(m)Ruby + PostgreSQL = <3

Holycorn makes it easy to implement a Foreign Data Wrapper using Ruby.

It is based on top of mruby, that provides sandboxing capabilities the regular Ruby VM "MRI/CRuby" does not implement.

Holycorn is the combination of the mruby VM, some supporting gems (some basic libraries and some more advanced ones), and custom code that implement the actual foreign data wrappers.

All the following wrappers are currently linked against Holycorn:

• Redis, using the mruby-redis gem

• PostgreSQL 9.4+

Simply run

rake

to vendor and build mruby.

Now that mruby is built, building holycorn requires to run

make

and installing it only requires to run

make install

Now connect to PostgreSQL and install the extension:

λ psql
psql (9.5.14)
Type "help" for help.

DROP EXTENSION holycorn CASCADE;
CREATE EXTENSION holycorn;

A set of builtin FDW are distributed with Holycorn for an easy setup. All one needs to provide are the options that will allow the FDW to be configured:

λ psql
psql (9.5.14)
Type "help" for help.

CREATE SERVER holycorn_server FOREIGN DATA WRAPPER holycorn;
CREATE FOREIGN TABLE redis_table (key text, value text)
  SERVER holycorn_server
  OPTIONS ( wrapper_class 'HolycornRedis'
          , host '127.0.0.1'
          , port '6379'
          , db '0');

Alternatively, Holycorn supports IMPORT FOREIGN SCHEMA and the same can be accomplished by using that statement instead:

λ psql
psql (9.5.14)

CREATE SERVER holycorn_server FOREIGN DATA WRAPPER holycorn;
IMPORT FOREIGN SCHEMA holycorn_schema
FROM SERVER holycorn_server
INTO holycorn_tables
OPTIONS ( wrapper_class 'HolycornRedis'
        , host '127.0.0.1'
        , port '6379'
        , db '0'
        , prefix 'holycorn_'
        );

Please note that custom data wrappers have to use the holycorn_schema schema as it would otherwise result in an error at runtime.

Note: IMPORT FOREIGN SCHEMA requires us to use a custom target schema and Holycorn encourages users to use a prefix to help avoiding name collisions.

As Holycorn doesn't support INSERTs yet, let's create some manually:

λ redis-cli
127.0.0.1:6379> select 0
OK
127.0.0.1:6379> keys *
(empty list or set)
127.0.0.1:6379> set foo 1
OK
127.0.0.1:6379> set bar 2
OK
127.0.0.1:6379> set baz 3
OK
127.0.0.1:6379> keys *
1) "bar"
2) "foo"
3) "baz"

Now that the table has been created and we have some data in Redis, we can select data from the foreign table.

SELECT * from redis_table;

 key | value
-----+-------
 bar | 2
 foo | 1
 baz | 3
(3 rows)

Alternatively, custom scripts can be used as the source for a Foreign Data Wrapper:

DROP EXTENSION holycorn CASCADE;
CREATE EXTENSION holycorn;
CREATE SERVER holycorn_server FOREIGN DATA WRAPPER holycorn;
CREATE FOREIGN TABLE holytable (some_date timestamptz) \
  SERVER holycorn_server
  OPTIONS (wrapper_path '/tmp/source.rb');

(IMPORT FOREIGN SCHEMA is also supported here.)

And the source file of the wrapper:

# /tmp/source.rb
class Producer
  def initialize(env = {}) # env contains informations provided by Holycorn
  end

  def each
    @enum ||= Enumerator.new do |y|
      10.times do |t|
        y.yield [ Time.now ]
      end
    end
    @enum.next
  end

  def import_schema(args = {})
    # Keys are:
    #   * local_schema: the target schema
    #   * server_name: name of the foreign data server in-use
    #   * wrapper_class: name of the current class
    #   * any other OPTIONS passed to IMPORT FOREIGN SCHEMA
  end

  self
end

For more details about #import_schema, please take a look at the examples located in in the builtin_wrappers directory.

Now you can select data out of the wrapper:

λ psql
psql (9.5.14)
Type "help" for help.

franck=# SELECT * FROM holytable;
      some_date
---------------------
 2015-06-21 22:39:24
 2015-06-21 22:39:24
 2015-06-21 22:39:24
 2015-06-21 22:39:24
 2015-06-21 22:39:24
 2015-06-21 22:39:24
 2015-06-21 22:39:24
 2015-06-21 22:39:24
 2015-06-21 22:39:24
 2015-06-21 22:39:24
(10 rows)

Pretty neat.

Any type of Ruby object can act as a FDW. The only requirements are that it can receive .new (with arity = 1) and return an object that can receive each (arity = 0).

It doesn't have to be a Class, and there's currently no will to provide a superclass to be inherited from.

In future versions, there will be many more callbacks to interact with PG's FDW infrastructure through Holycorn.

Also, the script can only be a single word - like MyClass - as long as MyClass has been defined and exists within your compilation of mruby.

A hash is passed by Holycorn to the Ruby script. Its current keys are:

• PG_VERSION
• PG_VERSION_NUM
• PACKAGE_STRING
• PACKAGE_VERSION
• MRUBY_RUBY_VERSION
• WRAPPER_PATH

• MRB_TT_FREE => null
• MRB_TT_FALSE => Boolean
• MRB_TT_TRUE => Boolean
• MRB_TT_FIXNUM => Int64
• MRB_TT_SYMBOL => Text
• MRB_TT_UNDEF => Unsupported
• MRB_TT_FLOAT => Float8
• MRB_TT_CPTR => Unsupported
• MRB_TT_OBJECT => Text (to_s is called)
• MRB_TT_CLASS => Text (class.to_s is called)
• MRB_TT_MODULE => Text (to_s is called)
• MRB_TT_ICLASS => Unsupported
• MRB_TT_SCLASS => Unsupported
• MRB_TT_PROC => Text (inspect is called)
• MRB_TT_ARRAY => Text (inspect is called)
• MRB_TT_HASH => Text (inspect is called)
• MRB_TT_STRING => Text
• MRB_TT_RANGE => Text (inspect is called)
• MRB_TT_EXCEPTION => Unsupported
• MRB_TT_FILE => Unsupported
• MRB_TT_ENV => Unsupported
• MRB_TT_DATA => See "Arbitraty Ruby objects" section
• MRB_TT_FIBER => Text (inspect is called)
• MRB_TT_MAXDEFINE => Unsupported

• Time (Ruby) => timestamptz

None (yet).

Either wrapper_class or wrapper_path can be used to defined whether an external script or a built-in wrapper will manage the foreign table.

• wrapper_class: Name of the built-in wrapper class
• wrapper_path: Path of a custom script

In both case, any other option will be pushed down to the wrapper class via the constructor.

• Array type
• JSON type
• Range type
• Support PG 9.5's IMPORT FOREIGN SCHEMA for easy setup

• Fork the project.
• Make your feature addition or bug fix.
• Add tests for it. This is important so I don't break it in a future version unintentionally.
• Commit, do not mess with version or history. (if you want to have your own version, that is fine but bump version in a commit by itself I can ignore when I pull)
• Send me a pull request. Bonus points for topic branches.

Copyright (c) 2015-2018 Franck Verrot

Holycorn is an Open Source project licensed under the terms of the LGPLv3 license. Please see http://www.gnu.org/licenses/lgpl-3.0.html for license text.

Franck Verrot, @franckverrot