@ -1,23 +1,25 @@
## Graph Data Base Basics
## What is Gremlin?
Gremlin is a graph traversal language: think of Gremlin as the SQL for graph databases. Gremlin is not
a graph database server, it is a language; but, there is a Gremlin Server and a Gremlin Console available for
interacting with graph databases. It is possible to use Gremlin on large database platforms
like [Titan ](https://www.digitalocean.com/community/tutorials/how-to-set-up-the-titan-graph-database-with-cassandra-and-elasticsearch-on-ubuntu-16-04 )
and [HBase ](https://docs.janusgraph.org/latest/hbase.html ).
A graph database is based on graph structures. A graph is composed of nodes, edges, and properties. A key
object/component in a graph database is stored as a node. Nodes are connected together via edges representing
relationships. For example, you may represent people as nodes and have edges representing who people are friends
with. You can assign properties to both nodes and edges. A person (node) may have the properties of age and name,
where a friendship (edge) may have a start date property.
#### Why Graph Databases?
## Graph Data Base Basics
Graph databases are great for modeling data where the value lies in the shape of the graph, or, it would be difficult
to model in a traditional table based database.
A graph database is based on graph theory. A graph is composed of nodes, edges, and properties. A key
object/component in a graph database is stored as a node. Nodes are connected via edges representing
relationships. For example, you may represent people as nodes and have edges representing friendships.
You can assign properties to both nodes and edges. A person (node) may have the properties of age and name,
where a friendship (edge) may have a start date property.
#### What is Gremlin?
## Why Graph Databases ?
Gremlin is a graph traversal language; think of Gremlin as SQL but for graph databases. Gremlin is not
a graph database server, it is a language; but, there is a Gremlin Server and Gremlin Console available for
interacting with graph databases using Gremlin. It is possible to use Gremlin on large database platforms
like [Titan ](https://www.digitalocean.com/community/tutorials/how-to-set-up-the-titan-graph-database-with-cassandra-and-elasticsearch-on-ubuntu-16-04 )
and [HBase ](https://docs.janusgraph.org/latest/hbase.html ).
Graph databases are great for modeling data where the value lies in the shape of the graph. Graph databases
also allow to to model more complex relationships which would be difficult to model in a normal table-based
database.
## Gremlin Installation
@ -25,17 +27,19 @@ Download and extract the following:
- [Gremlin Console ](https://www.apache.org/dyn/closer.lua/tinkerpop/3.3.3/apache-tinkerpop-gremlin-console-3.3.3-bin.zip )
- [Gremlin Server ](https://www.apache.org/dyn/closer.lua/tinkerpop/3.3.3/apache-tinkerpop-gremlin-server-3.3.3-bin.zip )
Start the Gremlin server by running it with the start script in the bin folder.
Start the Gremlin server by running it with the start script in the bin folder. As a prerequisite for running gremlin, you
must have Java installed on your computer.
```
./gremlin-server.sh
```
Start the Gremlin console by running the gremlin.sh or gremlin.bat script in the bin folder of the apache-tinkerpop folder .
Start the Gremlin console by running the gremlin.sh or gremlin.bat script in the bin folder.
```
./gremlin.sh
```
Now you need to instantiate a new graph on the server to use. To to that, execute the following commands.
```gremlin
Now you need to instantiate a new graph on the server to use. To to that, execute the following commands in
the Gremlin console.
```java
#Creates a empty graph
gremlin> graph = EmptyGraph.instance()
==>emptygraph[empty]
@ -54,80 +58,81 @@ gremlin> g = graph.traversal().withRemote(DriverRemoteConnection.using(cluster,
Now that you have your gremlin server and console set up, you are ready to start executing Gremlin queries.
#### Add a Vertex
### Adding a Vertex
For Gremlin, nodes are referred to as "Vertexes". To add a node/vertex to the graph, you simply use the
command addV('node label' ) on your graph traversal source. For consistency, most people and documentation
use "g" as their default graph traversal source. To append properties to your your node, you string a series of
.property('property_name', 'property_value') to the queries .
In Gremlin nodes are referred to as "Vertexes". To add a node/vertex to the graph, you simply use the
command addV() on your graph traversal source. For consistency, most people
use "g" as their default graph traversal source. To append properties to your your vertex, you add a series of
" .property('property_name', 'property_value')" strings to the add vertex query .
ex :
```gremlin
EX :
```java
g.addV('student').property('name', 'Jeffery').property('GPA', 4.0);
```
#### Update a Property
### Updating a Property
Unlike SQL, you are not limited to a specific schema for a graph database. If you want to add or change
a property of a vertex or edge, you simply call its .property('property_name', 'property_value') .
The g.V(1) in the example refers to a specific node with the primary id of 1, these ids are auto assigned by the graph database .
You can replace g.V(1) with a command to select a specific nod e.
Unlike SQL, you are not limited to a specific schema in a graph database. If you want to add or change
a property on a vertex or edge, you simply use the property command again .
The "g.V(1)" in the following example refers to a specific vertex with the primary id of 1-- the graph database auto assigns these ids .
You can replace "g.V(1)" with a command to select a specific vertex or edg e.
```gremlin
```java
g.V(1).property('name', 'Jeffery R');
```
#### Selection
### Selection
Selecting nodes and edges is the most complicated part of Gremlin. The concept is not particularly hard but, there
are dozens of ways to do traversals and selections. I will cover the most common and helpful ways to traverse a
graph.
Selecting nodes and edges is the most complicated part of Gremlin. The concept is not particularly hard, but, there
are dozens of ways to do graph traversals and selections. I will cover the most common aways to traverse a graph.
