PandasOnRay Execution#

Queries that perform data transformation, data ingress or data egress using the pandas on Ray execution pass through the Modin components detailed below.

To enable pandas on Ray execution, please refer to the usage section in pandas on Ray.

Data Transformation#

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When a user calls any DataFrame API, a query starts forming at the API layer to be executed at the Execution layer. The API layer is responsible for processing the query appropriately, for example, determining whether the final result should be a DataFrame or Series object. This layer is also responsible for sanitizing the input to the PandasQueryCompiler, e.g. validating a parameter from the query and defining specific intermediate values to provide more context to the query compiler. The PandasQueryCompiler is responsible for processing the query, received from the DataFrame API layer, to determine how to apply it to a subset of the data - either cell-wise or along an axis-wise partition backed by the pandas storage format. The PandasQueryCompiler maps the query to one of the Core Algebra Operators of the PandasOnRayDataframe which inherits generic functionality from the GenericRayDataframe and the PandasDataframe.

PandasOnRay Dataframe implementation#

Modin implements Dataframe, PartitionManager, VirtualPartition (a specific kind of AxisPartition with the capability to combine smaller partitions into the one “virtual”) and Partition classes specifically for the PandasOnRay execution:

Data Ingress#

../../../../../../../_images/pandas_on_ray_data_ingress.svg

Data Egress#

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When a user calls any IO function from the modin.pandas.io module, the API layer queries the FactoryDispatcher which defines a factory specific for the execution, namely, the PandasOnRayFactory. The factory, in turn, exposes the PandasOnRayIO class whose responsibility is to perform a parallel read/write from/to a file.

When reading data from a CSV file, for example, the PandasOnRayIO class forwards the user query to the _read() method of CSVDispatcher, where the query’s parameters are preprocessed to check if they are supported by the execution (defaulting to pandas if they are not) and computes some metadata common for all partitions to be read. Then, the file is split into row chunks, and this data is used to launch remote tasks on the Ray workers via the deploy() method of RayTask. On each Ray worker, the PandasCSVParser parses data. After the remote tasks are finished, additional result postprocessing is performed, and a new query compiler with the data read is returned.

When writing data to a CSV file, for example, the PandasOnRayIO processes the user query to execute it on Ray workers. Then, the PandasOnRayIO asks the PandasOnRayDataframe to decompose the data into row-wise partitions that will be written into the file in parallel in Ray workers.