This blog post describes my work on Graft.jl, a general purpose graph analysis package for Julia. For those unfamiliar with graph algorithms, a quick introduction might help.
My proposal, titled ParallelGraphs, was to develop a parallelized/distributed graph algorithms library. However, in the first month or so, we decided to work towards a more general framework that supports data analysis on networks (graphs with attributes defined on vertices and edges). Our change in direction was mainly motivated by:
The modified proposal could be summarized as the development of a package that supports:
LightGraphsParallelGraphs turned out to be a misnomer, since we were moving towards a more general purpose data analysis framework. So we chose the name Graft, a kind of abbreviation for Graph Toolkit. The following sections detail Graft's features:
Graphs are often representations of real world entities, and the relationships between them. Such entities (and their relationships), often have data attached to them. While it is quite straightforward to store vertex data (a simple table will suffice), storing edges and their data is very tricky. The data should be structured on the source and target vertices, should support random access and should be vectorized for queries.
At first we tried placing the edge data in a SparseMatrixCSC. This turned out to be a bad idea, because sparse matrices are designed for numeric storage. A simpler solution is to store edge metadata in a DataFrame, and have a SparseMatrixCSC map edges onto indices for the DataFrame. This strategy needed a lot less code, and the benchmarks were more promising. Mutations such as the addition or removal of vertices and edges become more complicated however.
Most graph libraries do not support vertex labelling. It can be very confusing to refer to a vertex by its (often long) integer identifier. It is also computationally expensive to use non-integer labels in the implementation of the package (any such implementation would involve dictionaries). There is no reason, however, for the user to have to use integer labels externally. Graft supports two modes of vertex labelling. By default, a vertex is identified by its internal identifier. A user can assign labels of any arbitrary Julia type to identify vertices, overriding the internal identifiers. This strategy, we feel, makes a reasonable compromise between user experience and performance.
If vertex labels were used in the internal implementation, the graph data structure would probably look like this:
Dict(
"Alice" => Dict(
"age" => 34,
"occupation" => "Doctor",
"adjacencies" => Dict("Bob" => Dict("relationship" => "follow")))
),
"Bob" => Dict(
"age" => 36,
"occupation" => "Software Engineer",
"adjacencies" => Dict("Charlie" => Dict("relationship" => "friend"))
),
"Charlie" => Dict(
"age" => 30,
"occupation" => "Lawyer",
"adjacencies" => Dict("David" => Dict("relationship" => "follow"))
),
"David" => Dict(
"age" => 29,
"occupation" => "Athlete",
"adjacencies" => Dict("Alice" => Dict("relationship" => "friend"))
)
)
Cleary, using labels internally is a very bad idea. Any sort of data access would set off multiple dictionary look-ups. Instead, if a bidirectional map could be used to translate labels into vertex identifiers and back, the number of dictionary lookups could be reduced to one. The data would also be better structured for query processing.
# Label Map to resolve queries
LabelMap(
# Forward map : labels to vertex identifiers
Dict("Alice" => 1, "David" => 4", "Charlie" => 3, Bob" => 2),
# Reverse map : vertex identifiers to labels
String["Alice, "Bob", "Charlie", "David"]
)
# Vertex DataFrame
4×2 DataFrames.DataFrame
│ Row │ age │ occupation │
├─────┼─────┼─────────────────────┤
│ 1 │ 34 │ "Doctor" │
│ 2 │ 36 │ "Software Engineer" │
│ 3 │ 30 │ "Lawyer" │
│ 4 │ 29 │ "Athlete" │
# SparseMatrixCSC : maps edges onto indices into Edge DataFrame
4×4 sparse matrix with 4 Int64 nonzero entries:
[4, 1] = 1
[1, 2] = 2
[2, 3] = 3
[3, 4] = 4
# Edge DataFrame
4×1 DataFrames.DataFrame
│ Row │ relationship │
├─────┼──────────────┤
│ 1 │ "follow" │
│ 2 │ "friend" │
│ 3 │ "follow" │
│ 4 │ "friend" │
Graft’s query notation is borrowed from Jplyr. The @query macro is used to simplify the query syntax, and
accepts a pipeline of abstractions separated by the pipe operator |>. The stages are described through abstractions:
Accepts an expression, that is run over every vertex. Vertex properties can be expressed using the dot notation. Some reserved properties are v.id, v.label,
v.adj, v.indegree and v.outdegree.
Examples:
# Check if the user has overridden the default labels
julia> @query(g |> eachvertex(v.id == v.label)) |> all
# Kirchoff's law :P
julia> @query(g |> eachvertex(v.outdegree - v.indegree)) .== 0
Accepts an expression, that is run over every edge. The symbol s is used to denote
the source vertex, and t is used to denote the target vertex in the edge. The symbol e is used to denote
the edge itself. Edge properties can be expressed through the dot notation. Some reserved properties are e.source, e.target, e.mutualcount, and e.mutual.
Examples:
# Arithmetic expression on edge, source and target properties
julia> @query g |> eachedge(e.p1 - s.p1 - t.p1)
# Check if constituent vertices have the same outdegree
julia> @query g |> eachedge(s.outdegree == t.outdegree)
# Count the number of "mutual friends" between the source and target vertices in each edge
julia> @query g |> eachedge(e.mutualcount)
Accepts vertex or edge expressions and computes subgraphs with a subset of vertices, or a subset of edges, or both. Examples:
# Remove vertices where property p1 equals property p2
@query g |> filter(v.p1 != v.p2)
# Remove self loops from the graph
@query g |> filter(e.source != e.target)
Returns a subgraph with a subset of vertex properties, or a subset of edge properties or both. Examples:
# Preserve vertex properties p1, p2 and nothing else
@query g |> select(v.p1, v.p2)
# Preserve vertex property p1 and edge property p2
@query g |> select(v.p1, e.p2)
The typical workflow we hope to support with Graft is:
LightGraphs and run computationally expensive algorithms there.Graft as a new property, or use it to modify the graphs structure.The following examples should demonstrate this workflow:
More information can be found here
This work was carried out as part of the Google Summer of Code program, under the guidance of mentors: Viral B Shah and Shashi Gowda.