Querying a Named Cache

The ScaleOut StateServer service allows you to query a named cache for Protocol Buffer objects whose indexed fields satisfy a specified filter. The NamedCache::query method returns a smart pointer to a query result object (boost::shared_ptr<sosscli::QueryResult>), which in turn contains a vector of keys to matching objects. 

#include "soss_client/named_protobuf_cache.h"
#include "soss_client/reference_comparand.h"  // 1
#include "soss_client/value_comparand.h"
#include "StockQuote.pb.h"

using namespace NativeClientSample;

int main(int argc, char* argv[])
{
    sosscli::NamedProtobufCache<StockQuote> quote_cache("Stock Quotes");

    // Create two StockQuote protobuf objects and insert them into the StateServer service:
    auto goog_quote = boost::make_shared<StockQuote>();
    goog_quote->set_ticker("GOOG");
    goog_quote->set_price(1054.48f);
    goog_quote->set_volume(1373206);
    quote_cache.put("GOOG", goog_quote);

    auto msft_quote = boost::make_shared<StockQuote>();
    msft_quote->set_ticker("MSFT");
    msft_quote->set_price(38.45f);
    msft_quote->set_volume(42927723);
    quote_cache.put("MSFT", msft_quote);

    // Query for stocks whose price is less than $100 and whose volume
    // is greater than 10 million shares traded:
    auto query_result = quote_cache.query(sosscli::ReferenceComparand("price")  < 100.0f  &&  // 2
                                          sosscli::ReferenceComparand("volume") > 10000000);

    // Print query results:
    for (sosscli::SossKey key : query_result->keys())
    {
        // Retrieve the object from the named cache so we can get its ticker symbol:
        auto get_result = quote_cache.get(key);
        auto retrieved_quote = get_result.object_ptr(); // a boost::shared_ptr
        std::cout << retrieved_quote->ticker() << std::endl;
    }

    return 0;
}

// Output:
// MSFT

1

reference_comparand.h and value_comparand.h must be included to allow construction of query filter expressions.

2

The ReferenceComparand class is used to create a filter for a query—pass field names from your protobuf classes into the ReferenceComparand’s constructor when building up a filter expression for your query. Note that filters can be composed using intuitive operators, which have been overloaded in the ReferenceComparand and Filter classes.

The NamedCache::query method that’s illustrated in the example above accepts a sosscli::Filter object as a parameter. Filter objects are not intended to be created directly—instead, operator overloads from comparison expressions act as factories for creating a Filter objects. Those filters may then be logically combined with other filter clauses using overloaded logical operators (&&, ||) to create a single Filter object that is sent to the StateServer service. These operator overloads allow multiple search predicates to be sent to the StateServer service using straightforward C++ syntax.

[Note] Note

Query operations are very resource-intensive compared to normal key-based operations: queries involve every host in the distributed data grid, and all of the hosts must coordinate to merge the results before returning to the caller. Favor key-based operations whenever possible, as they will run faster by orders of magnitude.