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Solr is an open-source search server based on the Lucene Java search library. You can host the opensourced code yourself, on EC2 or use a service such as WebSolr or SolrHQ.
Specs
Pros
Pro Customizablity
A key differentiator of Solr is the level of customizability the SearchComponent feature provides.
SearchComponent provides the developer astonishing flexibility in the way search queries are assembled and executed. At the time of writing, there does not appear to be a ElasticSearch equivalent of SearchComponent. source
Whilst ElasticSearch has a number of plugin-points there doesn't appear to be an equivalent of Solr's SearchComponent that enables you to modify the workflow of existing API endpoints.
Pro Local params
Solr has a great feature that enables you to use LocalParams to perform more advanced faceting. They provide a way to "localize" information about a specific argument that is being sent to Solr. In other words, LocalParams provide a way to add meta-data to certain argument types such as query strings. From the Solr Wiki:
LocalParams are expressed as prefixes to arguments to be sent to Solr. For example:
Assume we have the existing query parameter
q=solr rocks
We can prefix this query string with LocalParams to provide more information to the query parser, for example changing the default operator type to "AND" and the default field to "title" for the lucene query parser:
q={!q.op=AND df=title}solr rocks
Cons
Con General missing features
Solr is currently missing the following general features:
- Per-doc/query analyzer chain
- Support for nested documents
- Support for multiple document types per schema
- Ability to modify document scores with custom scripts
- Equivalent to Elasticsearch's percolation
Con Missing some useful features for cloud distribution
Solr is currently missing the following features that are useful when managing a distributed system:
- Automatic shard rebalancing
- Ability to re-locate shards and replicas on demand
- Ability to change the schema without restarting the server
- Ability to search across multiple indexes.