Delegating Translators

Translator "delegator"

A translator by name "delegator" is available in core Teiid installation, that can be used to modify the capabilities of a existing translator. Often times for debugging purposes or in special situations, one may require to either turn on/off certain capability of translator. For example, assume Hive database in their latest version supporting the ORDER BY construct, however Teiid’s current version of the Hive translator does not have this capability, you can use the "delegator" translator to turn ON the "ORDER BY" support without actually writing any code. Sometimes you may want to do the reverse, you want turn off certain capability to produce a better plan. In these situations, you can use this translator.

To use this translator, you need to define this translator in the VDB. What follows is an example of overriding the "hive" translator and turning off the ORDER BY support.

CREATE DATABASE myvdb;
USE DATABASE myvdb;
CREATE FOREIGN DATA WRAPPER delegator;
CREATE FOREIGN DATA WRAPPER "hive-delegator" TYPE delegator OPTIONS (delegateName 'hive', supportsOrderBy 'false');
CREATE SERVER source FOREIGN DATA WRAPPER "hive-delegator" OPTIONS ("resource-name" 'java:hive-ds');
CREATE SCHEMA mymodel SERVER source;
SET SCHEMA mymodel;
IMPORT FROM SERVER source INTO mymodel;
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<vdb name="myvdb" version="1">

    <model name="mymodel">
        <source name="source" translator-name="hive-delegator" connection-jndi-name="java:hive-ds"/>
    </model>

    <!-- the below it is called translator overriding, where you can set different properties -->
    <translator name="hive-delegator" type="delegator" >
        <property name="delegateName" value="hive" />
        <property name="supportsOrderBy" value="false"/>
   </translator>
</vdb>

You can override any/all the translator capabilities defined here Translator Capabilities as Execution Properties to override. Example of "supportsOrderBy" is shown in above example.

Extending the "delegator" translator

You may create a delegating translator by extending the  org.teiid.translator.BaseDelegatingExecutionFactory . Once your classes are then packaged as a custom translator, you will be able to wire another translator instance into your delegating translator at runtime in order to intercept all of the calls to the delegate. This base class does not provide any functionality on its own, other than delegation. The difference here from previous "delegator" translator is, you can hard code the capabilities instead of defining as configuration inside the vdb, as well as override methods to provide alternate behavior.

Execution Properties

Name

Description

Default

delegateName

Translator instance name to delegate to

n/a

cachePattern

Regex pattern of queries that should be cached using the translator caching API

n/a

cacheTtl

Time to live in milliseconds for queries matching the cache pattern

n/a

Lets say you are currently using "oracle" translator in your VDB, you want to intercept the calls going through this translator, then you first write a custom delegating translator like

@Translator(name="interceptor", description="interceptor")
public class InterceptorExecutionFactory extends org.teiid.translator.BaseDelegatingExecutionFactory{
    @Override
    public void getMetadata(MetadataFactory metadataFactory, C conn) throws TranslatorException {
        // do intercepting code here..

        // If you want call the original delegate, do not call if do not need to.
        // but if you did not call the delegate fullfill the method contract
        super.getMetadata(metadataFactory, conn);

        // do more intercepting code here..
    }
}

Now deploy this translator in Teiid engine. Then in your vdb file define like below.

CREATE DATABASE myvdb VERSION '1';
USE DATABASE myvdb VERSION '1';
CREATE FOREIGN DATA WRAPPER interceptor;
CREATE FOREIGN DATA WRAPPER "oracle-interceptor";
CREATE FOREIGN DATA WRAPPER "orcle-interceptor" TYPE interceptor OPTIONS (delegateName 'oracle');
CREATE SERVER source FOREIGN DATA WRAPPER "oracle-interceptor" OPTIONS ("resource-name" 'java:oracle-ds');
CREATE SCHEMA mymodel SERVER source;
SET SCHEMA mymodel;
IMPORT FROM SERVER source INTO mymodel;
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<vdb name="myvdb" version="1">

    <model name="mymodel">
        <source name="source" translator-name="oracle-interceptor" connection-jndi-name="java:oracle-ds"/>
    </model>

    <!-- the below it is called translator overriding, where you can set different properties -->
    <translator name="orcle-interceptor" type="interceptor" >
        <property name="delegateName" value="oracle" />
   </translator>
</vdb>

We have defined a "translator" override called "oracle-interceptor", which is based on the custom translator "interceptor" from above, and supplied the translator it needs to delegate to "oracle" as its delegateName. Then, we used this override translator "oracle-interceptor" in your VDB. Now any calls going into this VDB model’s translator will be intercepted by YOUR code to do whatever you want to do.

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