20.3. Example HTTP Connector

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

Code

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

Plugin Implementation

The plugin implementation in the Example HTTP connector looks very similar to other plugin implementations. Most of the implementation is devoted to handling optional configuration and the only function of interest is the following:

@Override
public 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() method and returns an ExampleConnectorFactory.

ConnectorFactory Implementation

In Presto, the primary object that handles the connection between Presto 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 the name of this connector. This is the same string used to reference this connector in Presto configuration.

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

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

// A plugin is not required to use Guice; it is just very convenient
Bootstrap app = new Bootstrap(
        new JsonModule(),
        new ExampleModule(connectorId));

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 services provided by the connector.

Metadata: ExampleMetadata

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

The ExampleMetadata implementation delegates many of these calls to ExampleClient, a class that implements much of the core functionality of the connector.

Split Manager: ExampleSplitManager

The split manager partitions the data for a table into the individual chunks that Presto will distribute to workers for processing. In the case of the Example HTTP connector, each table contains one or more 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 a record cursor that returns the actual data to Presto. ExampleRecordCursor reads data from a URI via HTTP. Each line corresponds to a single row. Lines are split on comma into individual field values which are then returned to Presto.