An aggregation summarizes your data as metrics, statistics, or other analytics.

A simple aggregationedit

In the example below we run an aggregation that creates a price histogram from a product index, for the products whose name match a user-provided text. To achieve this, we use a search request that has a query (explained in Searching for documents) and an aggregation definition.

This example is an analytics-type aggregation where we do not want to use the matching documents. A general pattern for search requests used for analytics is to set the result size to zero and the target class for search results to Void.

If that same aggregation was used for to display products and the price histogram as drill-down facets, we would have set size to a non-zero value and used Product as the target class to process the results.

String searchText = "bike";

Query query = MatchQuery.of(m -> m
    .field("name")
    .query(searchText)
)._toQuery();

SearchResponse<Void> response = esClient.search(b -> b
    .index("products")
    .size(0) 
    .query(query) 
    .aggregations("price-histogram", a -> a 
        .histogram(h -> h 
            .field("price")
            .interval(50.0)
        )
    ),
    Void.class 
);

The response contains an aggregation result for each aggregation in the request.

List<HistogramBucket> buckets = response.aggregations()
    .get("price-histogram") 
    .histogram() 
    .buckets().array(); 

for (HistogramBucket bucket: buckets) {
    logger.info("There are " + bucket.docCount() +
        " bikes under " + bucket.key());
}

The source code for the examples above can be found in the Java API Client tests.