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1. Create a logical operator, which holds the necessary configuration parameters, e.g. a predicate for a filter or an attribute list for a projection. ILogicalOperator is the interface that needs to be implemented, AbstractLogicalOperator the base class that should be extended. Important: If the logical operator contains predicates, it must implement the interface IProvidesPredicates!
2. Create (at least) one physical operator implementation that is initialized by the information provided by the logical operator and can process the input data.
3. Optionally, create a rewrite rule. These rules are used to modify the logical query plan. In most cases this is the switch of operators, e.g. to push selections and projections close to the sources. If you create a new operator where the placement in the query plan does not matter. Provide rules to allow switching or else plans with the new operator may be optimizable.
4. Create a transformation rule. A transformation rule translates a logical operator into one of its physical counterparts.

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First, you need to decide if you want to place the operator in a new OSGi bundle or in an existing one. Independently of that, the operator must be placed in a package ending with logicaloperator if it should be integrated automatically.
To create your own logical operator you need to extend AbstractLogicalOperator (which implements ILogicalOperator). For convenience reasons there are base implementations for operators with single input (UnaryLogicalOp) and binary input (BinaryLogicalOp). As a naming convention, the class name should end with AO (for algebra operator).
This class must provide (at least) two constructors and the clone method. The default constructor is required as instances of logical operators are created by newInstance(). Clone must call the copy constructor and the copy constructor must call the super copy constructor! If at runtime an error like "has no owner" is called, in most cases this is because of the missing call to the super copy constructor.
Finally, the class needs setters and getters for the parameter it should keep

package de.uniol.inf.is.odysseus.core.server.logicaloperator;

import java.util.List;

import de.uniol.inf.is.odysseus.core.predicate.IPredicate;
import de.uniol.inf.is.odysseus.core.server.logicaloperator.annotations.LogicalOperator;
import de.uniol.inf.is.odysseus.core.server.logicaloperator.annotations.Parameter;
import de.uniol.inf.is.odysseus.core.server.logicaloperator.builder.PredicateParameter;

@LogicalOperator(name="ROUTE", minInputPorts=1, maxInputPorts=1, doc = "My operator is doing ...", url = "http://example.com/MyOperator.html", category = { "Category" })
public class RouteAO extends UnaryLogicalOp {

    private static final long serialVersionUID = -8015847502104587689L;
    
    public RouteAO(){
        super();
    }
    
    public RouteAO(RouteAO routeAO){
        super(routeAO);
    }

    @Override
    @Parameter(type=PredicateParameter.class, isList=true=true, optional = false, doc = "This parameter sets the selection predicate")
    public void setPredicates(List<IPredicate<?>> predicates) {
        super.setPredicates(predicates);
    }
    
    @Override
    public AbstractLogicalOperator clone() {
        return new RouteAO(this);
    }
    
}

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Every operator needs to provide an output schema, i.e. what is the schema of the elements that are produced. As a default implementation AbstractLogicalOperator delivers the input schema as the output schema. Route does not change the input schema, so this implementation is sufficient. If the input schema is not the same as the output schema, the class needs to implement getOutputSchemaIntern(int port), where port is the output port of the operator.

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For the easy integration of new logical operators into to PQL query language, annotations should be used. Another way is to extend AbstractOperatorBuilder. In these annotations the name of the operator and number of minimal and maximal inputs and the parameters are described. In addition, the annotations may include some documentation and a URL for further information about the operator. These information are also available in the Odysseus Studio GUI.
On the right side, the whole implementation of the RouteAO can be found. Because predicates need to be initialized before the processing, they should be saved by the AbstractLogicalOperator.
Further information about the annotations can be found in the PQL documentation.

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Until now, only descriptive information about the new operator is given. Next the concrete implementation of its functionality needs to be provided.
To create an own physical operator, AbstractPipe<R,W> needs to be extended. To allow generic implementations we utilize the Java generics approach. R is the type of the objects that are read by the operator and W is the type of the objects that are written. Although in most cases, this will be the same class (e.g. Tuple) some operators may read other objects than they write.
The following methods need to be overwritten:

• getOutputMode(): Needed for locking, Goal: Reduce object copies
  
  • INPUT: read element will not be modified (e.g. selection)
  • MODIFIED_INPUT: read element will be modified (e.g. projection)
  • NEW_ELEMENT: operator creates a new element (e.g. join)

    @Override
    public OutputMode getOutputMode() {
        return OutputMode.INPUT;
    }

• process_next(R input, int port)
  • Process new input element on input port, a new created element can be send to the next operator with the transfer-() method
    
    

    @Override
    protected void process_next(T object, int port) {
        for (int i=0;i<predicates.size();i++){
            if (predicates.get(i).evaluate(object)) {
                transfer(object,i);
                return;
            }
        }
        transfer(object,predicates.size());
    }

If necessary: process_open, process_close. These methods are called when the query is initialized and terminated, respectively.

