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From Sergio <>
Subject Re: Cassandra Rack - Datacenter Load Balancing relations
Date Wed, 23 Oct 2019 19:33:39 GMT
Hi Reid,

Thank you very much for clearing these concepts for me. I posted this
question on the datastax forum regarding our cluster that it is unbalanced
and the reply was related that the *number of racks should be a multiplier
of the replication factor *in order to be balanced or 1. I thought then if
I have 3 availability zones I should have 3 racks for each datacenter and
not 2 (us-east-1b, us-east-1a) as I have right now or in the easiest way, I
should have a rack for each datacenter.

   1. Datacenter: live
   |/ State=Normal/Leaving/Joining/Moving
   --  Address      Load       Tokens       Owns    Host ID
   UN   289.75 GiB  256          ?
   be5a0193-56e7-4d42-8cc8-5d2141ab4872  us-east-1a
   UN  103.03 GiB  256          ?
   e5108a8e-cc2f-4914-a86e-fccf770e3f0f  us-east-1b
   UN  129.61 GiB  256          ?
   3c2efdda-8dd4-4f08-b991-9aff062a5388  us-east-1a
   UN  145.28 GiB  256          ?
   0a8f07ba-a129-42b0-b73a-df649bd076ef  us-east-1b
   UN  149.04 GiB  256          ?
   71563e86-b2ae-4d2c-91c5-49aa08386f67  us-east-1a
   DN  52.41 GiB  256          ?
   613b43c0-0688-4b86-994c-dc772b6fb8d2  us-east-1b
   UN   195.17 GiB  256          ?
   3647fcca-688a-4851-ab15-df36819910f4  us-east-1b
   UN  100.67 GiB  256          ?
   f43532ad-7d2e-4480-a9ce-2529b47f823d  us-east-1b
   So each rack label right now matches the availability zone and we have 3
   Datacenters and 2 Availability Zone with 2 racks per DC but the above is
   clearly unbalanced
   If I have a keyspace with a replication factor = 3 and I want to
   minimize the number of nodes to scale up and down the cluster and keep it
   balanced should I consider an approach like OPTION A)
   2. Node DC RACK AZ 1 read ONE us-east-1a 2 read ONE us-east-1a
   3. 3 read ONE us-east-1a
   4. 4 write ONE us-east-1b 5 write ONE us-east-1b
   5. 6 write ONE us-east-1b
   6. OPTION B)
   7. Node DC RACK AZ 1 read ONE us-east-1a 2 read ONE us-east-1a
   8. 3 read ONE us-east-1a
   9. 4 write TWO us-east-1b 5 write TWO us-east-1b
   10. 6 write TWO us-east-1b
   11. *7 read ONE us-east-1c 8 write TWO us-east-1c*
   12. *9 read ONE us-east-1c* Option B looks to be unbalanced and I would
   exclude it OPTION C)
   13. Node DC RACK AZ 1 read ONE us-east-1a 2 read ONE us-east-1b
   14. 3 read ONE us-east-1c
   15. 4 write TWO us-east-1a 5 write TWO us-east-1b
   16. 6 write TWO us-east-1c

   so I am thinking of A if I have the restriction of 2 AZ but I guess that
   option C would be the best. If I have to add another DC for reads because
   we want to assign a new DC for each new microservice it would look like:
      OPTION EXTRA DC For Reads
      1. Node DC RACK AZ 1 read ONE us-east-1a 2 read ONE us-east-1b
      2. 3 read ONE us-east-1c
      3. 4 write TWO us-east-1a 5 write TWO us-east-1b
      4. 6 write TWO us-east-1c 7 extra-read THREE us-east-1a
      5. 8 extra-read THREE us-east-1b

   1. 9 extra-read THREE us-east-1c
   The DC for *write* will replicate the data in the other datacenters. My
   scope is to keep the *read* machines dedicated to serve reads and *write*
   machines to serve writes. Cassandra will handle the replication for me. Is
   there any other option that is I missing or wrong assumption? I am thinking
   that I will write a blog post about all my learnings so far, thank you very
   much for the replies Best, Sergio

Il giorno mer 23 ott 2019 alle ore 10:57 Reid Pinchback <> ha scritto:

