Let’s Write a Dang ElasticSearch Plugin

ElasticSearch is awesome, but when it doesn't do quite what you need, it's time to write some Java.

One of our current projects involves a complex interactive query builder to search a large collection of news items. Some of the conditionals fall outside of the sweet spot of Postgres (e.g. word X must appear within Y words of word Z), and so we opted to pull in ElasticSearch alongside it. It's worked perfectly, hitting all of our condition and grouping needs with one exception: we need to be able to filter for articles that contain a term a minimum number of times (so "Apple" must appear in the article 3 times, for example). Frustratingly, Elastic totally has this information via its term_vector feature, but you can't use that data inside a query, as least as far as I can tell.

The solution, it seems, is to write a custom plugin. I figured it out, eventually, but it was a lot of trial-and-error as the documentation I was able to find is largely outdated or incomplete. So I figured I'd take what I learned while it's still fresh in my mind in the hopes that someone else might have an easier time of it. That's what internet friends are for, after all.

Quick note before we start: all the version numbers you see are current and working as of February 25, 2021. Hopefully this post ages well, but if you try this out and hit issues, bumping the versions of Elastic, Gradle, and maybe even Java is probably a good place to start. Also, I use projectname a lot in the code examples — that's not a special word and you should change it to something that makes sense for you.

1. Set up a Java development environment

First off, you're gonna be writing some Java. That's not my usual thing, so the first step was to get a working environment to compile my code. To do that, we'll use Docker. Here's a Dockerfile:

FROM adoptopenjdk/openjdk12:jdk-12.0.2_10-ubuntu

RUN apt-get update && \
  apt-get install -y zip unzip && \
  rm -rf /var/lib/apt/lists/*

SHELL ["/bin/bash", "-c"]

RUN curl -s "https://get.sdkman.io" | bash && \
  source "/root/.sdkman/bin/sdkman-init.sh" && \
  sdk install gradle 6.8.2

WORKDIR /plugin

We use a base image with all the Java stuff but also a working Ubuntu install so that we can do normal Linux-y things inside our container. From your terminal, build the image:

> docker build . -t projectname-java

Then, spin up the container and start an interactive shell, mounting your local working directory into /plugin:

> docker run --rm -it -v ${PWD}:/plugin projectname-java bash

2. Configure Gradle

Gradle is a "build automation tool for multi-language software development," and what Elastic recommends for plugin development. Configuring Gradle to build the plugin properly was the hardest part of this whole endeavor. Throw this into build.gradle in your project root:

buildscript {
  repositories {
    mavenLocal()
    mavenCentral()
    jcenter()
  }

  dependencies {
    classpath "org.elasticsearch.gradle:build-tools:7.11.1"
  }
}

apply plugin: 'java'

compileJava {
  sourceCompatibility = JavaVersion.VERSION_12
  targetCompatibility = JavaVersion.VERSION_12
}

apply plugin: 'elasticsearch.esplugin'

group = "com.projectname"
version = "0.0.1"

esplugin {
  name 'contains-multiple'
  description 'A script for finding documents that match a term a certain number of times'
  classname 'com.projectname.containsmultiple.ContainsMultiplePlugin'
  licenseFile rootProject.file('LICENSE.txt')
  noticeFile rootProject.file('NOTICE.txt')
}

validateNebulaPom.enabled = false

You'll also need files named LICENSE.txt and NOTICE.txt — mine are empty, since the plugin is for internal use only. If you're going to be releasing your plugin in some public way, maybe talk to a lawyer about what to put in those files.

3. Write the dang plugin

To write the actual plugin, I started with this example plugin which scores a document based on the frequency of a given term. My use case was fortunately quite similar, though I'm using a filter query, meaning I just want a boolean, i.e. does this document contain this term the requisite number of times? As such, I implemented a FilterScript rather than the ScoreScript implemented in the example code.

This file lives in (deep breath) src/main/java/com/projectname/containsmultiple/ContainsMultiplePlugin.java:

package com.projectname.containsmultiple;

import org.apache.lucene.index.LeafReaderContext;
import org.apache.lucene.index.PostingsEnum;
import org.apache.lucene.index.Term;
import org.elasticsearch.common.settings.Settings;
import org.elasticsearch.plugins.Plugin;
import org.elasticsearch.plugins.ScriptPlugin;
import org.elasticsearch.script.FilterScript;
import org.elasticsearch.script.FilterScript.LeafFactory;
import org.elasticsearch.script.ScriptContext;
import org.elasticsearch.script.ScriptEngine;
import org.elasticsearch.script.ScriptFactory;
import org.elasticsearch.search.lookup.SearchLookup;

import java.io.IOException;
import java.io.UncheckedIOException;
import java.util.Collection;
import java.util.Map;
import java.util.Set;

/**
 * A script for finding documents that match a term a certain number of times
 */
public class ContainsMultiplePlugin extends Plugin implements ScriptPlugin {

    @Override
    public ScriptEngine getScriptEngine(
        Settings settings,
        Collection<ScriptContext<?>> contexts
    ) {
        return new ContainsMultipleEngine();
    }

    // tag::contains_multiple
    private static class ContainsMultipleEngine implements ScriptEngine {
        @Override
        public String getType() {
            return "expert_scripts";
        }

        @Override
        public <T> T compile(
            String scriptName,
            String scriptSource,
            ScriptContext<T> context,
            Map<String, String> params
        ) {
            if (context.equals(FilterScript.CONTEXT) == false) {
                throw new IllegalArgumentException(getType()
                        + " scripts cannot be used for context ["
                        + context.name + "]");
            }
            // we use the script "source" as the script identifier
            if ("contains_multiple".equals(scriptSource)) {
                FilterScript.Factory factory = new ContainsMultipleFactory();
                return context.factoryClazz.cast(factory);
            }
            throw new IllegalArgumentException("Unknown script name "
                    + scriptSource);
        }

