Levels breadth-first search (BFS) algorithm

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The levels variant of breadth-first search is an algorithm for searching nodes from a starting node or nodes in breadth-first order. From there it performs a breadth-first search and records the hop level from the starting node of each node that it finds.

It returns a key column of nodes, and a value column containing the level values of those key nodes.

The level of a source node is 0. Note that because every source node passed into breadth-first search levels initiates its own execution of the algorithm, your queries should filter to a subset of the graph before executing BFS levels whenever possible.

.bfs.levels   syntax

CALL neptune.algo.bfs.levels(
  [source-node list (required)],
  {
    edgeLabels: [list of edge labels for filtering (optional)],
    vertexLabel: a node label for filtering (optional),
    maxDepth: maximum number of hops to traverse from a source node (optional),
    traversalDirection: traversal direction (optional),
    concurrency: number of threads to use (optional)
  }
)
YIELD the outputs to generate (source and/or node)
RETURN the outputs to return

.bfs.levels   inputs

• a source node list   (required)   –   type: Node[] or NodeId[];   default: none.

  The source-node list contains the node or nodes used as the starting locations for the algorithm.

  • Each starting node triggers its own execution of the algorithm.

  • If the source-node list is empty then the query result is also empty.

  • If the algorithm is called following a MATCH clause (this is known as query-algorithm integration), the output of the MATCH clause is used as the source-node list for the algorithm.

• a configuration object that contains:

  • edgeLabels   (optional)   –   type: a list of edge label strings;   example: ["route", ...];   default: no edge filtering.

    To filter on one more edge labels, provide a list of the ones to filter on. If no edgeLabels field is provided then all edge labels are processed during traversal.

  • vertexLabel   (optional)   –   type: string;   example: "airport";  default: no node filtering.

    If you provide a node label to filter on then only nodes matching that label will be traversed. This does not, however, filter out any nodes in the source node list.

  • maxDepth   (optional)   –   type: positive integer or 0 or -1;   default: -1.

    The maximum number of hops to traverse from a source node. If set at -1 then there's no maximum depth limit. If set to 0, only the nodes in the source node list are returned.

  • traversalDirection   (optional)   –   type: string;   default: "outbound".

    The direction of edge to follow. Must be one of: "inbound", "outbound", or "both".

  • concurrency   (optional)   –   type: 0 or 1;   default: 0.

    Controls the number of concurrent threads used to run the algorithm.

    If set to 0, uses all available threads to complete execution of the individual algorithm invocation. If set to 1, uses a single thread. This can be useful when requiring the invocation of many algorithms concurrently.

.bfs.levels   outputs

The .bfs.levels algorithm returns:

• source   –   type: Node[].

  The source nodes.

• node   –   type: Node[].

  The nodes that the algorithm traversed from each source node.

• level   –   type: integer[].

  The hop levels of those traversed nodes.

.bfs.levels   standalone query examples

The examples below are standalone examples, where the query provides an explicit source node list.

A query like this one would return an empty result because the source list is empty:

CALL neptune.algo.bfs.levels(
  ["101", "102"],
  {
    edgeLabels: ["route"],
    vertexLabel: "airport",
    maxDepth: 6,
    traversalDirection: "both",
    concurrency: 2
  }
)
YIELD node

You can run the algorithm using the execute-query operation in the AWS CLI like this:

aws neptune-graph execute-query \
  --graph-identifier ${graphIdentifier} \
  --query-string 'CALL neptune.algo.bfs.levels(["101", "102"], {edgeLabels: ["route"]})' \
  --language open_cypher \
  /tmp/out.txt

By default, all the outputs are generated. You can use YIELD to specify which of those outputs to generate. For example, to generate only the "node" and level outputs:

CALL neptune.algo.bfs.levels(["101"], {edgeLabels: ["route"]}) YIELD node, level

.bfs.levels   query integration examples

The examples below are query integration examples, where .bfs.levels follows a MATCH clause and uses the output of the MATCH clause as its source node list:

MATCH (n) WITH n LIMIT 5
CALL neptune.algo.bfs.levels(n, {edgeLabels: ["route"]})
YIELD node, level
RETURN n, node, level

This query illustrates various ways to constrain the input and output:

MATCH (n) where id(n)="101"
CALL neptune.algo.bfs.levels(n, { edgeLabel: "route", maxDepth: 2})
YIELD node, level WHERE node.city CONTAINS "New"
RETURN n.city, node.city, level

Warning

It is not good practice to use MATCH(n) without restriction in query integrations. Keep in mind that every node returned by the MATCH(n) clause invokes the algorithm once, which can result a very long-running query if a large number of nodes is returned. Use LIMIT or put conditions on the MATCH clause to restrict its output appropriately.

Sample .bfs.levels output

Here is an example of the output returned by .bfs.levels when run against the sample air-routes dataset [nodes], and sample air-routes dataset [edges], when using the following query:

aws neptune-graph execute-query \
  --graph-identifier ${graphIdentifier} \
  --query-string "CALL neptune.algo.bfs.levels(['101'], {maxDepth: 1}) yield source, node, level return source, node, level limit 2" \
  --language open_cypher \
  /tmp/out.txt
  
cat /tmp/out.txt   
{
  "results": [
    {
      "source": {
        "~id": "101",
        "~entityType": "node",
        "~labels": ["airport"],
        "~properties": {
          "lat": 13.6810998916626,
          "elev": 5,
          "longest": 13123,
          "city": "Bangkok",
          "type": "airport",
          "region": "TH-10",
          "desc": "Suvarnabhumi Bangkok International Airport",
          "code": "BKK",
          "lon": 100.747001647949,
          "country": "TH",
          "icao": "VTBS",
          "runways": 2
        }
      },
      "node": {
        "~id": "1483",
        "~entityType": "node",
        "~labels": ["airport"],
        "~properties": {
          "lat": 39.49,
          "elev": 4557,
          "longest": 9186,
          "city": "Ordos",
          "type": "airport",
          "region": "CN-15",
          "desc": "Ordos Ejin Horo Airport",
          "code": "DSN",
          "lon": 109.861388889,
          "country": "CN",
          "icao": "ZBDS",
          "runways": 1
        }
      },
      "level": 1
    },
    {
      "source": {
        "~id": "101",
        "~entityType": "node",
        "~labels": ["airport"],
        "~properties": {
          "lat": 13.6810998916626,
          "elev": 5,
          "longest": 13123,
          "city": "Bangkok",
          "type": "airport",
          "region": "TH-10",
          "desc": "Suvarnabhumi Bangkok International Airport",
          "code": "BKK",
          "lon": 100.747001647949,
          "country": "TH",
          "icao": "VTBS",
          "runways": 2
        }
      },
      "node": {
        "~id": "103",
        "~entityType": "node",
        "~labels": ["airport"],
        "~properties": {
          "lat": 55.972599029541,
          "elev": 622,
          "longest": 12139,
          "city": "Moscow",
          "type": "airport",
          "region": "RU-MOS",
          "desc": "Moscow, Sheremetyevo International Airport",
          "code": "SVO",
          "lon": 37.4146003723145,
          "country": "RU",
          "icao": "UUEE",
          "runways": 2
        }
      },
      "level": 1
    }
  ]
}