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This page provides an introduction to using Hadrian. It deliberately mirrors the Titus Basic Use page.

Before you begin…

Download any pre-built Hadrian JAR that includes dependencies. This article was tested with Hadrian 0.8.3; newer versions should work with no modification. Scala >= 2.10 is required.

Launch a Scala prompt using that JAR as a classpath:

> scala -cp hadrian-standalone-0.8.3-jar-with-dependencies.jar

and import com.opendatagroup.hadrian.jvmcompiler.PFAEngine:

Welcome to Scala version 2.10.5 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_45).
Type in expressions to have them evaluated.
Type :help for more information.

scala> import com.opendatagroup.hadrian.jvmcompiler.PFAEngine

Simplest possible scoring engines

Let’s start with an engine that merely adds 10 to each input. That’s something we can write inline.

scala> val engine = PFAEngine.fromJson("""
     | {"input": "double",
     |  "output": "double",
     |  "action": {"+": ["input", 100]}}
     | """).head
engine: com.opendatagroup.hadrian.jvmcompiler.PFAEngine[AnyRef,AnyRef] = PFA_Engine_1@3f792b9b

For convenience, we could have written it in YAML (all of Hadrian’s unit tests are written this way).

scala> val engine = PFAEngine.fromYaml("""
     | input: double
     | output: double
     | action: {+: [input, 100]}
     | """).head
engine: com.opendatagroup.hadrian.jvmcompiler.PFAEngine[AnyRef,AnyRef] = PFA_Engine_2@53e211ee

Note in both cases that we asked for the .head of what PFAEngine.fromJson and PFAEngine.fromYaml produces. In general, these functions produce a collection of PFAEngine objects from one PFA file (pass multiplicity = 4 and drop .head to see that). These scoring engines can run in parallel and share memory. For now, though, we’re only interested in one scoring engine.

By virtue of having created an engine, the PFA has been fully validated. If the PFA is not valid, you would see

Now run the scoring engine on some sample input:

scala> engine.action(java.lang.Double.valueOf(3.14))
res0: AnyRef = 103.14

For Java accessibility, the action method takes and returns boxed values of type AnyRef (Object in Java). See Hadrian data format for a complete menu.

You should only ever see one of the following exceptions at runtime

Emit-type engines

Of the three types of PFA scoring engine (map, emit, and fold), emit requires special attention in scoring. Map and fold engines yield results as the return value of the function (and fold does so cumulatively), but emit engines always return null. The only way to get results from them is by passing a callback function.

scala> val engine2 = PFAEngine.fromYaml("""
     | input: double
     | output: double
     | method: emit
     | action:
     |   - if:
     |       ==: [{"%": [input, 2]}, 0]
     |     then:
     |       - emit: input
     |       - emit: {/: [input, 2]}
     | """).head
engine2: com.opendatagroup.hadrian.jvmcompiler.PFAEngine[AnyRef,AnyRef] = PFA_Engine_2@4cacccbf

scala> import com.opendatagroup.hadrian.jvmcompiler.PFAEmitEngine
scala> val engine2AsEmit = engine2.asInstanceOf[PFAEmitEngine[AnyRef, AnyRef]]
engine2AsEmit: com.opendatagroup.hadrian.jvmcompiler.PFAEmitEngine[AnyRef,AnyRef] = PFA_Engine_2@4cacccbf

scala> def newEmit(x: AnyRef) =
     |   println("output: " + x.toString)
newEmit: (x: AnyRef)Unit

scala> engine2AsEmit.emit = newEmit
engine2AsEmit.emit: AnyRef => Unit = <function1>

scala> for (x <- 1 to 5) {
     |   println("input: " + x.toString)
     |   engine2.action(java.lang.Double.valueOf(x))
     | }
input: 1
input: 2
output: 2.0
output: 1.0
input: 3
input: 4
output: 4.0
output: 2.0
input: 5

Data from serialized sources

The PFAEngine interface has methods for streaming data in and out of JSON, Avro, and CSV. Thus, we don’t have to create data in Hadrian’s internal format.

scala> val engine3 = PFAEngine.fromYaml("""
     | input: {type: map, values: int}
     | output: {type: array, items: int}
     | action: {map.values: input}
     | """).head
engine3: com.opendatagroup.hadrian.jvmcompiler.PFAEngine[AnyRef,AnyRef] = PFA_Engine_3@1d6a0962

scala> import com.opendatagroup.hadrian.data._

// the hard way

scala> val input = PFAMap.fromMap(Map("one" -> java.lang.Integer.valueOf(1), "two" -> java.lang.Integer.valueOf(2), "three" -> java.lang.Integer.valueOf(3)))
input: com.opendatagroup.hadrian.data.PFAMap[Integer] = {one: 1, two: 2, three: 3}

scala> val output = engine3.action(input)
output: AnyRef = [1, 2, 3]

scala> output.getClass.getName
res0: String = com.opendatagroup.hadrian.data.PFAArray

