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What is Aggregation?

Aggregation - noun

"The process of combining things or amounts into a single group or total"

In a software context Aggregation can have a variety of interpretations, in Gaffer this specifically refers to ingest or query aggregation.

Why Aggregate?

Aggregation allows us to take a set of elements and group them and their properties together to form a new result. This allows us to get quick insights into our data and generate valuable outputs from our graphing queries.

There are also some key benefits when using aggregation with sketches as we can store aggregated data in a compact format to reduce storage requirements and improve throughput. Further reading on sketches is available in the reference guide.

How is Aggregation Applied?

The application of aggregation can be done at either data ingest time or added to a specific query.

For ingest time the configuration is specified via the graph schema so that as data is loaded into a graph it is aggregated and stored in that state. To demonstrate what this would look like take the simple graph below where we can apply aggregation to merge the multiple edges together by summing the properties.

If we don't apply ingest aggregation all elements will be stored separately in graph.

flowchart LR
    A(A)
    --
    "Edge
    prop: 1"
    --> B(B)
    A(A)
    --
    "Edge
    prop: 1"
    --> B(B)

Now if we have apply aggregation to sum the same properties on the Edge they now get stored together as one element.

flowchart LR
    A(A)
    --
    "Edge
    prop: 2"
    --> B(B)

Tip

Other aggregation functions are available, for an in-depth guide into ingest aggregation please see the administration guide.

The other place you can apply aggregation is at query time. As a user this is the most common use case, typically an administrator will have set up the ingest time aggregation to summarise the input data into a more manageable size and saving disk space in the process. Users can then use query time aggregation to further summarise the data without having to edit the graph schema.

The main difference between query-time aggregation and ingest aggregation is the aggregation applied on a query will only affect the elements in that query so the overall graph data is left intact. There is more information on how to apply query-time aggregation in the querying guide on filtering.