shortest

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The shortest path between a source (from) node and destination (to) node can be found using the keyword shortest for the query block name. It requires the source node UID, destination node UID and the predicates (at least one) that have to be considered for traversal. A shortest query block returns the shortest path under _path_ in the query response. The path can also be stored in a variable which is used in other query blocks.

K-Shortest Path queries

By default the shortest path is returned. With numpaths: k, and k > 1, the k-shortest paths are returned. Cyclical paths are pruned out from the result of k-shortest path query. With depth: n, the paths up to n depth away are returned.

If no predicates are specified in the shortest block, no path can be fetched as no edge is traversed.

If you’re seeing queries take a long time, you can set a gRPC deadline to stop the query after a certain amount of time.

For example:

curl localhost:8080/alter -XPOST -d $'
    name: string @index(exact) .
' | python -m json.tool | less

{
  set {
    _:a <friend> _:b (weight=0.1) .
    _:b <friend> _:c (weight=0.2) .
    _:c <friend> _:d (weight=0.3) .
    _:a <friend> _:d (weight=1) .
    _:a <name> "Alice" .
    _:a <dgraph.type> "Person" .
    _:b <name> "Bob" .
    _:b <dgraph.type> "Person" .
    _:c <name> "Tom" .
    _:c <dgraph.type> "Person" .
    _:d <name> "Mallory" .
    _:d <dgraph.type> "Person" .
  }
}

The shortest path between Alice and Mallory (assuming UIDs 0x2 and 0x5 respectively) can be found with this query:

{
 path as shortest(from: 0x2, to: 0x5) {
  friend
 }
 path(func: uid(path)) {
   name
 }
}

Which returns the following results.

without considering the weight facet, each edges’ weight is considered as 1

{
  "data": {
    "path": [
      {
        "name": "Alice"
      },
      {
        "name": "Mallory"
      }
    ],
    "_path_": [
      {
        "uid": "0x2",
        "friend": [
          {
            "uid": "0x5"
          }
        ]
      }
    ]
  }
}

We can return more paths by specifying numpaths. Setting numpaths: 2 returns the shortest two paths:

{

 A as var(func: eq(name, "Alice"))
 M as var(func: eq(name, "Mallory"))

 path as shortest(from: uid(A), to: uid(M), numpaths: 2) {
  friend
 }
 path(func: uid(path)) {
   name
 }
}

In the query above, instead of using UID literals, we query both people using var blocks and the uid() function. You can also combine it with GraphQL Variables.

Edge weight

The shortest path implementation in Dgraph relies on facets to provide weights. Using facets on the edges let you define the edges’ weight as follows:

Only one facet per predicate is allowed in the shortest query block.

{
 path as shortest(from: 0x2, to: 0x5) {
  friend @facets(weight)
 }

 path(func: uid(path)) {
  name
 }
}

{
  "data": {
    "path": [
      {
        "name": "Alice"
      },
      {
        "name": "Bob"
      },
      {
        "name": "Tom"
      },
      {
        "name": "Mallory"
      }
    ],
    "_path_": [
      {
        "uid": "0x2",
        "friend": [
          {
            "uid": "0x3",
            "friend|weight": 0.1,
            "friend": [
              {
                "uid": "0x4",
                "friend|weight": 0.2,
                "friend": [
                  {
                    "uid": "0x5",
                    "friend|weight": 0.3
                  }
                ]
              }
            ]
          }
        ]
      }
    ]
  }
}

Traverse example

Here is a graph traversal example that allows you to find the shortest path between friends using a Car or a Bus.

Car and Bus movement for each relation is modeled as facets and specified in the shortest query

{
  set {
    _:a <friend> _:b (weightCar=10, weightBus=1 ) .
    _:b <friend> _:c (weightCar=20, weightBus=1) .
    _:c <friend> _:d (weightCar=11, weightBus=1.1) .
    _:a <friend> _:d (weightCar=70, weightBus=2) .
    _:a <name> "Alice" .
    _:a <dgraph.type> "Person" .
    _:b <name> "Bob" .
    _:b <dgraph.type> "Person" .
    _:c <name> "Tom" .
    _:c <dgraph.type> "Person" .
    _:d <name> "Mallory" .
    _:d <dgraph.type> "Person" .
  }
}

Query to find the shortest path relying on Car and Bus:

{

 A as var(func: eq(name, "Alice"))
 M as var(func: eq(name, "Mallory"))

 sPathBus as shortest(from: uid(A), to: uid(M)) {
  friend
  @facets(weightBus)
 }

 sPathCar as shortest(from: uid(A), to: uid(M)) {
  friend
  @facets(weightCar)
 }

 pathBus(func: uid(sPathBus)) {
   name
 }

 pathCar(func: uid(sPathCar)) {
   name
 }
}

The response contains the following paths conforming to the specified weights:

    "pathBus": [
      {
        "name": "Alice"
      },
      {
        "name": "Mallory"
      }
    ],
    "pathCar": [
      {
        "name": "Alice"
      },
      {
        "name": "Bob"
      },
      {
        "name": "Tom"
      },
      {
        "name": "Mallory"
      }
    ]

Constraints

Constraints can be applied to the intermediate nodes as follows.

{
  path as shortest(from: 0x2, to: 0x5) {
    friend @filter(not eq(name, "Bob")) @facets(weight)
    relative @facets(liking)
  }

  relationship(func: uid(path)) {
    name
  }
}

The k-shortest path algorithm (used when numpaths > 1) also accepts the arguments minweight and maxweight, which take a float as their value. When they are passed, only paths within the weight range [minweight, maxweight] will be considered as valid paths. This can be used, for example, to query the shortest paths that traverse between 2 and 4 nodes.

{
 path as shortest(from: 0x2, to: 0x5, numpaths: 2, minweight: 2, maxweight: 4) {
  friend
 }
 path(func: uid(path)) {
   name
 }
}

Notes

Some points to keep in mind for shortest path queries:

• Weights must be non-negative. Dijkstra’s algorithm is used to calculate the shortest paths.
• Only one facet per predicate in the shortest query block is allowed.
• Only one shortest path block is allowed per query. Only one _path_ is returned in the result. For queries with numpaths > 1, _path_ contains all the paths.
• Cyclical paths are not included in the result of k-shortest path query.
• For k-shortest paths (when numpaths > 1), the result of the shortest path query variable will only return a single path which will be the shortest path among the k paths. All k paths are returned in _path_.