Titan 1.0 Data Migration

The release of Titan 1.0 brought with it many important features, but it also came with the need to do a major code and data migration given the upgrade to Apache TinkerPop 3 and the finalized changes to the schema used to store the graph data in Cassandra, HBase, etc. As a result, upgrading from older versions of Titan to this new version has its challenges.

This README and the associated code in this repository are designed to help with the data migration aspect of upgrading to Titan 1.0. It provides a model for data migration by using the OpenFlights data set and is aimed at users looking to upgrade from Titan 0.5.4 to Titan 1.0.0. It presents methods for dealing with the hardest aspects of data migration in this context and it is meant to offer guidance for those migrating graph data of any size.

Approach

This repository contains all the code required to simulate a data migration from Titan 0.5.4 to Titan 1.0.0. It provides Groovy scripts and Java code that together will generate the Titan 0.5.4 instance from the OpenFlights data set and then migrate it to Titan 1.0.0.

Prerequisites

The following list are prerequisites for executing the data migration model for OpenFlights. Actual requirements for a data migration may be different depending on the user's environment:

• Java 1.8.0_40+ (required by TinkerPop 3.x)
• Maven 3.x
• Titan 0.5.4 and Titan 1.0.0
• Cassandra as compatible with the Titan versions above (while the tutorial focuses on Cassandra, there should be no reason that these steps will not also work for other Titan backends)
• Hadoop 2
• git clone this repository and set an environment variable to the location of where it was cloned export OPENFLIGHTS_HOME="/projects/openflights")`

Assumptions

This tutorial assumes that the reader is knowledgeable with respect to Titan 0.5.x and has some basic knowledge of Titan 1.x and TinkerPop 3.x.

The Tutorial

For those wishing to get right to the abbreviated steps required to run this tutorial, they can be found at the bottom.

The Schema

OpenFlights is an open data set containing airport, airline and route data. While the dataset is not large, it bears sufficient complexity so as to provide a good model for a real-world data migration. Specifically, it allows for the modelling of multi-properties and mixed indices containing Geoshape data.

Loading Titan 0.5.4

Before the migration process can be started, the Titan 0.5.4 database needs to be loaded wth data. The first step in the process is to download the OpenFlights source data which is in CSV format. The data/download.sh shell script will help with that:

$ data/download.sh
--2015-11-25 07:20:28--  https://raw.githubusercontent.com/jpatokal/openflights/master/data/airports.dat
Resolving raw.githubusercontent.com (raw.githubusercontent.com)... 23.235.46.133
Connecting to raw.githubusercontent.com (raw.githubusercontent.com)|23.235.46.133|:443... connected.
HTTP request sent, awaiting response... 200 OK
...
Downloaded: 3 files, 3.4M in 1.2s (2.92 MB/s)
$ ls data
airlines.dat  airports.dat  download.sh  routes.dat

Note that Windows users can manually download these files to the data directory as the above script is meant for execution on Linux compatible systems.

The next step is to use the Titan 0.5.4 Gremlin Console to run the scripts/load-openflights-tp2.groovy script. This script will initialize the schema and parse the data into Titan. This script configures the target Titan instance to which data will be loaded and uses the Titan configuration at conf/openflights-tp2.properties for this purpose. This file defaults to local Cassandra usage, but could be changed to support a remote Cassandra cluster, HBase or other Titan backend.

Start the Titan 0.5.4 Gremlin Console and execute the script (in this case, be sure to replace ${OPENFLIGHTS_HOME} with the full path to that file):

gremlin> . ${OPENFLIGHTS_HOME}/scripts/load-openflights-tp2.groovy

Depending on what the reader hopes to learn from this tutorial, there are two interesting points to possibly consider in this script:

• The location property on the airport vertex is a Geoshape data type.
• The equipment property on the route vertex is a SET multi-property.

With that script executed, Titan now has the graph generated, but that script did something else at the very end. It dumped that data to GraphSON format using Titan's HadoopGraph as shown in the final lines of the script:

g = HadoopFactory.open(PROJECT_DIR + "/conf/hadoop/openflights-tp2.properties")
g._()

It is this step that is most pertient to those attempting to do data migrations from Titan 0.5.4, in that their best approach to a migration will be to dump their data to GraphSON. The contents of conf/hadoop/openflights-tp2.properties shows that it uses the TitanCassandraInputFormat to read the data from Cassandra and the GraphSONOutputFormat to write it to the file system. In this case, Hadoop's local job runner will write the output to the local file system. In a production environment, given a large scale graph migration, it would be preferred to have a Hadoop cluster with HDFS up and running to house this output. As a side note, if this process is being executed against a different Titan backend (i.e. not Cassandra), then the input format should be changed to match the backend being used.

Now that the OpenFlights graph data is available on the filesystem as GraphSON, it becomes possible to read that format into Titan 1.0.0.

Migrating to Titan 1.0.0

Apache TinkerPop 3.x introduced features for generalized bulk loading over different graph processors, by way of a VertexProgram called the BulkLoaderVertexProgram or BLVP. A VertexProgram can be executed over a GraphComputer like Spark and therefore provides a way to load graphs of any size, as the work can be distributed across a cluster.

