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[[compilers]]
= Gremlin Compilers

There are many languages built to query data. SQL is typically used to query relational data. There is SPARQL for RDF
data. Cypher is used to do pattern matching in graph data. The list could go on. Compilers convert languages like
these to Gremlin so that it becomes possible to use them in any context that Gremlin is used. In other words, a
Gremlin Compiler enables a particular query language to work on any TinkerPop-enabled graph system.

[[sparql-gremlin]]
== SPARQL-Gremlin

image::gremlintron.png[width=225]

The SPARQL-Gremlin compiler, transforms link:https://en.wikipedia.org/wiki/SPARQL[SPARQL] queries into Gremlin
traversals. It uses the https://jena.apache.org/index.html[Apache Jena] SPARQL processor
link:https://jena.apache.org/documentation/query/index.html[ARQ], which provides access to a syntax tree of a
SPARQL query.

The goal of this work is to bridge the query interoperability gap between the two famous, yet fairly disconnected,
graph communities: Semantic Web (which relies on the RDF data model) and Graph database (which relies on property graph
data model).

NOTE: The foundational research work on SPARQL-Gremlin compiler (aka Gremlinator) can be found in the
link:https://arxiv.org/pdf/1801.02911.pdf[Gremlinator paper]. This paper presents the graph query language semantics of
SPARQL and Gremlin, and a formal mapping between SPARQL pattern matching graph patterns and Gremlin traversals.

[source,xml]
----
<dependency>
   <groupId>org.apache.tinkerpop</groupId>
   <artifactId>sparql-gremlin</artifactId>
   <version>x.y.z</version>
</dependency>
----

The SPARQL-Gremlin compiler converts link:https://en.wikipedia.org/wiki/SPARQL[SPARQL] queries into Gremlin so that
they can be executed across any TinkerPop-enabled graph system. To use this compiler in the Gremlin Console, first
install and activate the "tinkerpop.sparql" plugin:

[source,text]
----
gremlin> :install org.apache.tinkerpop sparql-gremlin x.y.z
==>Loaded: [org.apache.tinkerpop, sparql-gremlin, x.y.z]
gremlin> :plugin use tinkerpop.sparql
==>tinkerpop.sparql activated
----

Installing this plugin will download appropriate dependencies and import certain classes to the console so that they
may be used as follows:

[gremlin-groovy,modern]
----
graph = TinkerFactory.createModern()
g = traversal(SparqlTraversalSource).withEmbedded(graph)                                               <1>
g.sparql("""SELECT ?name ?age
            WHERE { ?person v:name ?name . ?person v:age ?age }
            ORDER BY ASC(?age)""")                                                                     <2>
----

<1> Define `g` as a `TraversalSource` that uses the `SparqlTraversalSource` - by default, the `traversal()` method
usually returns a `GraphTraversalSource` which includes the standard Gremlin starts steps like `V()` or `E()`. In this
case, the `SparqlTraversalSource` enables starts steps that are specific to SPARQL only - in this case the `sparql()`
start step.
<2> Execute a SPARQL query against the TinkerGraph instance. The `SparqlTraversalSource` uses a
<<traversalstrategy,TraversalStrategy>> to transparently converts that SPARQL query into a standard Gremlin traversal
and then when finally iterated, executes that against the TinkerGraph.

[[prefixes]]
=== Prefixes

The SPARQL-Gremlin compiler supports the following prefixes to traverse the graph:

[cols=",",options="header",]
|====================================
|Prefix |Purpose
|`v:<id\|label\|<name>>` |access to vertex id, label or property value
|`e:<label>` |out-edge traversal
|`p:<name>` |property traversal
|====================================

Note that element IDs and labels are treated like normal properties, hence they can be accessed using the same pattern:

[gremlin-groovy,existing]
----
g.sparql("""SELECT ?name ?id ?label 
	WHERE { 
	?element v:name ?name . 
	?element v:id ?id . 
	?element v:label ?label .}""")
----

[[supported-queries]]
=== Supported Queries

The SPARQL-Gremlin compiler currently supports translation of the SPARQL 1.0 specification, especially `SELECT`
queries, though there is an on-going effort to cover the entire SPARQL 1.1 query feature spectrum. The supported
SPARQL query types are:

* Union 
* Optional 
* Order-By 
* Group-By 
* STAR-shaped or _neighbourhood queries_ 
* Query modifiers, such as: 
** Filter with _restrictions_ 
** Count 
** LIMIT 
** OFFSET

