In this section, we will lay out the overall system architecture for Modin, as well as go into detail about the component design, implementation and other important details. This document also contains important reference information for those interested in contributing new functionality, bugfixes and enhancements.
High-Level Architectural View¶
The diagram below outlines the general layered view to the components of Modin with a short description of each major section of the documentation following.
Modin is logically separated into different layers that represent the hierarchy of a typical Database Management System. Abstracting out each component allows us to individually optimize and swap out components without affecting the rest of the system. We can implement, for example, new compute kernels that are optimized for a certain type of data and can simply plug it in to the existing infrastructure by implementing a small interface. It can still be distributed by our choice of compute engine with the logic internally.
If we look to the overall class structure of the Modin system from very top, it will look to something like this:
The user - Data Scientist interacts with the Modin system by sending interactive or batch commands through API and Modin executes them using various backend execution engines: Ray, Dask and MPI are currently supported.
If we click down to the next level of details we will see that inside Modin the layered architecture is implemented using several interacting components:
For the simplicity the other backend systems - Dask and MPI are omitted and only Ray backend is shown.
- Dataframe subsystem is the backbone of the dataframe holding and query compilation. It is responsible for dispatching the ingress/egress to the appropriate module, getting the Pandas API and calling the query compiler to convert calls to the internal intermediate Dataframe Algebra.
- Data Ingress/Egress Module is working in conjunction with Dataframe and Partitions subsystem to read data split into partitions and send data into the appropriate node for storing.
- Query Planner is subsystem that translates the Pandas API to intermediate Dataframe Algebra representation DAG and performs an initial set of optimizations.
- Query Executor is responsible for getting the Dataframe Algebra DAG, performing further optimizations based on a selected backend execution subsystem and mapping or compiling the Dataframe Algebra DAG to and actual execution sequence.
- Backends module is responsible for mapping the abstract operation to an actual executor call, e.g. Pandas, PyArrow, custom backend.
- Orchestration subsystem is responsible for spawning and controlling the actual execution environment for the selected backend. It spawns the actual nodes, fires up the execution environment, e.g. Ray, monitors the state of executors and provides telemetry
The Modin DataFrame architecture follows in the footsteps of modern architectures for database and high performance matrix systems. We chose a partitioning schema that partitions along both columns and rows because it gives Modin flexibility and scalability in both the number of columns and the number of rows supported. The following figure illustrates this concept.
We currently use the
pandas.Index object for both indexing columns and rows. In the
future, we will implement a distributed, pandas-compatible Index object in order remove
this scaling limitation from the system. It does not start to become a problem until you
are operating on more than 10’s of billions of columns or rows, so most workloads will
not be affected by this scalability limit. Important note: If you are using the
default index (
pandas.RangeIndex) there is a fixed memory overhead (~200 bytes) and
there will be no scalability issues with the index.
The API is the outer-most layer that faces users. The majority of our current effort is spent implementing the components of the pandas API. We have implemented a toy example for a sqlite API as a proof of concept, but this isn’t ready for usage/testing. There are also plans to expose the Modin DataFrame API as a reduced API set that encompasses the entire pandas/dataframe API.
The Query Compiler receives queries from the pandas API layer. The API layer’s responsibility is to ensure clean input to the Query Compiler. The Query Compiler must have knowledge of the compute kernels/in-memory format of the data in order to efficiently compile the queries.
The Query Compiler is responsible for sending the compiled query to the Modin DataFrame. In this design, the Query Compiler does not have information about where or when the query will be executed, and gives the control of the partition layout to the Modin DataFrame.
In the interest of reducing the pandas API, the Query Compiler layer closely follows the pandas API, but cuts out a large majority of the repetition.
At this layer, operations can be performed lazily. Currently, Modin executes most
operations eagerly in an attempt to behave as pandas does. Some operations, e.g.
transpose are expensive and create full copies of the data in-memory. In these
cases, we can wait until another operation triggers computation. In the future, we plan
to add additional query planning and laziness to Modin to ensure that queries are
The structure of the Modin DataFrame is extensible, such that any operation that could be better optimized for a given backend can be overridden and optimized in that way.
This layer has a significantly reduced API from the QueryCompiler and the user-facing API. Each of these APIs represents a single way of performing a given operation or behavior. Some of these are expanded for convenience/understanding. The API abstractions are as follows:
Modin DataFrame API¶
mask: Indexing/masking/selecting on the data (by label or by integer index).
copy: Create a copy of the data.
mapreduce: Reduce the dimension of the data.
foldreduce: Reduce the dimension of the data, but entire column/row information is needed.
map: Perform a map.
fold: Perform a fold.
apply_<type>: Apply a function that may or may not change the shape of the data.
full_axis: Apply a function requires knowledge of the entire axis.
full_axis_select_indices: Apply a function performed on a subset of the data that requires knowledge of the entire axis.
select_indices: Apply a function to a subset of the data. This is mainly used for indexing.
binary_op: Perform a function between two dataframes.
concat: Append one or more dataframes to either axis of this dataframe.
transpose: Swap the axes (columns become rows, rows become columns).
groupby_reduce: Perform a reduction on each group.
groupby_apply: Apply a function to each group.
- take functions
head: Take the first
tail: Take the last
front: Take the first
back: Take the last
- import/export functions
from_pandas: Convert a pandas dataframe to a Modin dataframe.
to_pandas: Convert a Modin dataframe to a pandas dataframe.
to_numpy: Convert a Modin dataframe to a numpy array.
More documentation can be found internally in the code. This API is not complete, but represents an overwhelming majority of operations and behaviors.
The Modin DataFrame is responsible for the data layout and shuffling, partitioning, and serializing the tasks that get sent to each partition. Other implementations of the Modin DataFrame interface will have to handle these as well.
This layer is what Modin uses to perform computation on a partition of the data. The Modin DataFrame is designed to work with task parallel frameworks, but with some effort, a data parallel framework is possible.
These abstractions are not included in the above architecture, but are important to the internals of Modin.
The Partition Manager can change the size and shape of the partitions based on the type of operation. For example, certain operations are complex and require access to an entire column or row. The Partition Manager can convert the block partitions to row partitions or column partitions. This gives Modin the flexibility to perform operations that are difficult in row-only or column-only partitioning schemas.
Another important component of the Partition Manager is the serialization and shipment
of compiled queries to the Partitions. It maintains metadata for the length and width of
each partition, so when operations only need to operate on or extract a subset of the
data, it can ship those queries directly to the correct partition. This is particularly
important for some operations in pandas which can accept different arguments and
operations for different columns, e.g.
fillna with a dictionary.
This abstraction separates the actual data movement and function application from the DataFrame layer to keep the DataFrame API small and separately optimize the data movement and metadata management.
Partitions are responsible for managing a subset of the DataFrame. As is mentioned above, the DataFrame is partitioned both row and column-wise. This gives Modin scalability in both directions and flexibility in data layout. There are a number of optimizations in Modin that are implemented in the partitions. Partitions are specific to the execution framework and in-memory format of the data. This allows Modin to exploit potential optimizations across both of these. These optimizations are explained further on the pages specific to the execution framework.
Supported Execution Frameworks and Memory Formats¶
This is the list of execution frameworks and memory formats supported in Modin. If you would like to contribute a new execution framework or memory format, please see the documentation page on Contributing.