Apache Solr Reference Guide Covering Apache Solr 6.0

Limitations to Distributed Search
Distributed searching in Solr has the following limitations:
Each document indexed must have a unique key.
If Solr discovers duplicate document IDs, Solr selects the first document and discards subsequent ones.
The index for distributed searching may become momentarily out of sync if a commit happens between
the first and second phase of the distributed search. This might cause a situation where a document that
once matched a query and was subsequently changed may no longer match the query but will still be
retrieved. This situation is expected to be quite rare, however, and is only possible for a single query
request.
The number of shards is limited by number of characters allowed for GET method's URI; most Web
servers generally support at least 4000 characters, but many servers limit URI length to reduce their
vulnerability to Denial of Service (DoS) attacks.
Shard information can be returned with each document in a distributed search by including fl=i
d, [shard] in the search request. This returns the shard URL.
In a distributed search, the data directory from the core descriptor overrides any data directory in solrco
nfig.xml.
Update commands may be sent to any server with distributed indexing configured correctly. Document
adds and deletes are forwarded to the appropriate server/shard based on a hash of the unique document
id. commit commands and deleteByQuery commands are sent to every server in shards.
Formerly a limitation was that TF/IDF relevancy computations only used shard-local statistics. This is still the
case by default. If your data isn't randomly distributed, or if you want more exact statistics, then remember to
configure the ExactStatsCache.
Avoiding Distributed Deadlock
Like in SolrCloud mode, inter-shard requests could lead to a distributed deadlock. It can be avoided by following
the instructions here.
Testing Index Sharding on Two Local Servers
For simple functionality testing, it's easiest to just set up two local Solr servers on different ports. (In a production
environment, of course, these servers would be deployed on separate machines.)
1.
Make two Solr home directories:
mkdir example/nodes
mkdir example/nodes/node1
# Copy solr.xml into this solr.home
cp server/solr/solr.xml example/nodes/node1/.
# Repeat the above steps for the second node
mkdir example/nodes/node2
cp server/solr/solr.xml example/nodes/node2/.
2.
Start the two Solr instances
# Start first node on port 8983
bin/solr start -s example/nodes/node1 -p 8983
# Start second node on port 8984
bin/solr start -s example/nodes/node2 -p 8984
3.
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