// the easy way

scala> val input = engine3.jsonInput("""{"one": 1, "two": 2, "three": 3}""")               
input: AnyRef = {one: 1, two: 2, three: 3}

scala> val output = engine3.action(input)
output: AnyRef = [1, 2, 3]

scala> engine3.jsonOutput(output)
res1: String = [1,2,3]

Snapshots and reverting

Snapshots are representations of a PFA engine’s state at a moment in time. They are only relevant if the engine has mutable state. Let’s start by making a mutable scoring engine and filling it with some state.

scala> val engine4 = PFAEngine.fromYaml("""
     | input: int
     | output: {type: array, items: int}
     | cells:
     |   history:
     |     type: {type: array, items: int}
     |     init: []
     | action:
     |   cell: history
     |   to: {a.append: [{cell: history}, input]}
     | """).head

scala> engine4.action(java.lang.Integer.valueOf(1))
res0: AnyRef = [1]
scala> engine4.action(java.lang.Integer.valueOf(2))
res1: AnyRef = [1, 2]
scala> engine4.action(java.lang.Integer.valueOf(3))
res2: AnyRef = [1, 2, 3]
scala> engine4.action(java.lang.Integer.valueOf(4))
res3: AnyRef = [1, 2, 3, 4]
scala> engine4.action(java.lang.Integer.valueOf(5))
res4: AnyRef = [1, 2, 3, 4, 5]

The snapshot method locks the scoring engine and turns the state of the engine into a new AST that could be immediately serialized as a PFA file.

scala> engine4.snapshot
res5: com.opendatagroup.hadrian.ast.EngineConfig = {"name":"Engine_4","method":"map","input":"int","output":{"type":"array","items":"int"},"action":[{"cell":"history","to":{"a.append":[{"cell":"history"},"input"]}}],"cells":{"history":{"type":{"type":"array","items":"int"},"init":[1,2,3,4,5],"shared":false,"rollback":false}}}

The snapshotCell and snapshotPool methods are more focused: they do not lock the whole scoring engine and only report one cell or pool. The value is returned in Hadrian’s internal format, so they would need to be explicitly converted into a serialized format.

scala> engine4.snapshotCell("history")
res6: AnyRef = [1, 2, 3, 4, 5]

The revert method rolls back a scoring engine to the state described by its original PFA file. It is used by reducers in map-reduce to ensure that each key in a key-value stream is computed independently.

scala> engine4.revert()

scala> engine4.action(java.lang.Integer.valueOf(6))
res7: AnyRef = [6]
scala> engine4.action(java.lang.Integer.valueOf(7))
res8: AnyRef = [6, 7]
scala> engine4.action(java.lang.Integer.valueOf(8))
res9: AnyRef = [6, 7, 8]
scala> engine4.action(java.lang.Integer.valueOf(9))
res10: AnyRef = [6, 7, 8, 9]
scala> engine4.action(java.lang.Integer.valueOf(10))
res11: AnyRef = [6, 7, 8, 9, 10]

External function calls

PFA user-defined functions are intended to simplify repetitive tasks, but they can also be used as an alternate entry point into the scoring engine, distinct from the normal input-output stream. It is only relevant to do so on scoring engines with mutable state.

val engine5 = PFAEngine.fromYaml("""
input: int
output: {type: array, items: int}
    type: {type: array, items: int}
    init: []
  cell: history
  to: {a.append: [{cell: history}, input]}
    params: [{i: int}]
    ret: int
      cell: history
      path: [i]
    params: []
    ret: "null"
      - cell: history
        to: {a.reverse: {cell: history}}
      - null

scala> engine5.action(java.lang.Integer.valueOf(100))
res0: AnyRef = [100]
scala> engine5.action(java.lang.Integer.valueOf(101))
res1: AnyRef = [100, 101]
scala> engine5.action(java.lang.Integer.valueOf(102))
res2: AnyRef = [100, 101, 102]
scala> engine5.action(java.lang.Integer.valueOf(103))
res3: AnyRef = [100, 101, 102, 103]
scala> engine5.action(java.lang.Integer.valueOf(104))
res4: AnyRef = [100, 101, 102, 103, 104]

