensemble with leaderServes turned ON. Turning on leader selection is highly recommended when you have more than three ZooKeeper servers in an ensemble. http://zookeeper.apache.org/doc/r3.3.2/zookeeperAdmin.html My need to run only 2 ZKS as I'm ok with have +1 copy of the data. Is there a way to run a dummy ZKS in any of the instance ? Thanks, Srini -Original Message- From: Rakesh R [mailto:rake...@huawei.com] Sent: Monday, July 13, 2015 1:43 PM To: user@zookeeper.apache.org; Srinivasan Veerapandian Subject: RE: ZooKeeper ensemble. Size and Impact ? Hi Srini, ZooKeeper service will be available if 'quorum' number of servers are running(simple majority voting factors). I could see, one of the reason to get a majority vote is to avoid split-brain problem. In a network failure we don't want the two parts of the system to continue as usual. We need only one part to continue and the other to understand that it is out of the cluster and keep quiet. The main reason for suggesting odd number is, with even there won't get much benefit to the tolerated failures in terms of majority. With 3 and 4 servers, we could see the majority is 2 and 3. But in both the cases, the tolerated number of failure is 1. Quorum = Leader + Followers, (2n+1) nodes can tolerate failure of 'n' nodes. For example, n=0, (2*0+1) - 1 server = standalone. Here there is no quorum majority. - 2 servers = majority is 2. So it needs min 2 servers to form quorum. Tolerated failure is 0, if 0 failure will drop quorum automatically. n=1, (2*1+1) - 3 servers = majority is 2. So it needs min 2 servers to form quorum. Tolerated failure is 1, if 1 failures will drop quorum automatically. - 4 servers
Re: ZooKeeper ensemble. Size and Impact ?
In 3.4 releases you can't connect an observer to a standalone zookeeper server, but in 3.5.0 if you set standaloneEnabled=false your server will run in a distributed mode even if its the only one and you'll be able to have observers or reconfigure adding more servers later if needed. On Mon, Jul 13, 2015 at 5:34 AM, Rakesh R rake...@huawei.com wrote: Is it so that only ensemble would be down but other functions would be up and running like data-sync ... ? Say, if a ZooKeeper server lost connection with the quorum. It will shutdown all the services and try to join the quorum by starting internal election algo. There is a special type of read-only server, on connection lost, it will automatically transition to r-o mode and serve only the requests from r-o client. Please visit http://zookeeper.apache.org/doc/trunk/zookeeperAdmin.html for more details about r-o feature. My need to run only 2 ZKS as I'm ok with have +1 copy of the data. Is there a way to run a dummy ZKS in any of the instance ? There is a typical 'Observer' server mode which will act as an observer and only syncup data with the Leader server, but I'm not really sure whether it will work along with Standalone server. I haven't tried yet, probably you can do a try http://zookeeper.apache.org/doc/r3.5.0-alpha/zookeeperObservers.html To begin with, you can run both as Participant and later if you want to change servers you can use reconfig feature, http://zookeeper.apache.org/doc/trunk/zookeeperReconfig.html In 1+1 deployment, tolerated failure is 0 and you should ensure both servers are up running for the availability of ZooKeeper service. I could see one advantage of this approach is, you have a backup 'dataDir'. Administrator can use this if one is lost. -Rakesh -Original Message- From: Srinivasan Veerapandian [mailto:srinivasan.veerapand...@ericsson.com ] Sent: 13 July 2015 15:01 To: user@zookeeper.apache.org Subject: RE: ZooKeeper ensemble. Size and Impact ? Rakesh Garry, Thanks for the information and details. From both of your responses I can see that, more failures will cause drop of quorum automatically. Is it so that only ensemble would be down but other functions would be up and running like data-sync ... ? Sorry If this is very basic question. I see a note below note, does this means we can form ensemble with leaderServes turned ON. Turning on leader selection is highly recommended when you have more than three ZooKeeper servers in an ensemble. http://zookeeper.apache.org/doc/r3.3.2/zookeeperAdmin.html My need to run only 2 ZKS as I'm ok with have +1 copy of the data. Is there a way to run a dummy ZKS in any of the instance ? Thanks, Srini -Original Message- From: Rakesh R [mailto:rake...@huawei.com] Sent: Monday, July 13, 2015 1:43 PM To: user@zookeeper.apache.org; Srinivasan Veerapandian Subject: RE: ZooKeeper ensemble. Size and Impact ? Hi Srini, ZooKeeper service will be available if 'quorum' number of servers are running(simple majority voting factors). I could see, one of the reason to get a majority vote is to avoid split-brain problem. In a network failure we don't want the two parts of the system to continue as usual. We need only one part to continue and the other to understand that it is out of the cluster and keep quiet. The main reason for suggesting odd number is, with even there won't get much benefit to the tolerated failures in terms of majority. With 3 and 4 servers, we could see the majority is 2 and 3. But in both the cases, the tolerated number of failure is 1. Quorum = Leader + Followers, (2n+1) nodes can tolerate failure of 'n' nodes. For example, n=0, (2*0+1) - 1 server = standalone. Here there is no quorum majority. - 2 servers = majority is 2. So it needs min 2 servers to form quorum. Tolerated failure is 0, if 0 failure will drop quorum automatically. n=1, (2*1+1) - 3 servers = majority is 2. So it needs min 2 servers to form quorum. Tolerated failure is 1, if 1 failures will drop quorum automatically. - 4 servers = majority is 3. So it needs min 3 servers to form quorum. Tolerated failure is 1, if 1 failures will drop quorum automatically. n=2, (2*2+1) - 5 servers = majority is 3. So it needs min 3 servers to form quorum. Tolerated failure is 2, if 2 failures will drop quorum automatically. - 6 servers = majority is 4. So it needs min 4 servers to form quorum. Tolerated failure is 2, if 2 failures will drop quorum automatically. n=3, (2*3+1) - 7 servers = majority is 4. So it needs min 4 servers to form quorum. Tolerated failure is 3, if 3 failures will drop quorum automatically. - 8 servers = majority is 5. So it needs min 5 servers to form quorum. Tolerated failure is 3, if 3 failures will drop quorum automatically. -Rakesh -Original Message- From: Srinivasan Veerapandian
RE: ZooKeeper ensemble. Size and Impact ?
