Tutorial: Develop algorithms in Python¶

Users may write their own algorithms if the built-in algorithms do not meet their needs. graphscope enables users to write algorithms both in the PIE and in Pregel programming models in a pure Python manner.

Writing Your Own Algorithms in PIE¶

To implement an algorithm in PIE model, a user just need to fulfill this class.

from graphscope.analytical.udf.decorators import pie
from graphscope.framework.app import AppAssets

@pie(vd_type="double", md_type="double")
class YourAlgorithm(AppAssets):
    @staticmethod
    def Init(frag, context):
        pass

    @staticmethod
    def PEval(frag, context):
        pass

    @staticmethod
    def IncEval(frag, context):
        pass

As shown in the code, users need to implement a class decorated with @pie and provides three sequential graph functions. In the class, the Init is a function to set the initial status. PEval is a sequential method for partial evaluation, and IncEval is a sequential function for incremental evaluation over the partitioned fragment. The full API of fragment can be found in Cython SDK API.

Let’s take SSSP as example, a user defined SSSP in PIE model may be like this.

from graphscope.analytical.udf.decorators import pie
from graphscope.framework.app import AppAssets

@pie(vd_type="double", md_type="double")
class SSSP_PIE(AppAssets):
    @staticmethod
    def Init(frag, context):
        v_label_num = frag.vertex_label_num()
        for v_label_id in range(v_label_num):
            nodes = frag.nodes(v_label_id)
            context.init_value(
                nodes, v_label_id, 1000000000.0, PIEAggregateType.kMinAggregate
            )
            context.register_sync_buffer(v_label_id, MessageStrategy.kSyncOnOuterVertex)

    @staticmethod
    def PEval(frag, context):
        src = int(context.get_config(b"src"))
        graphscope.declare(graphscope.Vertex, source)
        native_source = False
        v_label_num = frag.vertex_label_num()
        for v_label_id in range(v_label_num):
            if frag.get_inner_node(v_label_id, src, source):
                native_source = True
                break
        if native_source:
            context.set_node_value(source, 0)
        else:
            return
        e_label_num = frag.edge_label_num()
        for e_label_id in range(e_label_num):
            edges = frag.get_outgoing_edges(source, e_label_id)
            for e in edges:
                dst = e.neighbor()
                # use the third column of edge data as the distance between two vertices
                distv = e.get_int(2)
                if context.get_node_value(dst) > distv:
                    context.set_node_value(dst, distv)

    @staticmethod
    def IncEval(frag, context):
        v_label_num = frag.vertex_label_num()
        e_label_num = frag.edge_label_num()
        for v_label_id in range(v_label_num):
            iv = frag.inner_nodes(v_label_id)
            for v in iv:
                v_dist = context.get_node_value(v)
                for e_label_id in range(e_label_num):
                    es = frag.get_outgoing_edges(v, e_label_id)
                    for e in es:
                        u = e.neighbor()
                        u_dist = v_dist + e.get_int(2)
                        if context.get_node_value(u) > u_dist:
                            context.set_node_value(u, u_dist)

As shown in the code, users only need to design and implement sequential algorithm over a fragment, rather than considering the communication and message passing in the distributed setting. In this case, the classic dijkstra algorithm and its incremental version works for large graphs partitioned on a cluster.

Writing Algorithms in Pregel¶

In addition to the sub-graph based PIE model, graphscope supports vertex-centric Pregel model as well. You may develop an algorithms in Pregel model by implementing this.

from graphscope.analytical.udf.decorators import pregel
from graphscope.framework.app import AppAssets

@pregel(vd_type='double', md_type='double')
class YourPregelAlgorithm(AppAssets):

    @staticmethod
    def Init(v, context):
        pass

    @staticmethod
    def Compute(messages, v, context):
        pass

    @staticmethod
    def Combine(messages):
        pass

Differ from the PIE model, the decorator for this class is @pregel. And the functions to be implemented is defined on vertex, rather than the fragment. Take SSSP as example, the algorithm in Pregel model looks like this.

from graphscope.analytical.udf import pregel
from graphscope.framework.app import AppAssets

# decorator, and assign the types for vertex data, message data.
@pregel(vd_type="double", md_type="double")
class SSSP_Pregel(AppAssets):
    @staticmethod
    def Init(v, context):
        v.set_value(1000000000.0)

    @staticmethod
    def Compute(messages, v, context):
        src_id = context.get_config(b"src")
        cur_dist = v.value()
        new_dist = 1000000000.0
        if v.id() == src_id:
            new_dist = 0
        for message in messages:
            new_dist = min(message, new_dist)
        if new_dist < cur_dist:
            v.set_value(new_dist)
            for e_label_id in range(context.edge_label_num()):
                edges = v.outgoing_edges(e_label_id)
                for e in edges:
                    v.send(e.vertex(), new_dist + e.get_int(2))
        v.vote_to_halt()

    @staticmethod
    def Combine(messages):
        ret = 1000000000.0
        for m in messages:
            ret = min(ret, m)
        return ret

Using `math.h` Functions in Algorithms¶

GraphScope supports using C functions from math.h in user-defined algorithms, via the context.math interface. E.g.,

@staticmethod
def Init(v, context):
    v.set_value(context.math.sin(1000000000.0 * context.math.M_PI))

will be translated to the following efficient C code

... Init(...)

    v.set_value(sin(1000000000.0 * M_PI));

Run Your Own Algorithms¶

To run your own algorithms, you may trigger it in place where you defined it.

import graphscope
from graphscope.dataset import load_p2p_network

g = load_p2p_network(generate_eid=False)

# load my algorithm
my_app = SSSP_Pregel()

# run my algorithm over a graph and get the result.
# Here the `src` is correspondent to the `context.get_config(b"src")`
ret = my_app(g, src="6")

After developing and testing, you may want to save it for the future use.

SSSP_Pregel.to_gar("/tmp/my_sssp_pregel.gar")

Later, you can load your own algorithm from the gar package.

from graphscope.framework.app import load_app

# load my algorithm from a gar package
my_app = load_app("/tmp/my_sssp_pregel.gar")

# run my algorithm over a graph and get the result.
ret = my_app(g, src="6")

Tutorial: Develop algorithms in Python¶

Writing Your Own Algorithms in PIE¶

Writing Algorithms in Pregel¶

Using math.h Functions in Algorithms¶

Run Your Own Algorithms¶

Using `math.h` Functions in Algorithms¶