User Defined Functions in Neo4j 3.1.0-M10

Posted by Michael Hunger on Oct 6, 2016 in apoc, cypher |

Neo4j 3.1 brings some really neat improvements in Cypher alongside other cool features

I already demonstrated the – GraphQL inspired – map projections and pattern comprehensions in my last blog post.

User Defined Procedures

In the 3.0 release my personal favorite was user defined procedures which can be implemented using Neo4j’s Java API and called directly from Cypher.
You can tell, because I wrote about half of the 270 procedures for the APOC procedure collection “, with the remainder provided by other contributors.

Remember the syntax: …​ CALL namespace.procedure(arg1, arg2) YIELD col1, col2 AS alias …​

MATCH (from:Place {coords:{from}}), (to:Place {coords:{to}})

CALL apoc.algo.dijkstra(from, to, "ROAD", "cost") YIELD path, weight

RETURN nodes(path)
ORDER BY weight LIMIT 10;

Procedures improvements in 3.1.

As of 3.1, the named procedure parameters can now have default values.
This means you can leave them off during the call (from right to left), e.g.

@Name(value="costProperty", defaultValue="cost") String prop

The annotations for the database mode were folded into the @Procedure annotation.
You can also now set DBMS as mode, which encompasses Schema operations like creating indexes or constraints. @Procedure(value="namespace.procedure",mode=READ|WRITE|DBMS).

Based on the security work in Neo4j Enterprise, procedures can now get an additional set of roles configured in the allowed attribute of the @Procedure annotation, that also permit users with that role (or only that role) to execute the procedure.

The last change is dropping the support for result objects without columns, which we sometimes used for changing the cardinality of a query (between 0 and 1) for boolean operations.
But that’s handled now much better by boolean functions (predicates) as discussed below.

User Defined Functions

If you used or wrote procedures in the past, you most probably came across instances where it felt quite unwieldy to call a procedure just to compute something, convert a value or provide a boolean decision.

For example:

CREATE (v:Value {id:{id}, data:{data}})
WITH v
CALL apoc.date.formatDefault(timestamp(), "ms") YIELD value as created
SET v.created = created

You’d rather write it as a function:

CREATE (v:Value {id:{id}, data:{data}, created: apoc.date.format(timestamp()) })

Now in 3.1 that’s possible, and you can also leave off the "ms" and use a single function name, because the unit and format parameters have a default value.

Functions are more limited than procedures: they can’t execute writes or schema operations and are expected to return a single value, not a stream of values.
But this makes it also easier to write and use them.

By having information about their types, the Cypher Compiler can also check for applicability.

The signature of the procedure above changed from:

@Procedure("apoc.date.format")
public Stream<StringResult> formatDefault(@Name("time") long time, @Name("unit") String unit) {
   return Stream.of(format(time, unit, DEFAULT_FORMAT));
}

to the much simpler function signature (ignoring the parameter name and value annotations):

@UserFunction("apoc.date.format")
public String format(@Name("time") long time,
                     @Name(value="unit", defaultValue="ms") String unit,
                     @Name(value="format", defaultValue=DEFAULT_FORMAT) String format) {
   return getFormatter().format(time, unit, format);
}

This can then be called in the manner outlined above.

In our APOC procedure library we already converted about 50 procedures into functions from the following areas:

package # of functions example function

date & time conversion

3

apoc.date.parse("time",["unit"],["format"])

number conversion

3

apoc.number.parse("number",["format"])

general type conversion

8

apoc.convert.toMap(value)

type information and checking

4

apoc.meta.type(value)

collection and map functions

25

apoc.map.fromList(["k1",v1,"k2",v2,"k3",v3])

JSON conversion

4

apoc.convert.toJson(value)

string functions

7

apoc.text.join(["s1","s2","s3"],"delim")

hash functions

2

apoc.util.md5(value)

You can list user defined functions with call dbms.functions()

dbms.functions

We also started to add default values for parameters and deprecate procedures that required alternative names because of a wider parameter set.

We also moved the Description annotation from APOC’s own to the one now provided within Neo4j, so you’ll see the descriptions for all functions and procedures in dbms.procedures().

All of this is available now, if you run Neo4j 3.1.0-M10 and use this latest 3.1.0.1 APOC release.

You can follow the development of APOC for Neo4j 3.1 in the 3.1 branch.

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