.. _guide-distributed-preprocessing:

7.1 Data Preprocessing
------------------------------------------

Before launching training jobs, DGL requires the input data to be partitioned
and distributed to the target machines. For relatively small graphs, DGL
provides a partitioning API :func:`~dgl.distributed.partition_graph` that
partitions an in-memory :class:`~dgl.DGLGraph` object. It supports
multiple partitioning algorithms such as random partitioning and
`Metis <http://glaros.dtc.umn.edu/gkhome/views/metis>`__.
The benefit of Metis partitioning is that it can generate partitions with
minimal edge cuts to reduce network communication for distributed training and
inference. DGL uses the latest version of Metis with the options optimized for
the real-world graphs with power-law distribution. After partitioning, the API
constructs the partitioned results in a format that is easy to load during the
training. For example,

.. code-block:: python

    import dgl

    g = ...  # create or load a DGLGraph object
    dgl.distributed.partition_graph(g, 'mygraph', 2, 'data_root_dir')

will outputs the following data file.

.. code-block:: none

    data_root_dir/
      |-- mygraph.json          # metadata JSON. File name is the given graph name.
      |-- part0/                # data for partition 0
      |  |-- node_feats.dgl     # node features stored in binary format
      |  |-- edge_feats.dgl     # edge features stored in binary format
      |  |-- graph.dgl          # graph structure of this partition stored in binary format
      |
      |-- part1/                # data for partition 1
         |-- node_feats.dgl
         |-- edge_feats.dgl
         |-- graph.dgl

Chapter :ref:`guide-distributed-partition` covers more details about the
partition format. To distribute the partitions to a cluster, users can either save
the data in some shared folder accessible by all machines, or copy the metadata
JSON as well as the corresponding partition folder ``partX`` to the X^th machine.

Using :func:`~dgl.distributed.partition_graph` requires an instance with large enough
CPU RAM to hold the entire graph structure and features, which may not be viable for
graphs with hundreds of billions of edges or large features. We describe how to use
the *parallel data preparation pipeline* for such cases next.

Parallel Data Preparation Pipeline
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

To handle massive graph data that cannot fit in the CPU RAM of a
single machine, DGL utilizes data chunking and parallel processing to reduce
memory footprint and running time. The figure below illustrates the
pipeline:

.. figure:: https://data.dgl.ai/asset/image/guide_7_distdataprep.png

* The pipeline takes input data stored in *Chunked Graph Format* and
  produces and dispatches data partitions to the target machines.
* **Step.1 Graph Partitioning:** It calculates the ownership of each partition
  and saves the results as a set of files called *partition assignment*.
  To speedup the step, some algorithms (e.g., ParMETIS) support parallel computing
  using multiple machines.
* **Step.2 Data Dispatching:** Given the partition assignment, the step then
  physically partitions the graph data and dispatches them to the machines user
  specified. It also converts the graph data into formats that are suitable for
  distributed training and evaluation.

The whole pipeline is modularized so that each step can be invoked
individually. For example, users can replace Step.1 with some custom graph partition
algorithm as long as it produces partition assignment files
correctly.

.. _guide-distributed-prep-chunk:
Chunked Graph Format
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

To run the pipeline, DGL requires the input graph to be stored in multiple data
chunks.  Each data chunk is the unit of data preprocessing and thus should fit
into CPU RAM.  In this section, we use the MAG240M-LSC data from `Open Graph
Benchmark <https://ogb.stanford.edu/docs/lsc/mag240m/>`__  as an example to
describe the overall design, followed by a formal specification and
tips for creating data in such format.

Example: MAG240M-LSC
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The MAG240M-LSC graph is a heterogeneous academic graph
extracted from the Microsoft Academic Graph (MAG), whose schema diagram is
illustrated below:

.. figure:: https://data.dgl.ai/asset/image/guide_7_mag240m.png

Its raw data files are organized as follows:

.. code-block:: none

    /mydata/MAG240M-LSC/
      |-- meta.pt   # # A dictionary of the number of nodes for each type saved by torch.save,
      |             # as well as num_classes
      |-- processed/
        |-- author___affiliated_with___institution/
        |  |-- edge_index.npy            # graph, 713 MB
        |
        |-- paper/
        |  |-- node_feat.npy             # feature, 187 GB, (numpy memmap format)
        |  |-- node_label.npy            # label, 974 MB
        |  |-- node_year.npy             # year, 974 MB
        |
        |-- paper___cites___paper/
        |  |-- edge_index.npy            # graph, 21 GB
        |
        |-- author___writes___paper/
           |-- edge_index.npy            # graph, 6GB

