Erick Erickson created SOLR-6512:
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Summary: Add a collections API call to attach arbitrary properties
to a replica
Key: SOLR-6512
URL: https://issues.apache.org/jira/browse/SOLR-6512
Project: Solr
Issue Type: Improvement
Reporter: Erick Erickson
Assignee: Erick Erickson
This is a sub-task for SOLR-6491, but seems generally useful.
Since this is in support of the "preferredLeader" functionality, I've run into
some considerations that I wanted some feedback on how to handle.
"preferredLeader" has the restriction that there should only be one per slice,
so setting this for a particular node means removing the property for all the
other replicas on the slice. Not a problem to do, my question is more whether
this is something reasonable to enforce on an arbitrary property based on what
that property is? Perfectly do-able, but "semantically challenged". Currently,
this is never a node with "preferedLeader" set to "false", it is forcibly
removed from other nodes in the slice when this property is assigned.
The problem here is that there's nothing about assigning an arbitrary property
to a node that would reasonably imply this kind of behavior. One could always
control this with secondary flags on the command, e.g.
"shardExclusive=true|false" for instance, perhaps with safety checks in for
known one-per-shard properties like "preferredLeader".
"preferredLeader" seems to fit more naturally into a "role", but currently
ADDROLE and DELTEROLE have nothing to do with the notion of setting a role for
a particular node relative to a collection/shard. Easy enough to add, but
enforcing the "only one node per slice may have this role" rule there is
similarly arbitrary and overloads the ADDROLE/DELETEROLE in a way that seems
equally confusing. Plus, checking whether the required collection/shard/node
params are present becomes based on the value of the property being set, which
is all equally arbitrary.
The other interesting thing is that setting an arbitrary property on a node
would allow one to mess things up royally by, say, changing properties like
"core", or "base_url" or node_name at will. Actually this is potentially
useful, but very, very dangerous and I'm not particularly interested in
supporting it ;). I suppose we could require a prefix, say the only settable
properties are "property.whatever".
We could also add something specific to nodes, something like
ADDREPLICAROLE/DELETEREPLICAROLE, perhaps with sub-params like
"onlyOneAllowedPerShard", but this gets messy and relies on the users "doing
the right thing". I prefer enforcing rules like this based on the role I
think. Or at least enforcing these kinds of requirements on the
"preferredLeader" role if we go that way.
What are people's thoughts here? I think I'm tending towards the
ADDREPLICAROLE/DELETEREPLICAROLE way of doing this, but it's not set in stone.
I have code locally for arbitrary properties that I can modify for the role
bits.
So, if I'm going to summarize the points I'd like feedback on:
1> Is setting arbitrary properties on a node desirable? If so, should we
require a prefix like "property" to prevent resetting values SolrCloud depends
on?
2> Is it better to piggyback on ADDROLE/DELETEROLE? Personally I'm not in favor
of this one. Too messy with requiring additional parameters to work right in
this case
3> Is the best option to create new collections API calls for
ADDREPLICAROLE/DELETEREPLICAROLE that
3.1> require collection/slice/node parameters
3.2> enforces the "onlyOnePerShard" rule for certain known roles
3.3> allows users to specify something like "onlyOnePerShard" as an optional
T|F parameter
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