[[limitations]]
=== Limitations

The current implementation of SPARQL-Gremlin compiler (i.e. SPARQL-Gremlin) does not support the following cases:

* SPARQL queries with variables in the predicate position are not currently covered, with an exception of the following
case:

[source,groovy]
----
g.sparql("""SELECT * WHERE { ?x ?y ?z . }""")
----

* A SPARQL Union query with un-balanced patterns, i.e. a gremlin union traversal can only be generated if the input
SPARQL query has the same number of patterns on both the side of the union operator. For instance, the following
SPARQL query cannot be mapped, since a union is executed between different number of graph patterns (two patterns
`union` 1 pattern).

[source,groovy]
----
g.sparql("""SELECT *
            WHERE {
                {?person e:created ?software .
                ?person v:name "josh" .}
                UNION
                {?software v:lang "java" .} }""")
----

* A non-Group key variable cannot be projected in a SPARQL query. This is a SPARQL language limitation rather than
that of Gremlin/TinkerPop. Apache Jena throws the exception "Non-group key variable in SELECT" if this occurs.
For instance, in a SPARQL query with GROUP-BY clause, only the variable on which the grouping is declared, can be
projected. The following query is valid:

[source,groovy]
----
g.sparql("""SELECT ?age
            WHERE {
                ?person v:label "person" .
                ?person v:age ?age .
                ?person v:name ?name .} GROUP BY (?age)""")
----

Whereas, the following SPARQL query will be invalid:

[source,groovy]
----
g.sparql("""SELECT ?person
            WHERE {
              ?person v:label "person" .
              ?person v:age ?age .
              ?person v:name ?name .} GROUP BY (?age)""")
----

* In a SPARQL query with an ORDER-BY clause, the ordering occurs with respect to the first projected variable in the
query. It is possible to choose any number of variable to be projected, however, the first variable in the selection
will be the ordering decider. For instance, in the query:

[source,groovy]
----
g.sparql("""SELECT ?name ?age
            WHERE {
                ?person v:label "person" .
                ?person v:age ?age .
                ?person v:name ?name . } ORDER BY (?age)""")
----

the result set will be ordered according to the `?name` variable (in ascending order by default) despite having passed
`?age` in the order by. Whereas, for the following query:

[source,groovy]
----
g.sparql("""SELECT ?age ?name
            WHERE {
                ?person v:label "person" .
                ?person v:age ?age .
                ?person v:name ?name . } ORDER BY (?age)""")
----

the result set will be ordered according to the `?age` (as it is the first projected variable). Finally, for the
select all case (`SELECT *`):

[source,groovy]
----
g.sparql("""SELECT *
            WHERE { ?person v:label "person" . ?person v:age ?age . ?person v:name ?name . } ORDER BY (?age)""")
----

the the variable encountered first will be the ordering decider, i.e. since we have `?person` encountered first,
the result set will be ordered according to the `?person` variable (which are vertex id).

* In the current implementation, `OPTIONAL` clause doesn't work under nesting with `UNION` clause (i.e. multiple optional
clauses with in a union clause) and `ORDER-By` clause (i.e. declaring ordering over triple patterns within optional
clauses). Everything else with SPARQL `OPTIONAL` works just fine.

[[examples]]
=== Examples

The following section presents examples of SPARQL queries that are currently covered by the SPARQL-Gremlin compiler.

==== Select All

Select all vertices in the graph.

[gremlin-groovy,existing]
----
g.sparql("""SELECT * WHERE { }""")
----

==== Match Constant Values

Select all vertices with the label `person`.

[gremlin-groovy,existing]
----
g.sparql("""SELECT * WHERE {  ?person v:label "person" .}""")
----

==== Select Specific Elements

Select the values of the properties `name` and `age` for each `person` vertex.

[gremlin-groovy,existing]
----
g.sparql("""SELECT ?name ?age
WHERE {
  ?person v:label "person" .
  ?person v:name ?name .
  ?person v:age ?age . }""")
----

==== Pattern Matching

Select only those persons who created a project.

[gremlin-groovy,existing]
----
g.sparql("""SELECT ?name ?age
WHERE {
  ?person v:label "person" .
  ?person v:name ?name .
  ?person v:age ?age .
  ?person e:created ?project . }""")
----

==== Filtering

Select only those persons who are older than 30.

