Custom Stellar Functions

Metron is fundamentally a programmable, extensible system and Stellar is the extension language. We have some great Stellar functions available out of the box and we’ll be adding more over time, but they may not quite scratch quite your particular itch.

Of course, we’d love to have your contribution inside of Metron if you think it general purpose enough, but not every function is general-purpose or it may rely on libraries those licenses aren’t acceptable for an Apache project. In that case, then you will be wondering how to add your custom function to a running instance of Metron.

Building Your Own Function

Let’s say that I need a function that returns the current time in milliseconds since the epoch. I notice that there’s nothing like that currently in Metron, so I embark on the adventure of adding it for my cluster.

I will presume that you have an installed Metron into your local maven repo via mvn install . In the future, when we publish to a maven repo, you will not need this. I will depend on 0.4.1 for the purpose of this demonstration

Hack, Hack, Hack

I like to use Maven, so we’ll use that for this demonstration, but you can use whatever build system that you like. Here’s my favorite way to build a project with groupId com.mycompany.stellar and artifactId of tempus mvn archetype:create -DgroupId=com.mycompany.stellar -DartifactId=tempus -DarchetypeArtifactId=maven-archetype-quickstart

First, we should depend on metron-common and we can do that by adjusting the pom.xml just created:

<project xmlns="" xmlns:xsi=""
  <name>Stellar Time Functions</name>
      <!-- NOTE: We will want to depend on the deployed common on the classpath. -->

Let’s add our implementation in src/main/java/com/mycompany/stellar/ with the following content:

package com.notmetron.stellar;
import org.apache.metron.stellar.dsl.Context;
import org.apache.metron.stellar.dsl.ParseException;
import org.apache.metron.stellar.dsl.Stellar;
import org.apache.metron.stellar.dsl.StellarFunction;
import java.util.List;
public class TimeFunction {
  @Stellar( name="NOW",
            description = "Right now!",
            params = {},
  public static class Now implements StellarFunction {
    public Object apply(List<Object> list, Context context) throws ParseException {
      return System.currentTimeMillis();
    public void initialize(Context context) { }
    public boolean isInitialized() {
      return true;

Now we can build the project via mvn package which will create a target/tempus-1.0-SNAPSHOT.jar file.

Install the Function

Now that we have a jar with our custom function, we must make Metron aware of it.

Deploy the Jar

First you need to place the jar in HDFS, if we have it on an access node, one way to do that is:

  • hadoop fs -put tempus-1.0-SNAPSHOT.jar /apps/metron/stellar This presumes that:
  • you’ve standardized on /apps/metron/stellar as the location for custom jars
  • you are running the command from an access node with the hadoop command installed
  • you are running from a user that has write access to /apps/metron/stellar

Set Global Config

You may not need this if your Metron administrator already has this setup.

With that dispensed with, we need to ensure that Metron knows to look at that location. We need to ensure that the stellar.function.paths property in the global.json is in place that makes Metron aware to look for Stellar functions in /apps/metron/stellar on HDFS.

This property looks like, the following for a vagrant install

  "es.clustername": "metron",
  "es.ip": "node1",
  "es.port": "9300",
  "": "yyyy.MM.dd.HH",
  "stellar.function.paths" : "hdfs://node1:8020/apps/metron/stellar/.*.jar",

The stellar.function.paths property takes a comma separated list of URIs or URIs with regex expressions at the end. Also, note path is prefaced by the HDFS default name, which, if you do not know, can be found by executing, hdfs getconf -confKey, such as

[root@node1 ~]# hdfs getconf -confKey

Use the Function

Now that we have deployed the function, if we want to use it, any running topologies that use Stellar will need to be restarted.

Beyond that, let’s take a look at it in the REPL:

Stellar, Go!
Please note that functions are loading lazily in the background and will be unavailable until loaded fully.
{es.clustername=metron, es.ip=node1, es.port=9300,, stellar.function.paths=hdfs://node1:8020/apps/metron/stellar/.*.jar, profiler.client.period.duration=1, profiler.client.period.duration.units=MINUTES}
[Stellar]>>> # Get the help for NOW
[Stellar]>>> ?NOW
Functions loaded, you may refer to functions now...
Description: Right now!
Returns: Timestamp
[Stellar]>>> # Try to run the NOW function, which we added:
[Stellar]>>> NOW()
[Stellar]>>> # Looks like I got a timestamp, success!