Introducing celluloid-io-pg-listener: Part 1

Introducing a new RubyGem

celluloid-io-pg-listener (on github)

Allows you to asynchronously LISTEN for Postgresql NOTIFY messages and do something with the payloads.

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Interesting Project!

Sure!

It should work if you just run bin/setup from the root directory of the gem, then you can follow along the examples in the readme.

All the data about db setup and structure is located in the spec/apps directory.

In spec/apps/Rakefile you will find a test_db_setup task. Check it out. ;)

A real Example

I am currently using a two stage approach to make my setup fault tolerant, redundant, and totally asynchronous.

Here is a slightly simplified, generic, version:

Note: In PostgreSQL a schema is more aptly understood as a “namespace”. Note: To use the PostgresSQL function json_build_object you will need to use Postgres >= 9.4, which is fairly recent.

2 Tables

Given a Heroku Connect (i.e. Salesforce) “account” table in the “salesforce” schema, something like this:

  create_table "salesforce.account", :force => true do |t|
    t.string    "sfid"
    t.string    "personemail"
    t.string    "firstname"
    t.string    "lastname"
    t.string    "phone"
    t.string    "postalcode"
    t.datetime  "createddate"
    # ... and whatever else
  end

Which resides in my app’s database, but in a different schema (“salesforce”, rather than “public”).

And given a "pg_notify_logs" table in your apps’ schema:

  create_table "pg_notify_logs", :force => true do |t|
    t.integer  "notify_column_unique_value"
    t.string   "notify_column"
    t.string   "notify_table"
    t.string   "notify_channel"
    t.text     "notify_payload"
    t.string   "notify_type"
    t.text     "notify_row"
    t.datetime "created_at",                 :null => false
    t.datetime "updated_at",                 :null => false
  end

A Function and a Trigger for Heroku Connect

Heroku Connect Function

CREATE OR REPLACE FUNCTION salesforce.push_account_insert_to_pg_notify_logs_function()
  RETURNS trigger AS
$BODY$
DECLARE
  notify_column_unique_val bigint;
  channel_name_value varchar;
  notify_payload_json_value text;
  notify_table_name varchar;
  timestamp_value timestamp;
BEGIN
  notify_column_unique_val = NEW.id;
  notify_table_name = TG_TABLE_SCHEMA || '.' || TG_TABLE_NAME;
  channel_name_value = lower(TG_TABLE_SCHEMA || '_' || TG_TABLE_NAME || '_' || TG_OP);
  notify_payload_json_value = json_build_object('table', notify_table_name, 'id', notify_column_unique_val, 'type', TG_OP)::text;
  timestamp_value = COALESCE(NEW.createddate, CURRENT_TIMESTAMP);
  SET TIME ZONE 'UTC';
  INSERT INTO
    public.pg_notify_logs
      (notify_column_unique_value,notify_column,notify_table,notify_channel,notify_payload,notify_type,notify_row,created_at,updated_at)
    VALUES
      (notify_column_unique_val,'id',notify_table_name,channel_name_value,notify_payload_json_value,TG_OP,row_to_json(NEW),timestamp_value,timestamp_value);
  RETURN NEW;
END;
$BODY$
LANGUAGE plpgsql VOLATILE
COST 100;

Heroku Connect Trigger

DROP TRIGGER IF EXISTS account_inserted ON salesforce.account;
CREATE TRIGGER account_inserted BEFORE INSERT ON salesforce.account
FOR EACH ROW EXECUTE PROCEDURE salesforce.push_account_insert_to_pg_notify_logs_function();

NOTE: To run the SQL above within Rails you have to switch the search path (and make sure to switch it back afterward, google to find more elegant solutions to this):

# before:
ActiveRecord::Base.connection.schema_search_path = "salesforce"
# after:
ActiveRecord::Base.connection.schema_search_path = %{"$user", public}

A Function and a Trigger for Your App

Function

CREATE OR REPLACE FUNCTION notify_on_pg_notify_logs_insert_function()
  RETURNS trigger AS
$BODY$
BEGIN
  SET TIME ZONE 'UTC';
  PERFORM pg_notify(NEW.notify_channel, json_build_object('id', NEW.id)::text);
  RETURN NEW;
END;
$BODY$
LANGUAGE plpgsql VOLATILE
COST 100;

Trigger

DROP TRIGGER IF EXISTS pg_notify_logs_inserted ON pg_notify_logs;
CREATE TRIGGER pg_notify_logs_inserted BEFORE INSERT ON pg_notify_logs
FOR EACH ROW EXECUTE PROCEDURE notify_on_pg_notify_logs_insert_function();

I use a “supervisor” to do the listening. See the bin/ directory of this project for an example.

What I have at the end is a pg_notify_logs table that fill up with logs of all the inserts into the “salesforce.account” table. I then have my listener client (built with this gem) insert jobs into Redis for working by my queue workers which do whatever I want with the data (Sidekiq, Resque). I can either delete the pg_notify_logs records, or mark them as ‘processed’. I could also record errors in processing there, but I haven’t gotten that far yet.

Let me know if this answered your questions!

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