> No, that’s not correct.  The point of racks is to help you distribute the
> replicas, not further-replicate the replicas.  Data centers are what do the
> latter.  So for example, if you wanted to be able to ensure that you always
> had quorum if an AZ went down, then you could have two DCs where one was in
> each AZ, and use one rack in each DC.  In your situation I think I’d be
> more tempted to consider that.  Then if an AZ went away, you could fail
> over your traffic to the remaining DC and still be perfectly fine.
> For background on replicas vs racks, I believe the information you want is
> under the heading ‘NetworkTopologyStrategy’ at:
> That should help you better understand how replicas distribute.
> As mentioned before, while you can choose to do the reads in one DC,
> except for concerns about contention related to network traffic and
> connection handling, you can’t isolate reads from writes.  You can _
> *mostly*_ insulate the write DC from the activity within the read DC, and
> even that isn’t an absolute because of repairs.  However, your mileage may
> vary, so do what makes sense for your usage pattern.
> R
> *From: *Sergio <>
> *Reply-To: *"" <>
> *Date: *Wednesday, October 23, 2019 at 12:50 PM
> *To: *"" <>
> *Subject: *Re: Cassandra Rack - Datacenter Load Balancing relations
> *Message from External Sender*
> Hi Reid,
> Thanks for your reply. I really appreciate your explanation.
> We are in AWS and we are using right now 2 Availability Zone and not 3. We
> found our cluster really unbalanced because the keyspace has a replication
> factor = 3 and the number of racks is 2 with 2 datacenters.
> We want the writes spread across all the nodes but we wanted the reads
> isolated from the writes to keep the load on that node low and to be able
> to identify problems in the consumers (reads) or producers (writes)
> applications.
> It looks like that each rack contains an entire copy of the data so this
> would lead to replicate for each rack and then for each node the
> information. If I am correct if we have  a keyspace with 100GB and
> Replication Factor = 3 and RACKS = 3 => 100 * 3 * 3 = 900GB
> If I had only one rack across 2 or even 3 availability zone I would save
> in space and I would have 300GB only. Please correct me if I am wrong.
> Best,
> Sergio
> Il giorno mer 23 ott 2019 alle ore 09:21 Reid Pinchback <
>> ha scritto:
> Datacenters and racks are different concepts.  While they don't have to be
> associated with their historical meanings, the historical meanings probably
> provide a helpful model for understanding what you want from them.
> When companies own their own physical servers and have them housed
> somewhere, the questions arise on where you want to locate any particular
> server.  It's a balancing act on things like network speed of related
> servers being able to talk to each other, versus fault-tolerance of having
> many servers not all exposed to the same risks.
> "Same rack" in that physical world tended to mean something like "all
> behind the same network switch and all sharing the same power bus".  The
> morning after an electrical glitch fries a power bus and thus everything in
> that rack, you realize you wished you didn't have so many of the same type
> of server together.  Well, they were servers.  Now they are door stops.
> Badness and sadness.
> That's kind of the mindset to have in mind with racks in Cassandra.  It's
> an artifact for you to separate servers into pools so that the disparate
> pools have hopefully somewhat independent infrastructure risks.  However,
> all those servers are still doing the same kind of work, are the same
> version, etc.
> Datacenters are amalgams of those racks, and how similar or different they
> are from each other depends on what you want to do with them.  What is true
> is that if you have N datacenters, each one of them must have enough disk
> storage to house all the data.  The actual physical footprint of that data
> in each DC depends on the replication factors in play.
> Note that you sorta can't have "one datacenter for writes" because the
> writes will replicate across the data centers.  You could definitely choose
> to have only one that takes read queries, but best to think of writing as
> being universal.  One scenario you can have is where the DC not taking live
> traffic read queries is the one you use for maintenance or performance
> testing or version upgrades.
> One rack makes your life easier if you don't have a reason for multiple
> racks. It depends on the environment you deploy into and your fault
> tolerance goals.  If you were in AWS and wanting to spread risk across
> availability zones, then you would likely have as many racks as AZs you
> choose to be in, because that's really the point of using multiple AZs.
> R
> On 10/23/19, 4:06 AM, "Sergio Bilello" <> wrote:
>      Message from External Sender
>     Hello guys!
>     I was reading about
>     I would like to understand a concept related to the node load
> balancing.
>     I know that Jon recommends Vnodes = 4 but right now I found a cluster
> with vnodes = 256 replication factor = 3 and 2 racks. This is unbalanced
> because the racks are not a multiplier of the replication factor.
>     However, my plan is to move all the nodes in a single rack to
> eventually scale up and down the node in the cluster once at the time.
>     If I had 3 racks and I would like to keep the things balanced I should
> scale up 3 nodes at the time one for each rack.
>     If I would have 3 racks, should I have also 3 different datacenters so
> one datacenter for each rack?
>     Can I have 2 datacenters and 3 racks? If this is possible one
> datacenter would have more nodes than the others? Could it be a problem?
>     I am thinking to split my cluster in one datacenter for reads and one
> for writes and keep all the nodes in the same rack so I can scale up once
> node at the time.
>     Please correct me if I am wrong
>     Thanks,
>     Sergio
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