GRAIL is an approach to run a common class of iterative graph analytics on top of existing relational database engines. So, you do not need a specialized graph engine! The original paper is at: http://pages.cs.wisc.edu/~jignesh/publ/Grail.pdf.

To use GRAIL, you have to load the graph data into two basic tables in your relational database system. These two tables are: vertex and edge.

The vertex table is created in SQL as:

    CREATE TABLE vertex {
      vertex_id INT
    }

The edge table is created as:

    CREATE TABLE edge {
      src_id INT,
      dest_id INT
    }

Additional primary key, indices and other constraints can also be added to the schema creating commands above.

Once you have load your graph data into these two tables, you then express the iterative graph analytics in a configuration file (see examples below). Then, you compile the configuration file using Grail to produce a SQL script. You then compute the graph analytics by running the SQL script against the database engine that has the graph data (stored in tables as described above).

The main top-level directory contains the implemetation of Grail in Python that generates PostgreSQL PL/pgSQL. The current version has been tested on PostgreSQL.

There are two directories under this top-level directory: analytics, and java.

1. The analytics directory contains configuration files for various graph analytics. This directory has the following files:

   Filename	# Description
   pagerank.grail	PageRank algorithm
   sssp.grail	Single-source shortest path
   wcc.grail	Weakly connected components

2. The java directory contains a Java-based Grail compiler to generate SQL code for Microsoft SQL Server (this is what was used in the original GRAIL paper). This directory is largely for historic purposes, and the main work has moved to the Python implementation.

1. VertexValueType: The type of value that a vertex represents. It should be one of the types supported by the relational database engine, e.g. int and float.

2. MessageValueType: The type of messages that a vertex can send and receive. It should also be one of the types supported by the database system, e.g. int and float.

3. InitiateVal: The initial value of the vertices. For example, if the vertices are of type int, then initial value could be zero.

4. Initial Message: The initial messages to send to all or specific vertices. (ALL, value) and (some_vertex_id, value) is used for sending the initial message to all vertices and to a specific vertex respectively.

5. CombineMessage: The method to compute an aggregate value on the "messages." The message will be automatically grouped on the destination vertex id. The aggregation should be supported by the database system. Some exmaples are built-in SQL aggregates like MIN and MAX. Many SQL system allow a user to define aggregate functions, i.e. they support user-defined aggregate functions (UDAF). UDAFs are also permitted here. Note that the UDAFs must be defined and registered with the database system separately.

6. UpdateAndSend: This part can be a combination of one or more of the following actions: mutate values, send messages, and flow control.

7. End: Ther are two possiblities: NO_MESSAGE or (ITER, max_iterations)

   • The NO_MESSAGE option terminates the graph computation, when there are no more messages that need to be processed.
   • The (ITER, max_iterations) option terminates the program after max_iterations number of iterations.

To get GRAIL running on PostgreSQL, open the file GRAIL_ON_POSTGRES.md.