• processPunctuation(Punctuation punctuation, int port)
  • This method is needed to process punctuations (e.g.heartbeat, Heartbeats allow determining the progress of time without the need to produce new elements). It is important, that punctuations and elements are timely ordered, i.e. if a punctuation is send, no other element is allowed to be send with an older timestamp anymore! If the operator has no state, it is enough to call sendPunctuation.

    @Override
    public void processPunctuation(PointInTime timestamp, int port) {
        sendPunctuation(timestamp);
    }

• process_isSemanticallyEqual(IPhysicalOperator ipo)
  
  • This method is needed for query sharing. Return true, if the operator given and the current operator are equivalent and can be replaced by each other.

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As a naming convention the class name should end with PO (physical operator).

package de.uniol.inf.is.odysseus.core.server.physicaloperator;

import java.util.ArrayList;
import java.util.List;

import de.uniol.inf.is.odysseus.core.metadata.PointInTime;
import de.uniol.inf.is.odysseus.core.physicaloperator.IPhysicalOperator;
import de.uniol.inf.is.odysseus.core.physicaloperator.OpenFailedException;
import de.uniol.inf.is.odysseus.core.predicate.IPredicate;

/**
 * @author Marco Grawunder
 */
@SuppressWarnings({"rawtypes"})
public class RoutePO<T> extends AbstractPipe<T, T> {

    private List<IPredicate<? super T>> predicates;

    public RoutePO(List<IPredicate<? super T>> predicates)  {
        super();
        initPredicates(predicates);
    }

    public RoutePO(RoutePO<T> splitPO) {
        super();
        initPredicates(splitPO.predicates);
    }
    
    private void initPredicates(List<IPredicate<? super T>> predicates) {
        this.predicates = new ArrayList<IPredicate<? super T>>(predicates.size());
        for (IPredicate<? super T> p: predicates){
            this.predicates.add(p.clone());
        }
    }
    


    @Override
    public OutputMode getOutputMode() {
        return OutputMode.INPUT;
    }

    @Override
    public void process_open() throws OpenFailedException{
        super.process_open();
        for (IPredicate<? super T> p: predicates){
            p.init();
        }        
    }
    
    @Override
    protected void process_next(T object, int port) {
        for (int i=0;i<predicates.size();i++){
            if (predicates.get(i).evaluate(object)) {
                transfer(object,i);
                return;
            }
        }
        transfer(object,predicates.size());
    }
    
    @Override
    public RoutePO<T> clone() {
        return new RoutePO<T>(this);
    }

    @Override
    public void processPunctuation(PointInTime timestamp, int port) {
        sendPunctuation(timestamp);
    }
    
    @Override
    public boolean process_isSemanticallyEqual(IPhysicalOperator ipo) {
        if(!(ipo instanceof RoutePO)) {
            return false;
        }
        RoutePO spo = (RoutePO) ipo;
        if(this.hasSameSources(spo) &&
                this.predicates.size() == spo.predicates.size()) {
            for(int i = 0; i<this.predicates.size(); i++) {
                if(!this.predicates.get(i).equals(spo.predicates.get(i))) {
                    return false;
                }
            }
            return true;
        }
        return false;
    }

}

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Query Rewrite

Rewriting is used to swtichswitch, add, remove or replace logical operators before they are transformed into their physical counterparts. Normally, the aim of the rewriting process is to optimize the query, for example, by pushing down selection operator before costly joins. The implementation is done by rewriting rules, which are implemented like transformation rules (see next section).

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Next, Odysseus needs to know this rule. This is done by a RuleProvider. Create a new class, which we call RuleProvider:

package de.uniol.inf.is.odysseus.transform.rules;
import java.util.ArrayList;
import java.util.List;
import de.uniol.inf.is.odysseus.ruleengine.rule.IRule;
import de.uniol.inf.is.odysseus.transform.flow.ITransformRuleProvider;

public class RuleProvider implements ITransformRuleProvider {

    @Override @Override
    public List<IRule<?, ?>> getRules() {
        List<IRule<?, ?>> rules = new ArrayList<IRule<?,?>>();       
        rules.add(new TRouteAORule());
        return rules;
    }
}

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• Create a folder (if does not exist yet) called "OSGI-INF" in the root directory of your bundle.
• Create a "Component Definition" (right click --> new).
• In the tab "Overview":
  
  • Name should be something unique (e.g. your package or bundle name + rule provider)
  • Choose your new RuleProvider as "Class"
• Switch to tab "Services":
  
  • Click on "on Add..." of " of Provided Services" and choose the interface "de.uniol.inf.is.odysseus.transform.flow.ITransformRuleProvider"

Attention:

If you copy the component definition, you have to add it to the MANIFEST.MF by hand! So, if the service is not started, check, if the manifest has the following line (assuming that your component definition file is named RuleService.xml)

Service-Component: OSGI-INF/RuleService.xml