World is targeting to decrease the response time, irrespective of the application developed and technology used. Searching is a core functionality being used in eCommerce as well in other application. Keeping the response time and scalability in high priority, Apache Lucene project introduced SOLR SEARCH ENGINE.

Solr is the popular, blazing fast open source enterprise search platform from the Apache Lucene project. Its major features are: powerful full-text search, hit highlighting, faceted search, dynamic clustering, database integration, rich document (e.g., Word, PDF) handling, and geospatial search. Solr is highly scalable, providing distributed search. Solr is written in Java and runs as a standalone full-text search server within a servlet container such as Tomcat. Solr uses the Lucene Java search library at its core for full-text indexing and search, and has REST-like HTTP/XML and JSON APIs that make it easy to use from virtually any programming language. Solr's powerful external configuration allows it to be tailored to almost any type of application without Java coding, and it has extensive plug-in architecture when more advanced customization is required.

Lucene uses 3 to 5 times less memory for when dealing with terms dictionary, so it's even less RAM consuming. So it provides quick search.
Faceted Searching.
Light weight application.
 
 
                        
Schema.xml:     This file is responsible to declare all fields of data feed with their type and scope for search. It first declare the data type which can be used for fields. It defines the field types and fields of documents. It can drive more intelligent processing Dynamic Fields enables on-the-fly addition of new fields in this field. CopyField functionality allows indexing a single field multiple ways, or combining multiple fields into a single searchable field. Explicit types eliminates the need for guessing types of fields. We can also provide customized data type . In that case we are required to create concerned class and need to point that class in the declaration of the data type .
It makes possible to specify that a field is a String, int, float, or other primitive, or a custom type.

It has a feature of  Dynamic field definitions.  If a field name is not found, dynamicFields will be used if the name matches any of the patterns. e.g. name="*_i" will match any field ending in _i (like myid_i, z_i). Longer patterns will be matched first.  If equal size patterns  are found then the first appearing in the schema will be used.
 
 

Solr Faceted Search Implementation:

Faceted search gives the facility to dig our searching upto bottom level. In the respect of user this helps to search from category to sub category and many more.  Each facet displayed also shows the number of hits within the search that match that category. Users can then "drill down" by applying specific contstraints to the search results. Faceted search is also called faceted browsing, faceted navigation, guided navigation and sometimes parametric search.
It's relatively simple to get faceting information from Solr, as there are few prerequisites. Faceting commands are added to any normal Solr query request, and the faceting counts come back in the same query response. Solr offers the following types of faceting, all of which can be requested with no prior configuration:
Field faceting - retrieve the counts for all terms, or just the top terms in any given field. The field must be indexed.
Query faceting - return the number of documents in the current search results that also match the given query.
Date faceting - return the number of documents that fall within certain date ranges.
Solr caches are associated with an Index Searcher ? a particular 'view' of the index that doesn't change. So as long as that Index Searcher is being used, any items in the cache will be valid and available for reuse. Caching in Solr is unlike ordinary caches in that Solr cached objects will not expire after a certain period of time; rather, cached objects will be valid as long as the Index Searcher is valid.
The current Index Searcher serves requests and when a new searcher is opened, the new one is auto-warmed while the current one is still serving external requests. When the new one is ready, it will be registered as the current searcher and will handle any new search requests. The old searcher will be closed after all request it was servicing finish. The current Searcher is used as the source of auto-warming. When a new searcher is opened, its caches may be prepopulated or "autowarmed" using data from caches in the old searcher.
 
Required attributes in caching.
 
Available  SolrCache Class for implementation:
solr.LRUCache
solr.FastLRUCache
solr.LFUCache
 
initialSize
The initial capacity (number of entries) of the cache.
 
autowarmCount
It is responsible to get the searched data from old cache instead of hitting solr.It defines the size or how many data can be cached. When a new searcher is opened, configurable searches are run against it in order to warm it up to avoid slow first hits. During warming, the current searcher handles live requests.When a new searcher is opened, its caches may be prepopulated or "autowarmed" with cached object from caches in the old searcher. autowarmCount is the number of cached items that will be regenerated in the new searcher. You will proably want to base the autowarmCount setting on how long it takes to autowarm. You must consider the trade-off ? time-to-autowarm versus how warm (i.e., autowarmCount) you want the cache to be. The autowarm parameter is set for the caches in solrconfig.xml.
 
This is Cache warming in background
 
   
      class="solr.LRUCache"
      size="512"
      initialSize="512"
      autowarmCount="0"/>
 
Caches configuration is view on static page of solr for a running Solr server.
Url for static page is http://localhost:8983/solr/admin/stats.jsp
 
 
 
Query created to hit Solr engine follows HTTP interface with configurable response formats (XML/XSLT, JSON, Python, Ruby, PHP, Velocity, binary).Sorting can be done by any number of fields, and by complex functions of numeric fields
Highlighted context snippets can be used.
Faceted Searching based on unique field values, explicit queries, date ranges, and numeric ranges is key feature of Solr. Spelling check and auto suggestions are also provided for user queries, in that case it is required to manipulate solrConfig.xml for spelling check.
Queries are creating using simple java code but one can use solr API apache-solr-solrj-1.4.1.jar. Inbuilt class SolrQuery is responsible to create queries. Its different methods can be used to append different parameters in query. The above API can be used for parsing response data also or one can write own code for parsing/handling response data.
Query Example:-
 
Basic Parameters of Solr Query:

1. q: This is the basic parameter in solr query which contains the searched content.
2. Start: This parameter is used to paginate results from a query.
3. Rows: This parameter is used to paginate results from a query.
4. Fq: This parameters contains the condition applied on the search query.
5. Fl: This parameter can be used to specify a set of fields to return, limiting the amount of information in the response.
6. Sort: This parameter says that we need the response in sorted form asc/desc on a particular field basis.
7. Facet: It defines weather facet search is true/false on the particular call.
8. Wt: The value of this parameter defines the format of solr response.