10.3. Example HTTP Connector

The Example HTTP connector has a simple goal: it reads comma-separateddata over HTTP. For example, if you have a large amount of data in aCSV format, you can point the example HTTP connector at this data andwrite a SQL query to process it.

Code

The Example HTTP connector can be found in the presto-example-httpdirectory in the root of the Presto source tree.

Plugin Implementation

The plugin implementation in the Example HTTP connector looks verysimilar to other plugin implementations. Most of the implementation isdevoted to handling optional configuration and the only function ofinterest is the following:

@Overridepublic Iterable<ConnectorFactory> getConnectorFactories(){    return ImmutableList.of(new ExampleConnectorFactory());}

Note that the ImmutableList class is a utility class from Guava.

As with all connectors, this plugin overrides the getConnectorFactories() methodand returns an ExampleConnectorFactory.

ConnectorFactory Implementation

In Presto, the primary object that handles the connection betweenPresto and a particular type of data source is the Connector object,which are created using ConnectorFactory.

This implementation is available in the class ExampleConnectorFactory.The first thing the connector factory implementation does is specify thename of this connector. This is the same string used to reference thisconnector in Presto configuration.

@Overridepublic String getName(){    return "example-http";}

The real work in a connector factory happens in the create()method. In the ExampleConnectorFactory class, the create() methodconfigures the connector and then asks Guice to create the object.This is the meat of the create() method without parameter validationand exception handling:

// A plugin is not required to use Guice; it is just very convenient
Bootstrap app = new Bootstrap(
        new JsonModule(),
        new ExampleModule(catalogName));
 
Injector injector = app
        .strictConfig()
        .doNotInitializeLogging()
        .setRequiredConfigurationProperties(requiredConfig)
        .initialize();
 
return injector.getInstance(ExampleConnector.class);

Connector: ExampleConnector

This class allows Presto to obtain references to the various servicesprovided by the connector.

Metadata: ExampleMetadata

This class is responsible for reporting table names, table metadata,column names, column metadata and other information about the schemasthat are provided by this connector. ConnectorMetadata is also calledby Presto to ensure that a particular connector can understand andhandle a given table name.

The ExampleMetadata implementation delegates many of these calls toExampleClient, a class that implements much of the core functionalityof the connector.

Split Manager: ExampleSplitManager

The split manager partitions the data for a table into the individualchunks that Presto will distribute to workers for processing.In the case of the Example HTTP connector, each table contains one ormore URIs pointing at the actual data. One split is created per URI.

Record Set Provider: ExampleRecordSetProvider

The record set provider creates a record set which in turn creates arecord cursor that returns the actual data to Presto.ExampleRecordCursor reads data from a URI via HTTP. Each linecorresponds to a single row. Lines are split on comma into individualfield values which are then returned to Presto.