The graph has three node types (``"paper"``, ``"author"`` and ``"institution"``),
three edge types/relations (``"cites"``, ``"writes"`` and ``"affiliated_with"``). The
``"paper"`` nodes have three attributes (``"feat"``, ``"label"``, ``"year"'``), while
other types of nodes and edges are featureless. Below shows the data files when
it is stored in DGL Chunked Graph Format:

.. code-block:: none

    /mydata/MAG240M-LSC_chunked/
      |-- metadata.json            # metadata json file
      |-- edges/                   # stores edge ID data
      |  |-- writes-part1.csv
      |  |-- writes-part2.csv
      |  |-- affiliated_with-part1.csv
      |  |-- affiliated_with-part2.csv
      |  |-- cites-part1.csv
      |  |-- cites-part1.csv
      |
      |-- node_data/               # stores node feature data
         |-- paper-feat-part1.npy
         |-- paper-feat-part2.npy
         |-- paper-label-part1.npy
         |-- paper-label-part2.npy
         |-- paper-year-part1.npy
         |-- paper-year-part2.npy

All the data files are chunked into two parts, including the edges of each relation
(e.g., writes, affiliates, cites) and node features. If the graph has edge features,
they will be chunked into multiple files too. All ID data are stored in
CSV (we will illustrate the contents soon) while node features are stored in
numpy arrays.

The ``metadata.json`` stores all the metadata information such as file names
and chunk sizes (e.g., number of nodes, number of edges).

.. code-block:: python

    {
       "graph_name" : "MAG240M-LSC",  # given graph name
       "node_type": ["author", "paper", "institution"],
       "num_nodes_per_chunk": [
           [61191556, 61191556],      # number of author nodes per chunk
           [61191553, 61191552],      # number of paper nodes per chunk
           [12861, 12860]             # number of institution nodes per chunk
       ],
       # The edge type name is a colon-joined string of source, edge, and destination type.
       "edge_type": [
           "author:writes:paper",
           "author:affiliated_with:institution",
           "paper:cites:paper"
       ],
       "num_edges_per_chunk": [
           [193011360, 193011360],    # number of author:writes:paper edges per chunk
           [22296293, 22296293],      # number of author:affiliated_with:institution edges per chunk
           [648874463, 648874463]     # number of paper:cites:paper edges per chunk
       ],
       "edges" : {
            "author:write:paper" : {  # edge type
                 "format" : {"name": "csv", "delimiter": " "},
                 # The list of paths. Can be relative or absolute.
                 "data" : ["edges/writes-part1.csv", "edges/writes-part2.csv"]
            },
            "author:affiliated_with:institution" : {
                 "format" : {"name": "csv", "delimiter": " "},
                 "data" : ["edges/affiliated_with-part1.csv", "edges/affiliated_with-part2.csv"]
            },
            "author:affiliated_with:institution" : {
                 "format" : {"name": "csv", "delimiter": " "},
                 "data" : ["edges/cites-part1.csv", "edges/cites-part2.csv"]
            }
       },
       "node_data" : {
            "paper": {       # node type
                 "feat": {   # feature key
                     "format": {"name": "numpy"},
                     "data": ["node_data/paper-feat-part1.npy", "node_data/paper-feat-part2.npy"]
                 },
                 "label": {   # feature key
                     "format": {"name": "numpy"},
                     "data": ["node_data/paper-label-part1.npy", "node_data/paper-label-part2.npy"]
                 },
                 "year": {   # feature key
                     "format": {"name": "numpy"},
                     "data": ["node_data/paper-year-part1.npy", "node_data/paper-year-part2.npy"]
                 }
            }
       },
       "edge_data" : {}  # MAG240M-LSC does not have edge features
    }

There are three parts in ``metadata.json``:

* Graph schema information and chunk sizes, e.g., ``"node_type"`` , ``"num_nodes_per_chunk"``, etc.
* Edge index data under key ``"edges"``.
* Node/edge feature data under keys ``"node_data"`` and ``"edge_data"``.

The edge index files contain edges in the form of node ID pairs:

.. code-block:: bash

    # writes-part1.csv
    0 0
    0 1
    0 20
    0 29
    0 1203
    ...

Specification
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In general, a chunked graph data folder just needs a ``metadata.json`` and a
bunch of data files. The folder structure in the MAG240M-LSC example is not a
strict requirement as long as ``metadata.json`` contains valid file paths.

