There is always a replication factor, but it could be 1 - meaning there is
only a single replica of the data for a shard. You can't have a replication
factor of 0 - that would mean the data does not exist.
Don't confuse the old pre-SolrCloud master/slave use of replica. There is no
"replication" process per se in SolrCloud as there was with master/slave -
the data is fully distributed.
There is no concept of "a shard's core", except when the replication factor
is 1. Otherwise, a shard consists of multiple cores, each a replica of the
shard and each a "copy" of the logical subset of the collection's data. Each
replica is a copy of the other replicas for that shard.
-- Jack Krupansky
-----Original Message-----
From: Darren Govoni
Sent: Thursday, January 03, 2013 9:17 AM
To: solr-user@lucene.apache.org
Subject: RE: Re: Terminology question: Core vs. Collection vs...
I think what's confusing about your explanation below is when you have a
situation where there is no replication factor. That's possible too, yes?
So in that case, is each core of a shard of a collection, still referred to
as a replica?
To me a replica is a duplicate/backup of a shard's core. Not the sharded
core itself. Or is there just no difference. Even a non-replicated core is
called a replica?
<br><br><br>------- Original Message -------
On 1/3/2013 09:08 AM Jack Krupansky wrote:<br>Oops... let me word that a
little more carefully:
<br>
<br>...we are "replicating the data of each shard".
<br>
<br>
<br>
<br>
<br>
<br>-- Jack Krupansky
<br>-----Original Message-----
<br>From: Jack Krupansky
<br>Sent: Thursday, January 03, 2013 9:03 AM
<br>To: solr-user@lucene.apache.org
<br>Subject: Re: Terminology question: Core vs. Collection vs...
<br>
<br>No, a shard is a subset (or "slice") of the collection. Sharding is a
way of
<br>"slicing" the original data, before we talk about how the shards get
stored
<br>and replicated on actual Solr cores. Replicas are instances of the data
for
<br>a shard.
<br>
<br>Sometimes people may loosely speak of a replica as being "a shard", but
<br>that's just loose use of the terminology.
<br>
<br>So, we're not "sharding shards", but we are "replicating shards".
<br>
<br>-- Jack Krupansky
<br>
<br>-----Original Message-----
<br>From: Darren Govoni
<br>Sent: Thursday, January 03, 2013 8:51 AM
<br>To: solr-user@lucene.apache.org
<br>Subject: RE: Re: Terminology question: Core vs. Collection vs...
<br>
<br>Thanks again. (And sorry to jump into this convo)
<br>
<br>But I had a question on your statement:
<br>
<br>On 1/3/2013 08:07 AM Jack Krupansky wrote:
<br> <br>Collection is the more modern term and incorporates the fact that
the
<br><br>collection may be sharded, with each shard on one or more cores,
with
<br>each <br>core being a replica of the other cores within that shard of
that
<br><br>collection.
<br>
<br>A collection is sharded, meaning it is distributed across cores. A shard
<br>itself is not distributed across cores in the same since. Rather a shard
<br>exist on a single core and is replicated on other cores. Is that right?
The
<br>way its worded above, it sounds like a shard can also be sharded...
<br>
<br>
<br><br><br><br>------- Original Message -------
<br>On 1/3/2013 08:28 AM Jack Krupansky wrote:<br>A node is a machine in a
<br>cluster or cloud (graph). It could be a real
<br><br>machine or a virtualized machine. Technically, you could have
multiple
<br><br>virtual nodes on the same physical "box". Each Solr replica would be
on
<br>a
<br><br>different node.
<br><br>
<br><br>Technically, you could have multiple Solr instances running on a
single
<br><br>hardware node, each with a different port. They are simply instances
of
<br><br>Solr, although you could consider each Solr instance a node in a
Solr
<br>cloud
<br><br>as well, a "virtual" node. So, technically, you could have multiple
<br>replicas
<br><br>on the same node, but that sort of defeats most of the purpose of
having
<br><br>replicas in the first place - to distribute the data for performance
and
<br><br>fault tolerance. But, you could have replicas of different shards on
the
<br><br>same node/box for a partial improvement of performance and fault
<br>tolerance.
<br><br>
<br><br>A Solr "cloud' is really a cluster.
<br><br>
<br><br>-- Jack Krupansky
<br><br>
<br><br>-----Original Message-----
<br><br>From: Darren Govoni
<br><br>Sent: Thursday, January 03, 2013 8:16 AM
<br><br>To: solr-user@lucene.apache.org
<br><br>Subject: RE: Re: Terminology question: Core vs. Collection vs...
<br><br>
<br><br>Good write up.
<br><br>
<br><br>And what about "node"?
<br><br>
<br><br>I think there needs to be an official glossary of terms that is
<br>sanctioned
<br><br>by the solr team and some terms still ni use may need to be labeled
<br><br>"deprecated". After so many years, its still confusing.