As stated earlier, the BulkLoaderVertexProgram that ships with TinkerPop 3.x is meant to provided generalized bulk loading capabilities. Unfortunately, a minor bug prevents it's usage, so that class is provided here with the patch. The issue is expected to be fixed for TinkerPop 3.1.1, at which point usage of the included OpenflightsBulkLoaderVertexProgram will no longer be necessary. This instance will still be referred to as "BLVP" in this tutorial.

BLVP utilizes an IncrementalBulkLoader for "getOrCreate" functionality. In other words, the IncrementalBulkLoader has implementations of methods that check for graph Element existence and, if present, "get" them or otherwise "create" them. By extending this class, it becomes possible to customize those "getOrCreate" operations. For OpenFlights, it is necessary to customize how VertexProperty elements are managed. Specifically, it is important to ensure that the correct Cardinality is used for migrating the equipment multi-property:

public class OpenflightsBulkLoader extends IncrementalBulkLoader {
    @Override
    public VertexProperty getOrCreateVertexProperty(final VertexProperty<?> property, final Vertex vertex, final Graph graph, final GraphTraversalSource g) {
        if (property.key().equals("equipment")) {
            return vertex.property(VertexProperty.Cardinality.set, property.key(), property.value());
        }
        return super.getOrCreateVertexProperty(property, vertex, graph, g);
    }
}

All other properties can be set by the standard method provided in the IncrementalBulkLoader.

Given this new familiarity that has been developed with the Java code portion of this repository, it is now time to build the project with Maven:

$ mvn clean install -DskipTests

Copy the resulting target/openflights-1.0-SNAPSHOT.jar file to the Titan 1.0.0 ext directory, which makes it available on Titan's path. Before it is possible to use BLVP and the custom jar file, the GraphSON file that was dumped from Cassandra to the local file system needs to be moved into an openflights directory of Hadoop 2 HDFS so that BLVP can operate on it through Spark.

$ hadoop fs -mkdir openflights
$ hadoop fs -copyFromLocal part-m-* openflights

As with the data load to Titan 0.5.4, a Groovy script will be used for the Titan 1.0.0 work. The scripts/load-openflights-tp3.groovy first creates the schema in Titan 1.0.0 and then loads the data using BLVP and Spark. It is assumed that experienced Titan users will recognize the syntax for schema creation, so the focus for this tutorial is on executing the BLVP portion of the script:

graph = GraphFactory.open(PROJECT_DIR + "/conf/hadoop/openflights-tp3.properties")

writeGraph = PROJECT_DIR + "/conf/openflights.properties"
blvp = OpenflightsBulkLoaderVertexProgram.build().keepOriginalIds(false).writeGraph(writeGraph).
        intermediateBatchSize(10000).bulkLoader(OpenflightsBulkLoader).create(graph)
graph.compute(SparkGraphComputer).program(blvp).submit().get()

The first line creates a HadoopGraph instance, which is configured by conf/hadoop/openflights-tp3.properties. This configuration tells HadoopGraph to read the Titan 0.5.4 GraphSON format using ScriptInputFormat. ScriptInputFormat takes an arbitrary Groovy script and uses it to read Vertex objects. It is a good format to use when the graph data being read is in a custom format. In a sense, the GraphSON generated from the Titan 0.5.4 dump is "custom", because the format changed between TinkerPop 2.x and TinkerPop 3.x and there is Geoshape data encoded in that GraphSON, which is a type specific to Titan. The script to be used is referenced in that properties file given to the HadoopGraph and is called scripts/openflights-script-input.groovy. Copy this file to HDFS alongside the data file copied previously:

gremlin> hdfs.copyFromLocal(System.getenv('OPENFLIGHTS_HOME') + '/scripts/openflights-script-input.groovy', 'openflights-script-input.groovy')

The environment should now be ready to execute the migration script in full:

gremlin> :load ${OPENFLIGHTS_HOME}/scripts/load-openflights-tp3.groovy

When that script completes successfully, the data will have been written to Titan 1.0.0 and the migration from Titan 0.5.4 will be complete.

For the Impatient

0. Clone the project and set an environment variable that holds the path to the projects (e.g. export OPENFLIGHTS_HOME="/projects/openflights")

1. Download data files (run ${OPENFLIGHTS_HOME}/data/download.sh)

2. In a Titan 0.5.4 Gremlin console type (be sure to replace${OPENFLIGHTS_HOME} with the full path in this case):

   gremlin> . ${OPENFLIGHTS_HOME}/scripts/load-openflights-tp2.groovy

3. Copy the output files (from the output/ directory in HDFS) into a Hadoop2 HDFS directory called openflights

4. Upload the script ${OPENFLIGHTS_HOME}/scripts/openflights-script-input.groovy into Hadoop2's HDFS home directory

5. Compile the project:

   $ mvn clean install -DskipTests

6. Copy the jar file ${OPENFLIGHTS_HOME}/target/openflights-1.0-SNAPSHOT.jar into ${TITAN_1_0_HOME}/ext/

7. In a Titan 1.0.0 Gremlin console:

   gremlin> :load ${OPENFLIGHTS_HOME}/scripts/load-openflights-tp3.groovy

Conclusion

This tutorial provides a working model for doing a data migration from Titan 0.5.4 to Titan 1.0.0. It demonstrates some important new features of TinkerPop 3 that would allow this same model to be applied on a far larger scale. For those who have read this tutorial, the task that lies ahead involves adapting the model to the specifics of their individual production environments. It may also require more detailed reading of the TinkerPop reference documentation to fully grasp all the aspects of what has been presented here.