JanusGraph provides the following backends for use with Cassandra:

Cassandra has two protocols for clients to use: CQL and Thrift. Thrift was the original interface, however it was deprecated starting with Cassandra 2.1. The core of JanusGraph was originally written before the deprecation of Thrift, and it has several classes that support Thrift. With Cassandra 4.0, Thrift support will be removed in Cassandra. JanusGraph users are recommended to use the cql storage backend.

	Note
	If you plan to use a Thrift-based driver and you are using Cassandra 2.2 or higher, you need to explicitly enable Thrift so that JanusGraph can connect to the cluster. Do so by running ./bin/nodetool enablethrift on every Cassandra node.

	Note
	If security is enabled on Cassandra, the user must have CREATE permission on <all keyspaces>, otherwise the keyspace must be created ahead of time by an administrator.

Cassandra can be run as a standalone database on the same local host as JanusGraph and the end-user application. In this model, JanusGraph and Cassandra communicate with one another via a localhost socket. Running JanusGraph over Cassandra requires the following setup steps:

In the Gremlin Console, you can not define the type of the variables conf and g. Therefore, simply leave off the type declaration.

Cassandra does not have a native install for Windows or OSX. One of the easiest ways to run Cassandra on OSX, Windows, or Linux is to use a Docker Container. You can download and run Cassandra with a single Docker command. It is important to install a version that is supported by the version of JanusGraph you intend to use. The compatible versions can be found under the Tested Compatibility section of the specific release on the Releases page. The Cassandra Docker Hub page can be referenced for the available versions and useful commands. In the command below an environment variable is being set to enable Cassandra Thrift with -e CASSANDRA_START_RPC=true. A description of the ports can be found here. Port 9160 is used for the Thrift client API. Port 9042 is for CQL native clients. Ports 7000, 7001 and 7099 are for inter-node communication. Version 3.11 of Cassandra was the latest compatible version for JanusGraph 0.2.0 and is specified in the reference command below.

docker run --name jg-cassandra -d -e CASSANDRA_START_RPC=true -p 9160:9160 \
-p 9042:9042 -p 7199:7199 -p 7001:7001 -p 7000:7000 cassandra:3.11

When the graph needs to scale beyond the confines of a single machine, then Cassandra and JanusGraph are logically separated into different machines. In this model, the Cassandra cluster maintains the graph representation and any number of JanusGraph instances maintain socket-based read/write access to the Cassandra cluster. The end-user application can directly interact with JanusGraph within the same JVM as JanusGraph.

For example, suppose we have a running Cassandra cluster where one of the machines has the IP address 77.77.77.77, then connecting JanusGraph with the cluster is accomplished as follows (comma separate IP addresses to reference more than one machine):

JanusGraph graph = JanusGraphFactory.build().
set("storage.backend", "cql").
set("storage.hostname", "77.77.77.77").
open();

In the Gremlin Console, you can not define the type of the variables conf and g. Therefore, simply leave off the type declaration.

Gremlin Server can be wrapped around each JanusGraph instance defined in the previous subsection. In this way, the end-user application need not be a Java-based application as it can communicate with Gremlin Server as a client. This type of deployment is great for polyglot architectures where various components written in different languages need to reference and compute on the graph.

Start Gremlin Server using bin/gremlin-server.sh and then in an external Gremlin Console session using bin/gremlin.sh you can send Gremlin commands over the wire:

:plugin use tinkerpop.server
:remote connect tinkerpop.server conf/remote.yaml
:> g.addV()

In this case, each Gremlin Server would be configured to connect to the Cassandra cluster. The following shows the graph specific fragment of the Gremlin Server configuration. Refer to Chapter 7, JanusGraph Server for a complete example and more information on how to configure the server.

...
graphs: {
  g: conf/janusgraph-cql.properties}
plugins:
  - janusgraph.imports
...

For more information about Gremlin Server see the Apache TinkerPop documentation

Finally, Cassandra can be embedded in JanusGraph, which means, that JanusGraph and Cassandra run in the same JVM and communicate via in process calls rather than over the network. This removes the (de)serialization and network protocol overhead and can therefore lead to considerable performance improvements. In this deployment mode, JanusGraph internally starts a cassandra daemon and JanusGraph no longer connects to an existing cluster but is its own cluster.

To use JanusGraph in embedded mode, simply configure embeddedcassandra as the storage backend. The configuration options listed below also apply to embedded Cassandra. In creating a JanusGraph cluster, ensure that the individual nodes can discover each other via the Gossip protocol, so setup a JanusGraph-with-Cassandra-embedded cluster much like you would a stand alone Cassandra cluster. When running JanusGraph in embedded mode, the Cassandra yaml file is configured using the additional configuration option storage.conf-file, which specifies the yaml file as a full url, e.g. storage.conf-file = file:///home/cassandra.yaml.

When running a cluster with JanusGraph and Cassandra embedded, it is advisable to expose JanusGraph through the Gremlin Server so that applications can remotely connect to the JanusGraph graph database and execute queries.

Note, that running JanusGraph with Cassandra embedded requires GC tuning. While embedded Cassandra can provide lower latency query answering, its GC behavior under load is less predictable.

Refer to Chapter 15, Configuration Reference for a complete listing of all Cassandra specific configuration options in addition to the general JanusGraph configuration options.

When configuring Cassandra it is recommended to consider the following Cassandra specific configuration options:

For more information on Cassandra consistency levels and acceptable values, please refer to the Cassandra Thrift API. In general, higher levels are more consistent and robust but have higher latency.

JanusGraph over Cassandra supports global vertex and edge iteration. However, note that all these vertices and/or edges will be loaded into memory which can cause OutOfMemoryException. Use Chapter 37, JanusGraph with TinkerPop’s Hadoop-Gremlin to iterate over all vertices or edges in large graphs effectively.

Follow these steps to setup a Cassandra cluster on EC2 and deploy JanusGraph over Cassandra. To follow these instructions, you need an Amazon AWS account with established authentication credentials and some basic knowledge of AWS and EC2.

--clustername [cassandra-cluster-name]
--totalnodes [number-of-instances]
--version community
--opscenter no

--clustername janusgraph
--totalnodes 4
--version community
--opscenter no