Pinot has many built-in Aggregation Functions such as MIN, MAX, SUM, AVG etc. See PQL page for the list of aggregation functions.
Adding a new AggregationFunction requires two things
Implement AggregationFunction interface and make it available as part of the classpath
Register the function in AggregationFunctionFactory. As of today, this requires code change in Pinot but we plan to add the ability to plugin Functions without having to change Pinot code.
To get an overall idea, see MAX Aggregation Function implementation. All other implementations can be found here.
Lets look at the key methods to implements in AggregationFunction
Before getting into the implementation, it's important to understand how Aggregation works in Pinot.
This is advanced topic and assumes you know Pinot concepts. All the data in Pinot is stored in segments across multiple nodes. The query plan at a high level comprises of 3 phases
1. Map phase
This phase works on the individual segments in Pinot.
Initialization: Depending on the query type the following methods are invoked to set up the result holder. While having different methods and return types adds complexity, it helps in performance.
AGGREGATION : createAggregationResultHolderThis must return an instance of type AggregationResultHolder. You can either use the DoubleAggregationResultHolder or ObjectAggregationResultHolder
GROUP BY: createGroupByResultHolderThis method must return an instance of type GroupByResultHolder. Depending on the type of result object, you might be able to use one of the existing implementations.
Callback: For every record that matches the filter condition in the query,
one of the following methods are invoked depending on the queryType(aggregation vs group by) and columnType(single-value vs multi-value). Note that we invoke this method for a batch of records instead of every row for performance reasons and allows JVM to vectorize some of parts of the execution if possible.
AGGREGATION: aggregate(int length, AggregationResultHolder aggregationResultHolder, Map<String,BlockValSet> blockValSetMap)
length: This represent length of the block. Typically < 10k
aggregationResultHolder: this is the object returned fromcreateAggregationResultHolder
blockValSetMap: Map of blockValSets depending on the arguments to the AggFunction
Group By Single Value: aggregateGroupBySV(int length, int[] groupKeyArray, GroupByResultHolder groupByResultHolder, Map blockValSets)
length: This represent length of the block. Typically < 10k
groupKeyArray: Pinot internally maintains a value to int mapping and this groupKeyArray maps to the internal mapping. These values together form a unique key.
groupByResultHolder: This is the object returned fromcreateGroupByResultHolder
blockValSetMap: Map of blockValSets depending on the arguments to the AggFunction
Group By Multi Value: aggregateGroupBySV(int length, int[] groupKeyArray, GroupByResultHolder groupByResultHolder, Map blockValSets)
length: This represent length of the block. Typically < 10k
groupKeyArray: Pinot internally maintains a value to int mapping and this groupKeyArray maps to the internal mapping. These values together form a unique key.
groupByResultHolder: This is the object returned fromcreateGroupByResultHolder
blockValSetMap: Map of blockValSets depending on the arguments to the AggFunction
2. Combine phase
In this phase, the results from all segments within a single pinot server are combined into IntermediateResult. The type of IntermediateResult is based on the Generic Type defined in the AggregationFunction implementation.
3. Reduce phase
There are two steps in the Reduce Phase
Merge all the IntermediateResult's from various servers using the merge function
Extract the final results by invoking the extractFinalResult method. In most cases, FinalResult is same type as IntermediateResult. AverageAggregationFunction is an example where IntermediateResult (AvgPair) is different from FinalResult(Double)
When Pinot segment files are created in external systems (Hadoop/Spark/etc), there are several ways to push those data to Pinot Controller and Server:
Push segment to shared NFS and let Pinot pull segment files from the location of that NFS.
Push segment to a Web server and let Pinot pull segment files from the Web server with http/https link.
Push segment to HDFS and let Pinot pull segment files from HDFS with hdfs location uri.
Push segment to other system and implement your own segment fetcher to pull data from those systems.
The first two options should be supported out of the box with Pinot package. As long your remote jobs send Pinot controller with the corresponding URI to the files it will pick up the file and allocate it to proper Pinot Servers and brokers. To enable Pinot support for HDFS, you will need to provide Pinot Hadoop configuration and proper Hadoop dependencies.
In your Pinot controller/server configuration, you will need to provide the following configs:
or
This path should point the local folder containing core-site.xml and hdfs-site.xml files from your Hadoop installation
or
These two configs should be the corresponding Kerberos configuration if your Hadoop installation is secured with Kerberos. Check Hadoop Kerberos guide on how to generate Kerberos security identification.
You will also need to provide proper Hadoop dependencies jars from your Hadoop installation to your Pinot startup scripts.
To push HDFS segment files to Pinot controller, you just need to ensure you have proper Hadoop configuration as we mentioned in the previous part. Then your remote segment creation/push job can send the HDFS path of your newly created segment files to the Pinot Controller and let it download the files.
For example, the following curl requests to Controller will notify it to download segment files to the proper table:
You can also implement your own segment fetchers for other file systems and load into Pinot system with an external jar. All you need to do is to implement a class that extends the interface of SegmentFetcher and provides config to Pinot Controller and Server as follows:
or
You can also provide other configs to your fetcher under config-root pinot.server.segment.fetcher.<protocol>