Question

I’m looking at using AVRO on hadoop. But I am concerned with serialization of large data-structures and how to add methods to the (data-) classes.

The example (taken from http://blog.voidsearch.com/bigdata/apache-avro-in-practice/) shows a model of facebook users.

{
  "namespace": "test.avro",
  "name": "FacebookUser",
  "type": "record",
  "fields": [
      {"name": "name", "type": "string"},
      ...,
      {"name": "friends", "type": "array", "items": "FacebookUser"} 
  ]
}

Does avro serialize the complete social graph of a facebookuser in this model?

[That is, if I want to serialize one user, does the serialization include all it’s friends and their friends and so on?]

If the answer is yes, I’d rather store ID’s of friends instead of references, to look up in my application whenever needed. In that case I would like to be able to add a method that returns the actual friends instead of ID’s.

How can I wrap/extend generated AVRO java classes to add methods?

(also to add methods that return for example friend-count)

Answer 1

Regarding the second question: How can I wrap/extend generated AVRO java classes to add methods?

You can use the AspectJ to inject new methods into an existing/generated class. AspectJ is required only at compile-time. Approach is illustrated below.

Define a Person record as Avro IDL (person.avdl):

@namespace("net.tzolov.avro.extend")
protocol PersonProtocol {
    record Person {
        string firstName;
        string lastName;
    }     
}

use maven and the avro-maven-plugin to generate java sources from the AVDL:

<dependency>
    <groupId>org.apache.avro</groupId>
    <artifactId>avro</artifactId>
    <version>1.6.3</version>
</dependency>
    ......
    <plugin>
        <groupId>org.apache.avro</groupId>
        <artifactId>avro-maven-plugin</artifactId>
        <version>1.6.3</version>
        <executions>
            <execution>
                <id>generate-avro-sources</id>
                <phase>generate-sources</phase>
                <goals>
                    <goal>idl-protocol</goal>
                </goals>
                <configuration>
                    <sourceDirectory>src/main/resources/avro</sourceDirectory>
                    <outputDirectory>${project.build.directory}/generated-sources/java</outputDirectory>
                </configuration>
            </execution>
        </executions>
    </plugin>

Above configuration presumes that the person.avid file is in src/main/resources/avro. Sources are generated in target/generated-sources/java.

Generated Person.java has two methods: getFirstName() and getLastName(). If you want to extend it with another method: getCompleteName() = firstName + lastName then you can inject this method with the following aspect:

package net.tzolov.avro.extend;

import net.tzolov.avro.extend.Person;

public aspect PersonAspect {

    public String Person.getCompleteName() {        
        return this.getFirstName() + " " + this.getLastName();
    }
}

Use the aspectj-maven-plugin maven plugin to weave this aspect with the generated code

<dependency>
    <groupId>org.aspectj</groupId>
    <artifactId>aspectjrt</artifactId>
    <version>1.6.12</version>
</dependency>
<dependency>
    <groupId>org.aspectj</groupId>
    <artifactId>aspectjweaver</artifactId>
    <version>1.6.12</version>
</dependency>
    ....
<plugin>
    <groupId>org.codehaus.mojo</groupId>
    <artifactId>aspectj-maven-plugin</artifactId>
    <version>1.2</version>
    <dependencies>
        <dependency>
            <groupId>org.aspectj</groupId>
            <artifactId>aspectjrt</artifactId>
            <version>1.6.12</version>
        </dependency>
        <dependency>
            <groupId>org.aspectj</groupId>
            <artifactId>aspectjtools</artifactId>
            <version>1.6.12</version>
        </dependency>
    </dependencies>
    <executions>
        <execution>
            <goals>
                <goal>compile</goal>
                <goal>test-compile</goal>
            </goals>
        </execution>
    </executions>
    <configuration>
        <source>6</source>
        <target>6</target>
    </configuration>
</plugin>

and the result:

@Test
public void testPersonCompleteName() throws Exception {

    Person person = Person.newBuilder()
            .setFirstName("John").setLastName("Atanasoff").build();

    Assert.assertEquals("John Atanasoff", person.getCompleteName());
}