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(catalogName));
Injector injector = app
.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.