This is a good explanation and makes sense. The one inconsistency is referring to a replica of a shard that has no replication. But its not that big of a problem. If you wove the term 'core' into your writeup below it would be complete and should be posted on the wiki.
Sent from my Verizon Wireless 4G LTE Smartphone -------- Original message -------- From: Jack Krupansky <j...@basetechnology.com> Date: To: solr-user@lucene.apache.org Subject: Re: Terminology question: Core vs. Collection vs... Replication makes perfect sense even if our explanations so far do not. A shard is an abstraction of a subset of the data for a collection. A replica is an instance of the data of the shard and instances of Solr servers that have indicated a readiness to service queries and updates for the data. Alternatively, a replica is a node which has indicated a readiness to receive and serve the data of a shard, but may not have any data at the moment. Lets describe it operationally for SolrCloud: If data comes in to any replica of a shard it will automatically and quickly be "replicated" to all other replicas of the shard. If a new replica of a shard comes up it will be streamed all of the data from the another replica of the shard. If an existing replica of a shard restarts or reconnects to the cluster, it will be streamed updates of any new data since it was last updated from another replica of the shard. Replication is simply the process of assuring that all replicas are kept up to date. That's the same abstract meaning as for Master/Slave even though the operational details are somewhat different. The goal remains the same. Replication factor is the number of instances of the data of the shard and instances of Solr servers that can service queries and updates for the data. Alternatively, the replication factor is the number of nodes of the SolrCloud cluster which have indicated a readiness to receive and serve the data of a shard, but may not have any data at the moment. A node is an instance of Solr running in a Java JVM that has indicated to the Zookeeper ensemble of a SolrCloud cluster that it is ready to be a replica for a shard of a collection. [The latter part of that is a bit too fuzzy - I'm not sure what the node tells Zookeeper and who does shard assignment. I mean, does a node explicitly say what shard it wants to be, or is that assigned by Zookeeper, or is that a node's choice/option? But none of that changes the fact that a node "registers" with Zookeeper and then somehow becomes a replica for a shard.] A node (instance of a Solr server) can be a replica of shards from multiple collections (potentially multiple shards per collection). A node is not a replica per se, but a container that can serve multiple collections. A node can serve as multiple replicas, each of a different collection. My only interest here on this user list is to understand and explain the terms we have today and that SEEM to be working for the most part, even though we may not have defined them carefully enough and used them consistently enough. If somebody want to propose an alternative terminology - fine, discuss that on the dev list and/or file a Jira. I won't claim that my definitions are perfect (yet), but perfecting the definitions (for users) should be separated from changing the terms themselves. -- Jack Krupansky -----Original Message----- From: Per Steffensen Sent: Friday, January 04, 2013 2:49 AM To: solr-user@lucene.apache.org Subject: Re: Terminology question: Core vs. Collection vs... On 1/3/13 5:58 PM, Walter Underwood wrote: > A "factor" is multiplied, so multiplying the leader by a replicationFactor > of 1 means you have exactly one copy of that shard. > > I think that recycling the term "replication" within Solr was confusing, > but it is a bit late to change that. > > wunder Yes, the term "factor" is not misleading, but the term "replication" is. If we keep calling shard-instances for "Replica" I guess "replicaFactor" will be ok - at least much better than "replicationFactor". But it would still be better with e.g. "ShardInstance" and "InstancesPerShard"