This example will select all vertexes which have the label "student". The .valueMap() appended to the end means
that it will returns a map of all the properties of the nodes it returns.
```gremlin
This example will select all vertexes which have the label "student". The ".valueMap()" command appended to the end of the query
makes Gremlin return a map of all the objects it returns with their properties.
```java
g.V().hasLabel('student').valueMap();
```
In this example instead of returning a ValueMap of values, we are just returning the names of the students
In this following example, instead of returning a ValueMap of values, we are just returning the names of the students
in the graph.
```gremlin
```java
g.V().hasLabel('student').values('name');
```
This example will return the GPA of the student with the name "Jeffery R".
```gremlin
```java
g.V().hasLabel('student').has('name', 'Jeffery R').values('gpa');
```
This command will all the students in order of their GPA.
```gremlin
This command will return all the students in order of their GPA.
```java
g.V().hasLabel('student').order().by('gpa', decr).value('name')
```
#### Adding Edges
### Adding Edges
If you want to add a edge (relationship/connection) between two nodes, the easiest way (my opinion) to do it in Gremlin is by
using something called aliasing. In this example we select two nodes and give them a name, in this case it is "a", and "b".
After we have selected two edges, we can add an edge to them using the addE('Relation_Name') command. The syntax of this is
nice because we know that "a" is friends with "b"-- it is easy to tell the direction
The easiest way (my opinion) to add edges in Gremlin is by
using aliasing. In this example we select two nodes and assign them a name: in this case it is "a", and "b".
After we have selected two edges, we can add an edge to them using the "addE()" command. The syntax of this is
nice because we know that "a" is friends with "b"-- it is easy to tell the direction of the edge.
```gremlin
```java
g.V(0).as('a').V(1).as('b').addE('knows')
.from('a').to('b');
```
## Using Gremlin W ith Java
## Using Gremlin w ith Java
Now that you know the syntax of Gremlin, you are ready to use it somewhere other than just the Gremlin console. If you
are trying to use Gremlin with Java, there is a nice Maven dependency for TinkerPop and Gremlin. If you want to quickly
connect to your server with Java, make sure your server is set up exactly as it was before this tutorial started discussing
Gremlin Syntax.
```maven
Now that you know the basic syntax of Gremlin, you are ready to use it somewhere other than the Gremlin console. If you
are trying to use Gremlin with Java, there is a great Maven dependency for TinkerPop and Gremlin. If you want to quickly
connect to your Gremlin server with Java, make sure your server is set up exactly as it was before this tutorial started discussing
Gremlin syntax.
#### Maven dependency for Java:
```html
<!-- https://mvnrepository.com/artifact/com.tinkerpop/gremlin - core -->
< dependency >
< groupId > com.tinkerpop< / groupId >
@ -150,7 +155,7 @@ Gremlin Syntax.
```
It is helpful to wrap everything relating to the graph database connection into a single Java class. This is roughly
the code that I usually use to interact with a Gremlin Server, anybody is free to use it.
the code that I usually use to interact with a Gremlin Server-- anybody is free to use it.
```java
public class GraphConnection
@ -182,7 +187,7 @@ public class GraphConnection
}
```
ex GraphConnection Usage:
#### Basic GraphConnection.java Usage:
```java
RemoteConnection con = new RemoteConnection()
String query = "g.V().hasLabel('player')" +
@ -193,14 +198,13 @@ String query = "g.V().hasLabel('player')" +
".as('p2')" +
".addE('friends')" +
".from('p1').to('p2')";
//System.out.println(query);
this.con.queryGraph(query);
```
Overly complex usage with lambda example.
#### Overly complex usage with a lambda statement
```java
/**
* Fetches a list of friends from the graph database
* Fetches the list of a player's friends.
*
* @param id steam id
* @return list of friends
@ -225,11 +229,11 @@ private List getFriendsFromGraph(String id)
```
The most important thing to do while playing around with Gremlin in Java is to keep an eye on the
return type. From experience, I can say that it is often easier to return the node/edges from your
query rather than doing a valueMap.
return type. From experience, I can say that it is often easier to return the vertex from your
query rather than returning the valueMap.
Without adding valueMap()/values() to the end of the query, you now can directly access the vertex or edge in the result rather than
doing some voodoo witchcraft and casting between ArrayLists and HashMaps.
Without returning the valueMap in the query, you can directly access the vertex
in the result rather than doing some voodoo witchcraft and casting between ArrayLists and HashMaps.
The previous example could be re-written as this:
```java
@ -242,16 +246,15 @@ String query = "g.V().hasLabel('player')" +
for(Result r: this.con.queryGraph(query))
{
friends.add(new Player(r.getVertex("name").value().toString),
r.getVertex("id").value().toString), )
r.getVertex("id").value().toString));
}
```
Now you know enough to be dangerous with Gremlin . Yay! If you want to do more than basic things with Gremlin,
I highly suggest that you take a look at the tutorial [SQL 2 Gremlin ](http://sql2gremlin.com/ ).
If you plan on deploying this to production, it is recommended to use HBase with JanusGraph for a persistent back end storage
You now know enough about Gremlin to be dangerous with it . Yay! If you want to do more than basic things with Gremlin,
I highly suggest that you look at the tutorial [SQL 2 Gremlin ](http://sql2gremlin.com/ ).
If you plan on deploying this to production, it is recommended that y ou use HBase for a persistent back end storage
server.
## Resources
- [SQL 2 Gremlin ](http://sql2gremlin.com/ )
jrtechs
6 years ago
