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https://issues.apache.org/jira/browse/CASSANDRA-18084?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17642823#comment-17642823
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Josh McKenzie commented on CASSANDRA-18084:
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bq. So factors related to topology/ownership/placement of data would move out
of gossip, but I don't see any real impediment to building this feature in the
proposed new system.
So my take here is that Sam's saying "Go for it, consider making it more
generic so we can use it for more things, and we'll forklift this into CEP-21
as we work on it". I agree with him for the record - attaching arbitrary
metadata to nodes seems like a really useful general purpose feature that could
lead to some interesting experimentation and innovation in the community. Sorry
if my comment in any way came across as "wait for CEP-21"; my intention was to
communicate "This sounds really interesting and powerful and like there's
perhaps a bigger set of things a generalized version of this enables we should
consider rather than coupling it with a specific subsystem".
bq. too bad major releases are so far apart, time-wise.
While 4.0 -> 4.1 is going to be about 18 months, we're moving in the right
direction and collectively decided a "fastest path floor" of 12 months was
about all most users could stomach since certifying major releases is a big
lift. Curious to hear if you have a different opinion on that front or are more
thinking of the delay leading up to 4.0.
> 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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