如果两个顶点之间存在路径，则称两个顶点为连通的。如果无向图G中任意两个顶点都是连通的，则为连通图，否则称为非连通图。其顶点个数极大的连通子图称为连通分量。
本算法计算每个点的连通分量成员，最后输出顶点值中包含最小顶点ID的连通分量。将最小顶点ID沿着边传播到连通分量的所有顶点。
代码示例
连通分量的代码，如下所示。
import java.io.IOException;
import com.aliyun.odps.data.TableInfo;
import com.aliyun.odps.graph.ComputeContext;
import com.aliyun.odps.graph.GraphJob;
import com.aliyun.odps.graph.GraphLoader;
import com.aliyun.odps.graph.MutationContext;
import com.aliyun.odps.graph.Vertex;
import com.aliyun.odps.graph.WorkerContext;
import com.aliyun.odps.graph.examples.SSSP.MinLongCombiner;
import com.aliyun.odps.io.LongWritable;
import com.aliyun.odps.io.NullWritable;
import com.aliyun.odps.io.WritableRecord;
/**
* Compute the connected component membership of each vertex and output
* each vertex which's value containing the smallest id in the connected
* component containing that vertex.
*
* Algorithm: propagate the smallest vertex id along the edges to all
* vertices of a connected component.
*
*/
public class ConnectedComponents {
public static class CCVertex extends
Vertex<LongWritable, LongWritable, NullWritable, LongWritable> {
@Override
public void compute(
ComputeContext<LongWritable, LongWritable, NullWritable, LongWritable> context,
Iterable<LongWritable> msgs) throws IOException {
if (context.getSuperstep() == 0L) {
this.setValue(getId());
context.sendMessageToNeighbors(this, getValue());
return;
}
long minID = Long.MAX_VALUE;
for (LongWritable id : msgs) {
if (id.get() < minID) {
minID = id.get();
}
}
if (minID < this.getValue().get()) {
this.setValue(new LongWritable(minID));
context.sendMessageToNeighbors(this, getValue());
} else {
this.voteToHalt();
}
}
/**
* Output Table Description:
* +-----------------+----------------------------------------+
* | Field | Type | Comment |
* +-----------------+----------------------------------------+
* | v | bigint | vertex id |
* | minID | bigint | smallest id in the connected component |
* +-----------------+----------------------------------------+
*/
@Override
public void cleanup(
WorkerContext<LongWritable, LongWritable, NullWritable, LongWritable> context)
throws IOException {
context.write(getId(), getValue());
}
}
/**
* Input Table Description:
* +-----------------+----------------------------------------------------+
* | Field | Type | Comment |
* +-----------------+----------------------------------------------------+
* | v | bigint | vertex id |
* | es | string | comma separated target vertex id of outgoing edges |
* +-----------------+----------------------------------------------------+
*
* Example:
* For graph:
* 1 ----- 2
* | |
* 3 ----- 4
* Input table:
* +-----------+
* | v | es |
* +-----------+
* | 1 | 2,3 |
* | 2 | 1,4 |
* | 3 | 1,4 |
* | 4 | 2,3 |
* +-----------+
*/
public static class CCVertexReader extends
GraphLoader<LongWritable, LongWritable, NullWritable, LongWritable> {
@Override
public void load(
LongWritable recordNum,
WritableRecord record,
MutationContext<LongWritable, LongWritable, NullWritable, LongWritable> context)
throws IOException {
CCVertex vertex = new CCVertex();
vertex.setId((LongWritable) record.get(0));
String[] edges = record.get(1).toString().split(",");
for (int i = 0; i < edges.length; i++) {
long destID = Long.parseLong(edges[i]);
vertex.addEdge(new LongWritable(destID), NullWritable.get());
}
context.addVertexRequest(vertex);
}
}
public static void main(String[] args) throws IOException {
if (args.length < 2) {
System.out.println("Usage: <input> <output>");
System.exit(-1);
}
GraphJob job = new GraphJob();
job.setGraphLoaderClass(CCVertexReader.class);
job.setVertexClass(CCVertex.class);
job.setCombinerClass(MinLongCombiner.class);
job.addInput(TableInfo.builder().tableName(args[0]).build());
job.addOutput(TableInfo.builder().tableName(args[1]).build());
long startTime = System.currentTimeMillis();
job.run();
System.out.println("Job Finished in "
+ (System.currentTimeMillis() - startTime) / 1000.0 + " seconds");
}
}