Hi, I claim no ZK expertise but did have a similar scenario so I'll share my conclusion and then the ZK experts can point out how wrong I am. :) The odd number of servers requirement is because a ZK consensus can only be formed by an *absolute* majority of nodes in the ensemble. With an even number of servers fault-tolerance isn't as expected. A cluster of 4 nodes can only survive 1 server loss before the ensemble goes down. Which is the same as an ensemble of 3. The added server in the group of 4 gives no additional fault tolerance in terms of the number of live nodes. An ensemble of 2 does though seem to be a special case in that it has a somewhat unintuitive quality; since it cannot survive the loss of either server (1 is *not* an absolute majority of 2) then it seems that the availability of a 2-node ensemble is worse than a single node setup. If either node in the 2-node ensemble is lost then the ensemble will go down. That's why when faced with a similar scenario to your own -- where 3 hosts was overkill/not available for the use case -- I went with a single node. Fortunately it was not a production system... Garry -Original Message- From: Srinivasan Veerapandian [mailto:srinivasan.veerapand...@ericsson.com] Sent: 13 July 2015 07:18 To: user@zookeeper.apache.org Subject: ZooKeeper ensemble. Size and Impact ? Hi, We know ZK demands odd number of servers to provide reliability. My requirement on having zookeeper in my application is to know the application status from all the clients(Max 100). And today my application can support deployment 1+1(=2) to N+1(=100) Given this I would like to go with 2 ZK servers in two different instances because adding one more server for this purpose would be demand one more instance itself in my 1+1 deployment model. Questions: 1. What would happen to ensemble formed ? Would the service goes down automatically ? 2. What would be the impact if number ZK server instances are even (E.g. 2) How do I size a Zoo Keeper ensemble (cluster)? https://cwiki.apache.org/confluence/display/ZOOKEEPER/FAQ Designing a Zoo Keeper Deployment http://zookeeper.apache.org/doc/r3.1.2/zookeeperAdmin.html Thanks, Srini - No virus found in this message. Checked by AVG - www.avg.com Version: 2014.0.4821 / Virus Database: 4365/10205 - Release Date: 07/11/15
RE: ZooKeeper ensemble. Size and Impact ?
Hi Srini, ZooKeeper service will be available if 'quorum' number of servers are running(simple majority voting factors). I could see, one of the reason to get a majority vote is to avoid split-brain problem. In a network failure we don't want the two parts of the system to continue as usual. We need only one part to continue and the other to understand that it is out of the cluster and keep quiet. The main reason for suggesting odd number is, with even there won't get much benefit to the tolerated failures in terms of majority. With 3 and 4 servers, we could see the majority is 2 and 3. But in both the cases, the tolerated number of failure is 1. Quorum = Leader + Followers, (2n+1) nodes can tolerate failure of 'n' nodes. For example, n=0, (2*0+1) - 1 server = standalone. Here there is no quorum majority. - 2 servers = majority is 2. So it needs min 2 servers to form quorum. Tolerated failure is 0, if 0 failure will drop quorum automatically. n=1, (2*1+1) - 3 servers = majority is 2. So it needs min 2 servers to form quorum. Tolerated failure is 1, if 1 failures will drop quorum automatically. - 4 servers = majority is 3. So it needs min 3 servers to form quorum. Tolerated failure is 1, if 1 failures will drop quorum automatically. n=2, (2*2+1) - 5 servers = majority is 3. So it needs min 3 servers to form quorum. Tolerated failure is 2, if 2 failures will drop quorum automatically. - 6 servers = majority is 4. So it needs min 4 servers to form quorum. Tolerated failure is 2, if 2 failures will drop quorum automatically. n=3, (2*3+1) - 7 servers = majority is 4. So it needs min 4 servers to form quorum. Tolerated failure is 3, if 3 failures will drop quorum automatically. - 8 servers = majority is 5. So it needs min 5 servers to form quorum. Tolerated failure is 3, if 3 failures will drop quorum automatically. -Rakesh -Original Message- From: Srinivasan Veerapandian [mailto:srinivasan.veerapand...@ericsson.com] Sent: 13 July 2015 11:48 To: user@zookeeper.apache.org Subject: ZooKeeper ensemble. Size and Impact ? Hi, We know ZK demands odd number of servers to provide reliability. My requirement on having zookeeper in my application is to know the application status from all the clients(Max 100). And today my application can support deployment 1+1(=2) to N+1(=100) Given this I would like to go with 2 ZK servers in two different instances because adding one more server for this purpose would be demand one more instance itself in my 1+1 deployment model. Questions: 1. What would happen to ensemble formed ? Would the service goes down automatically ? 2. What would be the impact if number ZK server instances are even (E.g. 2) How do I size a Zoo Keeper ensemble (cluster)? https://cwiki.apache.org/confluence/display/ZOOKEEPER/FAQ Designing a Zoo Keeper Deployment http://zookeeper.apache.org/doc/r3.1.2/zookeeperAdmin.html Thanks, Srini