[gremlin-groovy,existing]
----
g.sparql("""SELECT ?name ?age
WHERE {
  ?person v:label "person" .
  ?person v:name ?name .
  ?person v:age ?age .
    FILTER (?age > 30) }""")
----

==== Deduplication

Select the distinct names of the created projects.

[gremlin-groovy,existing]
----
g.sparql("""SELECT DISTINCT ?name
WHERE {
  ?person v:label "person" .
  ?person v:age ?age .
  ?person e:created ?project .
  ?project v:name ?name .
    FILTER (?age > 30)}""")
----

==== Multiple Filters

Select the distinct names of all Java projects.

[gremlin-groovy,existing]
----
g.sparql("""SELECT DISTINCT ?name
WHERE {
  ?person v:label "person" .
  ?person v:age ?age .
  ?person e:created ?project .
  ?project v:name ?name .
  ?project v:lang ?lang .
    FILTER (?age > 30 && ?lang = "java") }""")
----

==== Union

Select all persons who have developed a software in java using union.

[gremlin-groovy,existing]
----
g.sparql("""SELECT * 
WHERE {
  {?person e:created ?software .}
  UNION
  {?software v:lang "java" .} }""")
----

==== Optional

Return the names of the persons who have created a software in java and optionally python.

[source,groovy]
----
g.sparql("""SELECT ?person 
WHERE {
  ?person v:label "person" .
  ?person e:created ?software .
  ?software v:lang "java" .
  OPTIONAL {?software v:lang "python" . }}""")
----

==== Order By

Select all vertices with the label `person` and order them by their age.

[gremlin-groovy,existing]
----
g.sparql("""SELECT ?age ?name
WHERE {
  ?person v:label "person" .
  ?person v:age ?age .
  ?person v:name ?name .
} ORDER BY (?age)""")
----

==== Group By

Select all vertices with the label `person` and group them by their age.

[gremlin-groovy,existing]
----
g.sparql("""SELECT ?age 
WHERE {
  ?person v:label "person" .
  ?person v:age ?age .
} GROUP BY (?age)""")
----

==== Mixed/complex/aggregation-based queries

Count the number of projects which have been created by persons under the age of 30 and group them by age. Return only
the top two.

[source,groovy]
----
g.sparql("""SELECT (COUNT(?project) as ?p)
WHERE {
  ?person v:label "person" .
  ?person v:age ?age . FILTER (?age < 30)
  ?person e:created ?project .
} GROUP BY (?age) LIMIT 2""")
----

==== Meta-Property Access

Accessing the Meta-Property of a graph element. Meta-Property can be perceived as the reified statements in an RDF
graph.

[gremlin-groovy,theCrew]
----
g = traversal(SparqlTraversalSource).withEmbedded(graph)
g.sparql("""SELECT ?name ?startTime
WHERE {
  ?person v:name "daniel" .
  ?person p:location ?location .
  ?location v:value ?name .
  ?location v:startTime ?startTime }""")
----

==== STAR-shaped queries

STAR-shaped queries are the queries that form/follow a star-shaped execution plan. These in terms of graph traversals
can be perceived as path queries or neighborhood queries. For instance, getting all the information about a specific
`person` or `software`.

[gremlin-groovy,existing]
----
g.sparql("""SELECT ?age ?software ?lang ?name
WHERE {
  ?person v:name "josh" .
  ?person v:age ?age .
  ?person e:created ?software .
  ?software v:lang ?lang .
  ?software v:name ?name . }""")
----

[[sparql-with-gremlin]]
=== With Gremlin

The `sparql()`-step takes a SPARQL query and returns a result. That result can be further processed by standard Gremlin
steps as shown below:

[gremlin-groovy,modern]
----
g = traversal(SparqlTraversalSource).withEmbedded(graph)
g.sparql("SELECT ?name ?age WHERE { ?person v:name ?name . ?person v:age ?age }")
g.sparql("SELECT ?name ?age WHERE { ?person v:name ?name . ?person v:age ?age }").select("name")
g.sparql("SELECT * WHERE { }").out("knows").values("name")
g.withSack(1.0f).sparql("SELECT * WHERE { }").
  repeat(outE().sack(mult).by("weight").inV()).
    times(2).
  sack()
----

Mixing SPARQL with Gremlin steps introduces some interesting possibilities for complex traversals.