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https://issues.apache.org/jira/browse/CASSANDRA-18084?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17642639#comment-17642639
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Stefan Miklosovic commented on CASSANDRA-18084:
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You are 100% correct that if we ever going to do this, lets just do that
"properly". But on the other hand, if there is already on-going effort to do
that more robustly (whatever it means), I think that me doing anything which
would cover what you just wrote feels like duplication of the effort if it is
eventually going to be replaced by something else.
I think that it is, strategically, better if I ask you politely to think about
this while doing that CEP rather than trying to implement some solution which
might be eventually deprecated.
I agree that this just scratches our itch, too bad major releases are so far
apart, time-wise. It is always better to have something we depend on in
upstream rather than plumb something internally.
> Introduce tags to snitch for better decision making for replica placement in
> topology strategies
> ------------------------------------------------------------------------------------------------
>
> Key: CASSANDRA-18084
> URL: https://issues.apache.org/jira/browse/CASSANDRA-18084
> Project: Cassandra
> Issue Type: New Feature
> Components: Cluster/Gossip, Cluster/Schema, Legacy/Distributed
> Metadata
> Reporter: Stefan Miklosovic
> Assignee: Stefan Miklosovic
> Priority: Normal
>
> We would like to have extra meta-information in cassandra-rackdc.properties
> which would further differentiate nodes in dc / racks.
> The main motivation behind this is that we have special requirements around
> node's characteristics based on which we want to make further decisions when
> it comes to replica placement in topology strategies. (New topology strategy
> would be mostly derived from NTS / would be extended)
> The most reasonable way to do that is to introduce a new property into
> cassandra-rackdc.properties called "tags"
> {code:java}
> # Arbitrary tag to assign to this node, they serve as additional
> identificator of a node based on which operators might act.
> # Value of this property is meant to be a comma-separated list of strings.
> #tags=tag1,tag2
> {code}
> We also want to introduce new application state called TAGS. On startup of a
> node, this node would advertise its tags to cluster and vice versa, all nodes
> would tell to that respective node what tags they have so everybody would see
> the same state of tags across the cluster based on which topology strategies
> would do same decisions.
> These tags are not meant to be changed during whole runtime of a node,
> similarly as datacenter and rack is not.
> For performance reasons, we might limit the maximum size of tags (sum of
> their lenght), to be, for example, 64 characters and anything bigger would be
> either shortened or the start would fail.
> Once we have tags for all nodes, we can have access to them, cluster-wide,
> from TokenMetadata which is used quite heavily in topology strategies and it
> exposes other relevant topology information (dc's, racks ...). We would just
> add a new way to look at nodes.
> Tags would be a set.
> This would be persisted to the system.local to see what tags a local node has
> and it would be persisted to system.peers_v2 to see what tags all other nodes
> have. Column would be called "tags".
> {code:java}
> admin@cqlsh> select * from system.local ;
> @ Row 1
> key | local
> bootstrapped | COMPLETED
> broadcast_address | 172.19.0.5
> broadcast_port | 7000
> cluster_name | Test Cluster
> cql_version | 3.4.6
> data_center | dc1
> gossip_generation | 1669739177
> host_id | 54f8c6ea-a6ba-40c5-8fa5-484b2b4184c9
> listen_address | 172.19.0.5
> listen_port | 7000
> native_protocol_version | 5
> partitioner | org.apache.cassandra.dht.Murmur3Partitioner
> rack | rack1
> release_version | 4.2-SNAPSHOT
> rpc_address | 172.19.0.5
> rpc_port | 9042
> schema_version | ef865449-2491-33b8-95b0-47c09cb14ea9
> tags | {'tag1', 'tag2'}
> tokens | {'6504358681601109713'}
> {code}
> for system.peers_v2:
> {code:java}
> admin@cqlsh> select peer,tags from system.peers_v2 ;
> @ Row 1
> ------+-----------------
> peer | 172.19.0.15
> tags | {'tag2', 'tag3'}
> @ Row 2
> ------+-----------------
> peer | 172.19.0.11
> tags | null
> {code}
> the POC implementation doing exactly that is here:
> We do not want to provide our custom topology strategies which are using this
> feature as that will be the most probably a proprietary solution. This might
> indeed change in the future. For now, we just want to implement hooks we can
> base our in-house implementation on. All other people can benefit from this
> as well if they choose so as this feature enables them to do that.
> Adding tags is not only about custom topology strategies. Operators could tag
> their nodes if they wish to make further distinctions on topology level for
> their operational needs.
> [https://github.com/instaclustr/cassandra/commit/eddd4681d8678515454dabb926d5f56b0c225eea]
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