<br><br>
<br><br><br><br><br>------- Original Message -------
<br><br>On 1/3/2013 08:07 AM Jack Krupansky wrote:<br>Collection is the
more
<br>modern
<br><br>term and incorporates the fact that the
<br><br><br>collection may be sharded, with each shard on one or more cores,
<br>with
<br><br>each
<br><br><br>core being a replica of the other cores within that shard of
that
<br><br><br>collection.
<br><br><br>
<br><br><br>Instance is a general term, but is commonly used to refer to a
<br>running
<br><br>Solr
<br><br><br>server, each of which can service any number of cores. A sharded
<br><br>collection
<br><br><br>would typically require multiple instances of Solr, each with a
<br>shard of
<br><br>the
<br><br><br>collection.
<br><br><br>
<br><br><br>Multiple collections can be supported on a single instance of
Solr.
<br>They
<br><br><br>don't have to be sharded or replicated. But if they are, each
Solr
<br><br>instance
<br><br><br>will have a copy or replica of the data (index) of one shard of
each
<br><br>sharded
<br><br><br>collection - to the degree that each collection needs that many
<br>shards.
<br><br><br>
<br><br><br>At the API level, you talk to a Solr instance, using a host and
<br>port,
<br><br>and
<br><br><br>giving the collection name. Some operations will refer only to
the
<br><br>portion
<br><br><br>of a multi-shard collection on that Solr instance, but typically
<br>Solr
<br><br>will
<br><br><br>"distribute" the operation, whether it be an update or a query,
to
<br>all
<br><br>of
<br><br><br>the shards of the named collection. In the case of update, the
<br>update
<br><br>will
<br><br><br>be distributed to all replicas as well, but in the case of query
<br>only
<br><br>one
<br><br><br>replica of each shard of the collection is needed.
<br><br><br>
<br><br><br>Before SolrCloud we Solr had master and slave and the slaves
were
<br><br>replicas
<br><br><br>of the master, but with SolrCloud there is no master and all the
<br><br>replicas of
<br><br><br>the shard are peers, although at any moment of time one of them
will
<br>be
<br><br><br>considered the "leader" for coordination purposes, but not in
the
<br>sense
<br><br>that
<br><br><br>it is a master of the other replicas in that shard. A SolrCloud
<br>replica
<br><br>is a
<br><br><br>replica of the data, in an abstract sense, for a single shard of
a
<br><br><br>collection. A SolrCloud replica is more of an instance of the
<br><br>data/index.
<br><br><br>
<br><br><br>An index exists at two levels: the portion of a collection on a
<br>single
<br><br>Solr
<br><br><br>core will have a Lucene index, but collectively the Lucene
indexes
<br>for
<br><br>the
<br><br><br>shards of a collection can be referred to the index of the
<br>collection.
<br><br>Each
<br><br><br>replica is a copy or instance of a portion of the collection's
<br>index.
<br><br><br>
<br><br><br>The term slice is sometimes used to refer collectively to all of
the
<br><br><br>cores/replicas of a single shard, or sometimes to a single
replica
<br>as it
<br><br><br>contains only a "slice" of the full collection data.
<br><br><br>
<br><br><br>-- Jack Krupansky
<br><br><br>
<br><br><br>-----Original Message-----
<br><br><br>From: Alexandre Rafalovitch
<br><br><br>Sent: Thursday, January 03, 2013 4:42 AM
<br><br><br>To: solr-user@lucene.apache.org
<br><br><br>Subject: Terminology question: Core vs. Collection vs...
<br><br><br>
<br><br><br>Hello,
<br><br><br>
<br><br><br>I am trying to understand the core Solr terminology. I am
looking
<br>for
<br><br><br>correct rather than loose meaning as I am trying to teach an
example
<br><br>that
<br><br><br>starts from easy scenario and may scale to multi-core,
multi-machine
<br><br><br>situation.
<br><br><br>
<br><br><br>Here are the terms that seem to be all overlapping and/or
crossing
<br>over
<br><br>in
<br><br><br>my mind a the moment.
<br><br><br>
<br><br><br>1) Index
<br><br><br>2) Core
<br><br><br>3) Collection
<br><br><br>4) Instance
<br><br><br>5) Replica (Replica of _what_?)
<br><br><br>6) Others?
<br><br><br>
<br><br><br>I tried looking through documentation, but either there is a
<br>terminology
<br><br><br>drift or I am having trouble understanding the distinctions.
<br><br><br>
<br><br><br>If anybody has a clear picture in their mind, I would appreciate
a
<br><br><br>clarification.
<br><br><br>
<br><br><br>Regards,
<br><br><br> Alex.
<br><br><br>
<br><br><br>Personal blog: http://blog.outerthoughts.com/
<br><br><br>LinkedIn: http://www.linkedin.com/in/alexandrerafalovitch
<br><br><br>- Time is the quality of nature that keeps events from happening
all
<br>at
<br><br><br>once. Lately, it doesn't seem to be working. (Anonymous - via
GTD
<br><br>book)
<br><br><br>
<br><br><br>
<br><br>
<br><br>
<br>
<br>
