Distributed Requests

Limiting Which Shards are Queried

While one of the advantages of using SolrCloud is the ability to query very large collections distributed across various shards, in some cases you may have configured Solr so you know you are only interested in results from a specific subset of shards. You have the option of searching over all of your data or just parts of it.

A query across all shards for a collection is simply a query that does not define a shards parameter:

http://localhost:8983/solr/gettingstarted/select?q=*:*

If you want to search just one shard, use the shards parameter to specify the shard by its logical ID, as in:

http://localhost:8983/solr/gettingstarted/select?q=*:*&shards=shard1

If you want to search a group of shards, you can specify each shard separated by a comma in one request:

http://localhost:8983/solr/gettingstarted/select?q=*:*&shards=shard1,shard2

In both of the above examples, while only the specific shards are queried, any random replica of the shard will get the request.

Alternatively, you can specify a list of replicas you wish to use in place of a shard IDs by separating the replica IDs with commas:

http://localhost:8983/solr/gettingstarted/select?q=*:*&shards=localhost:7574/solr/gettingstarted,localhost:8983/solr/gettingstarted

Or you can specify a list of replicas to choose from for a single shard (for load balancing purposes) by using the pipe symbol (|) between different replica IDs:

http://localhost:8983/solr/gettingstarted/select?q=*:*&shards=localhost:7574/solr/gettingstarted|localhost:7500/solr/gettingstarted

Finally, you can specify a list of shards (separated by commas) each defined by a list of replicas (seperated by pipes).

In the following example, 2 shards are queried, the first being a random replica from shard1, the second being a random replica from the explicit pipe delimited list:

http://localhost:8983/solr/gettingstarted/select?q=*:*&shards=shard1,localhost:7574/solr/gettingstarted|localhost:7500/solr/gettingstarted

Configuring the ShardHandlerFactory

For finer-grained control, you can directly configure and tune aspects of the concurrency and thread-pooling used within distributed search in Solr. The default configuration favors throughput over latency.

This is done by defining a shardHandler in the configuration for your search handler.

To add a shardHandler to the standard search handler, provide a configuration in solrconfig.xml, as in this example:

<requestHandler name="/select" class="solr.SearchHandler">
  <!-- other params go here -->
  <shardHandler class="HttpShardHandlerFactory">
    <int name="socketTimeOut">1000</int>
    <int name="connTimeOut">5000</int>
  </shardHandler>
</requestHandler>

HttpShardHandlerFactory is the only ShardHandlerFactory implementation included out of the box with Solr, It accepts the following parameters:

Configuring statsCache (Distributed IDF)

Document and term statistics are needed in order to calculate relevancy. Solr provides four implementations out of the box when it comes to document stats calculation:

The implementation can be selected by setting <statsCache> in solrconfig.xml. For example, the following line makes Solr use the ExactStatsCache implementation:

<statsCache class="org.apache.solr.search.stats.ExactStatsCache"/>

Avoiding Distributed Deadlock

Each shard serves top-level query requests and then makes sub-requests to all of the other shards. Care should be taken to ensure that the max number of threads serving HTTP requests is greater than the possible number of requests from both top-level clients and other shards. If this is not the case, the configuration may result in a distributed deadlock.

For example, a deadlock might occur in the case of two shards, each with just a single thread to service HTTP requests. Both threads could receive a top-level request concurrently, and make sub-requests to each other. Because there are no more remaining threads to service requests, the incoming requests will be blocked until the other pending requests are finished, but they will not finish since they are waiting for the sub-requests. By ensuring that Solr is configured to handle a sufficient number of threads, you can avoid deadlock situations like this.

preferLocalShards Parameter

Deprecated, use shards.preference=replica.location:local instead (see below).

shards.preference Parameter

Solr allows you to pass an optional string parameter named shards.preference to indicate that a distributed query should sort the available replicas in the given order of precedence within each shard.

The syntax is: shards.preference=property:value. The order of the properties and the values are significant: the first one is the primary sort, the second is secondary, etc.

The properties that can be specified are as follows:

Examples:

Note that if you provide the settings in a query string, they need to be properly URL-encoded.