        @Override
        public void close() {
            // optionally close resources
        }

        @Override
        public Set<ScriptContext<?>> getSupportedContexts() {
            return Set.of(FilterScript.CONTEXT);
        }

        private static class ContainsMultipleFactory implements FilterScript.Factory,
                                                      ScriptFactory {
            @Override
            public boolean isResultDeterministic() {
                return true;
            }

            @Override
            public LeafFactory newFactory(
                Map<String, Object> params,
                SearchLookup lookup
            ) {
                return new ContainsMultipleLeafFactory(params, lookup);
            }
        }

        private static class ContainsMultipleLeafFactory implements LeafFactory {
            private final Map<String, Object> params;
            private final SearchLookup lookup;
            private final String field;
            private final String term;
            private final int count;

            private ContainsMultipleLeafFactory(
                        Map<String, Object> params, SearchLookup lookup) {
                if (params.containsKey("field") == false) {
                    throw new IllegalArgumentException(
                            "Missing parameter [field]");
                }
                if (params.containsKey("term") == false) {
                    throw new IllegalArgumentException(
                            "Missing parameter [term]");
                }
                if (params.containsKey("count") == false) {
                    throw new IllegalArgumentException(
                            "Missing parameter [count]");
                }
                this.params = params;
                this.lookup = lookup;
                field = params.get("field").toString();
                term = params.get("term").toString();
                count = Integer.parseInt(params.get("count").toString());
            }

            @Override
            public FilterScript newInstance(LeafReaderContext context)
                    throws IOException {
                PostingsEnum postings = context.reader().postings(
                        new Term(field, term));
                if (postings == null) {
                    /*
                     * the field and/or term don't exist in this segment,
                     * so always return 0
                     */
                    return new FilterScript(params, lookup, context) {
                        @Override
                        public boolean execute() {
                            return false;
                        }
                    };
                }
                return new FilterScript(params, lookup, context) {
                    int currentDocid = -1;
                    @Override
                    public void setDocument(int docid) {
                        /*
                         * advance has undefined behavior calling with
                         * a docid <= its current docid
                         */
                        if (postings.docID() < docid) {
                            try {
                                postings.advance(docid);
                            } catch (IOException e) {
                                throw new UncheckedIOException(e);
                            }
                        }
                        currentDocid = docid;
                    }
                    @Override
                    public boolean execute() {
                        if (postings.docID() != currentDocid) {
                            /*
                             * advance moved past the current doc, so this
                             * doc has no occurrences of the term
                             */
                            return false;
                        }
                        try {
                            return postings.freq() >= count;
                        } catch (IOException e) {
                            throw new UncheckedIOException(e);
                        }
                    }
                };
            }
        }
    }
    // end::contains_multiple
}

4. Add it to ElasticSearch

With our code in place (and synced into our Docker container with a mounted volume), it's time to compile it. In the Docker shell you started up in step #1, build your plugin:

> gradle build

Assuming that works, you should now see a build directory with a bunch of stuff in it. The file you care about is build/distributions/contains-multiple-0.0.1.zip (though that'll obviously change if you call your plugin something different or give it a different version number). Grab that file and copy it to where you plan to actually run ElasticSearch. For me, I placed it in a folder called .docker/elastic in the main project repo. In that same directory, create a new Dockerfile that'll actually run Elastic:

FROM docker.elastic.co/elasticsearch/elasticsearch:7.11.1

COPY .docker/elastic/contains-multiple-0.0.1.zip /plugins/contains-multiple-0.0.1.zip

RUN elasticsearch-plugin install \
  file:///plugins/contains-multiple-0.0.1.zip

Then, in your project root, create the following docker-compose.yml:

version: '3.2'

services: elasticsearch:
    image: projectname_elasticsearch
    build:
      context: .
      dockerfile: ./.docker/elastic/Dockerfile
      ports:
        - 9200:9200
      environment:
        - discovery.type=single-node
        - script.allowed_types=inline
        - script.allowed_contexts=filter

Those last couple lines are pretty important and your script won't work without them. Build your image with docker-compose build and then start Elastic with docker-compose up.

5. Use your plugin

To actually see the plugin in action, first create an index and add some documents (I'll assume you're able to do this if you've read this far into this post). Then, make a query with curl (or your Elastic wrapper of choice), substituting full_text, yabba and index_name with whatever makes sense for you:

> curl -H "content-type: application/json" \
-d '
{
  "query": {
    "bool": {
      "filter": {
        "script": {
          "script": {
            "source": "contains_multiple",
            "lang": "expert_scripts",
            "params": {
              "field": "full_text",
              "term": "yabba",
              "count": 3
            }
          }
        }
      }
    }
  }
}' \
"localhost:9200/index_name/_search?pretty"

The result should be something like:

{
  "took" : 6,
  "timed_out" : false,
  "_shards" : {
    "total" : 1,
    "successful" : 1,
    "skipped" : 0,
    "failed" : 0
  },
  "hits" : {
    "total" : {
      "value" : 1,
      "relation" : "eq"
    },
    "max_score" : 0.0,
    "hits" : [
      {
        "_index" : "index_name",
        "_type" : "_doc",
        "_id" : "10",
        ...

So that's that, an ElasticSearch plugin from start-to-finish. I'm sure there are better ways to do some of this stuff, and if you're aware of any, let us know in the comments or write your own dang blog.

David Eisinger

David is Viget's managing development director. From our Durham, NC, office, he builds high-quality, forward-thinking software for PUMA, the World Wildlife Fund, ID.me, and many others.

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