SimpleStrategy doesn’t take DC or rack into account at all. It simply places
replicas on subsequent tokens. You could end up with 3 copies in 1 DC and zero
in another.
/**
* This class returns the nodes responsible for a given
* key but does not respect rack awareness. Basically
* returns the RF nodes that lie right next to each other
* on the ring.
*/
public List<InetAddress> calculateNaturalEndpoints(Token token, TokenMetadata
metadata)
{
int replicas = getReplicationFactor();
ArrayList<Token> tokens = metadata.sortedTokens();
List<InetAddress> endpoints = new ArrayList<InetAddress>(replicas);
if (tokens.isEmpty())
return endpoints;
// Add the token at the index by default
Iterator<Token> iter = TokenMetadata.ringIterator(tokens, token, false);
while (endpoints.size() < replicas && iter.hasNext())
{
InetAddress ep = metadata.getEndpoint(iter.next());
if (!endpoints.contains(ep))
endpoints.add(ep);
}
return endpoints;
}
NTS keeps track of the replicas per-DC. The code is here, but it’s a bit
longer so I’ll just link to it.
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/locator/NetworkTopologyStrategy.java#L146
<https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/locator/NetworkTopologyStrategy.java#L146>
Jon
> On Jul 21, 2017, at 11:31 AM, Eric Stevens <migh...@gmail.com> wrote:
>
> > If using the SimpleStrategy replication class, it appears that
> > replication_factor is the only option, which applies to the entire
> > cluster, so only one node in both datacenters would have the data.
>
> This runs counter to my understanding, or else I'm not reading your statement
> correctly. When a user chooses SimpleStrategy they're saying that the same
> replication applies to _each DC_ in your cluster, not that _all DC's_
> contribute to the total replication.
>
> Put another way, my understanding is that if you have SimpleStrategy RF=1
> with two data centers, you have two copies of each piece of data - one in
> each DC.
>
> On Thu, Jul 20, 2017 at 2:56 PM Michael Shuler <mich...@pbandjelly.org
> <mailto:mich...@pbandjelly.org>> wrote:
> Datacenter replication is defined in the keyspace schema, so I believe that
> ...
> WITH replication = {'class': 'NetworkTopologyStrategy', 'DC1': 1,
> 'DC2': 1}
> ...
> you ought to be able to repair DC1 from DC2, once you have the DC1 node
> healthy again.
>
> If using the SimpleStrategy replication class, it appears that
> replication_factor is the only option, which applies to the entire
> cluster, so only one node in both datacenters would have the data.
>
> https://cassandra.apache.org/doc/latest/cql/ddl.html#create-keyspace
> <https://cassandra.apache.org/doc/latest/cql/ddl.html#create-keyspace>
>
> --
> Kind regards,
> Michael
>
> On 07/20/2017 03:23 PM, Roger Warner wrote:
> > Hi
> >
> >
> >
> > I’m a little dim on what multi datacenter implies in the 1 replica
> > case. I know about replica recovery, how about “node recovery”
> >
> >
> >
> > As I understand if there a node failure or disk crash with a single node
> > cluster with replication factor 1 I lose data. Easy.
> >
> >
> >
> > nodetool tells me each node in my 3 node X 2 datacenters is responsible
> > for ~1/3 of the data. If in this cluster with RF=1 a node fails in
> > dc1 what happens ? in 1 dc with data loss can the node be “restored”
> > from a node in dc2?. Automatically?
> >
> >
> >
> > I’m also asking tangentially how does the data map from nodes in dc1 to
> > dc2.
> >
> >
> >
> > I hope I made that coherent.
> >
> >
> >
> > Roger
> >
>
>
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