``metadata.json`` top-level keys:

* ``graph_name``: String. Unique name used by :class:`dgl.distributed.DistGraph`
  to load graph.
* ``node_type``: List of string. Node type names.
* ``num_nodes_per_chunk``: List of list of integer. For graphs with :math:`T` node
  types stored in :math:`P` chunks, the value contains :math:`T` integer lists.
  Each list contains :math:`P` integers, which specify the number of nodes
  in each chunk.
* ``edge_type``: List of string. Edge type names in the form of
  ``<source node type>:<relation>:<destination node type>``.
* ``num_edges_per_chunk``: List of list of integer. For graphs with :math:`R` edge
  types stored in :math:`P` chunks, the value contains :math:`R` integer lists.
  Each list contains :math:`P` integers, which specify the number of edges
  in each chunk.
* ``edges``: Dict of ``ChunkFileSpec``. Edge index files.
  Dictionary keys are edge type names in the form of
  ``<source node type>:<relation>:<destination node type>``.
* ``node_data``: Dict of ``ChunkFileSpec``. Data files that store node attributes.
  Dictionary keys are node type names.
* ``edge_data``: Dict of ``ChunkFileSpec``. Data files that store edge attributes.
  Dictionary keys are edge type names in the form of
  ``<source node type>:<relation>:<destination node type>``.

``ChunkFileSpec`` has two keys:

* ``format``: File format. Depending on the format ``name``, users can configure more
  details about how to parse each data file.
    - ``"csv"``: CSV file. Use the ``delimiter`` key to specify delimiter in use.
    - ``"numpy"``: NumPy array binary file created by :func:`numpy.save`.
* ``data``: List of string. File path to each data chunk. Support absolute path.

Tips for making chunked graph data
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Depending on the raw data, the implementation could include:

* Construct graphs out of non-structured data such as texts or tabular data.
* Augment or transform the input graph struture or features. E.g., adding reverse
  or self-loop edges, normalizing features, etc.
* Chunk the input graph structure and features into multiple data files so that
  each one can fit in CPU RAM for subsequent preprocessing steps.

To avoid running into out-of-memory error, it is recommended to process graph
structures and feature data separately. Processing one chunk at a time can also
reduce the maximal runtime memory footprint. As an example, DGL provides a
`tools/chunk_graph.py
<https://github.com/dmlc/dgl/blob/master/tools/chunk_graph.py>`_ script that
chunks an in-memory feature-less :class:`~dgl.DGLGraph` and feature tensors
stored in :class:`numpy.memmap`.


.. _guide-distributed-prep-partition:
Step.1 Graph Partitioning
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

This step reads the chunked graph data and calculates which partition each node
should belong to. The results are saved in a set of *partition assignment files*.
For example, to randomly partition MAG240M-LSC to two parts, run the
``partition_algo/random.py`` script in the ``tools`` folder:

.. code-block:: bash

    python /my/repo/dgl/tools/partition_algo/random.py
        --metadata /mydata/MAG240M-LSC_chunked/metadata.json
        --output_path /mydata/MAG240M-LSC_2parts/
        --num_partitions 2

, which outputs files as follows:

.. code-block:: none

    MAG240M-LSC_2parts/
      |-- paper.txt
      |-- author.txt
      |-- institution.txt

Each file stores the partition assignment of the corresponding node type.
The contents are the partition ID of each node stored in lines, i.e., line i is
the partition ID of node i.

.. code-block:: bash

    # paper.txt
    0
    1
    1
    0
    0
    1
    0
    ...

.. note::

    DGL currently requires the number of data chunks and the number of partitions to be the same.

Despite its simplicity, random partitioning may result in frequent
cross-machine communication.  Check out chapter
:ref:`guide-distributed-partition` for more advanced options.

Step.2 Data Dispatching
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

DGL provides a ``dispatch_data.py`` script to physically partition the data and
dispatch partitions to each training machines. It will also convert the data
once again to data objects that can be loaded by DGL training processes
efficiently. The entire step can be further accelerated using multi-processing.

.. code-block:: bash

    python /myrepo/dgl/tools/dispatch_data.py         \
       --in-dir /mydata/MAG240M-LSC_chunked/          \
       --partitions-dir /mydata/MAG240M-LSC_2parts/   \
       --out-dir data/MAG_LSC_partitioned            \
       --ip-config ip_config.txt

* ``--in-dir`` specifies the path to the folder of the input chunked graph data produced
* ``--partitions-dir`` specifies the path to the partition assignment folder produced by Step.1.
* ``--out-dir`` specifies the path to stored the data partition on each machine.
* ``--ip-config`` specifies the IP configuration file of the cluster.

An example IP configuration file is as follows:

.. code-block:: bash

    172.31.19.1
    172.31.23.205

During data dispatching, DGL assumes that the combined CPU RAM of the cluster
is able to hold the entire graph data. Moreover, the number of machines (IPs) must be the
same as the number of partitions. Node ownership is determined by the result
of partitioning algorithm where as for edges the owner of the destination node
also owns the edge as well.