You can get references to these functions by name. It is necessary to specify the number of arguments.

scala> val getItem = engine5.fcn1("getItem")
getItem: AnyRef => AnyRef = <function1>

scala> val flipList = engine5.fcn0("flipList")
flipList: () => AnyRef = <function0>

When you call them, you have to pass them data in Hadrian’s internal format.

scala> getItem(java.lang.Integer.valueOf(1))
res5: AnyRef = 101

scala> getItem(java.lang.Integer.valueOf(3))
res6: AnyRef = 103

The functions may or may not return a value, and they may or may not have side-effects. This is the only recommended way to change cell/pool values from outside of a scoring engine.

scala> flipList()
res7: AnyRef = null

scala> engine5.action(java.lang.Integer.valueOf(0))
res8: AnyRef = [104, 103, 102, 101, 100, 0]

Abstract Syntax Tree

The PFA AST is an immutable tree structure built from the serialized JSON, stored in engine.config, which is an EngineConfig. You can query anything about the original PFA file in a structured way through this AST. For instance,

scala> engine.config.action.head
res0: com.opendatagroup.hadrian.ast.Expression = {"+":["input",100]}

scala> engine.config.action.head.getClass.getName
res1: String = com.opendatagroup.hadrian.ast.Call

scala> engine.config.input.avroType
res2: com.opendatagroup.hadrian.datatype.AvroType = "double"

There are also a few methods for recursively walking over the AST. The collect method applies a partial function to all nodes in the tree and produces a list of matches. For instance, to get all Expressions (function calls like “+”, symbol references like “input”, and literal values like “100”), do

scala> import com.opendatagroup.hadrian.ast.Expression
scala> engine.config collect {case x: Expression => x}
res3: Seq[com.opendatagroup.hadrian.ast.Expression] = List({"+":["input",100]}, "input", 100)

You can also build new scoring engines by passing a replacement function. This one turns instances of 100 into 999. You can do quite a lot just by crafting the right partial function.

scala> import com.opendatagroup.hadrian.ast.LiteralInt
scala> engine.config replace {case x: LiteralInt if (x.value == 100) => LiteralInt(999)}
res4: com.opendatagroup.hadrian.ast.Ast = {"name":"Engine_2","method":"map","input":"double","output":"double","action":[{"+":["input",999]}]}

In fact, this is how Hadrian generates code in general. A walk over the tree checks for semantic errors while calling a Task at each node. Usually, this Task is to create Java code, but it could be anything. This small example generates Lisp.

scala> import com.opendatagroup.hadrian.ast._
scala> import com.opendatagroup.hadrian.datatype._
scala> import com.opendatagroup.hadrian.options.EngineOptions
scala> import com.opendatagroup.hadrian.signature.PFAVersion

scala> trait LispCode extends TaskResult

scala> case class LispFunction(car: String, cdr: Seq[LispCode]) extends LispCode {
     |   override def toString() = "(" + car + cdr.map(" " + _.toString).mkString + ")"
     | }

scala> case class LispSymbol(name: String) extends LispCode {
     |   override def toString() = name
     | }

scala> object GenerateLisp extends Task {
     |   def apply(context: AstContext, engineOptions: EngineOptions, resolvedType: Option[Type]): TaskResult = context match {
     |     case Call.Context(_, _, fcn: LibFcn, args: Seq[TaskResult], _, _, _) => LispFunction(fcn.name, args.map(_.asInstanceOf[LispCode]))
     |     case Ref.Context(_, _, name: String) => LispSymbol(name)
     |     case LiteralInt.Context(_, _, value: Int) => LispSymbol(value.toString)
     |   }
     | }

scala> val symbolTable = SymbolTable.blank
scala> symbolTable.put("input", AvroDouble())
scala> engine.config.action.head.walk(GenerateLisp, symbolTable, FunctionTable.blank, new EngineOptions(Map(), Map()), PFAVersion(0, 8, 1))._2
res5: com.opendatagroup.hadrian.ast.TaskResult = (+ input 100)

scala> val engine6 = PFAEngine.fromYaml("""
     | input: double
     | output: double
     | action: {+: [{/: [input, 2]}, {m.sqrt: input}]}
     | """).head

scala> engine6.config.action.head.walk(GenerateLisp, symbolTable, FunctionTable.blank, new EngineOptions(Map(), Map()))._2
res6: com.opendatagroup.hadrian.ast.TaskResult = (+ (/ input 2) (m.sqrt input))