Postgresql extension: running periodic jobs

Why ?

Just for fun ¯\(ツ)/¯

I wanted to discover PostgreSQL extension API and pg background workers.

What ?

A simple PostgreSQL extension for scheduling SQL queries.

Supports cron expressions with second field.

Scheduling is based on UTC time.

How ?

• pg_cron_create_task(schedule VARCHAR, sql_query VARCHAR): schedules a new task
• pg_cron_get_task_list(): lists existing tasks
• pg_cron_get_task(task_id UUID): gets a task
• pg_cron_drop_task(task_id UUID): deletes a task

Insert a new row into table foo every weekday at 7am:

SELECT * FROM pg_cron_create_task('0 0 7 ? * MON-FRI', 'INSERT INTO foo (bar) VALUES (42);');
                  id                  |     schedule      |               query                |       next_exec
--------------------------------------+-------------------+------------------------------------+------------------------
 a63b67fd-f739-48de-bcea-86e67f112c41 | 0 0 7 ? * MON-FRI | INSERT INTO foo (bar) VALUES (42); | 2018-06-06 07:00:00+00
(1 row)

List tasks:

SELECT * FROM pg_cron_get_task_list();
                  id                  |     schedule      |               query               |       next_exec
--------------------------------------+-------------------+-----------------------------------+------------------------
 a63b67fd-f739-48de-bcea-86e67f112c41 | 0 0 7 ? * MON-FRI | INSERT INTO foo (bar) VALUES (42); | 2018-06-06 07:00:00+00
(1 row)

Drop task:

SELECT * FROM pg_cron_drop_task('a63b67fd-f739-48de-bcea-86e67f112c41');
 pg_cron_drop_task
-------------------
                 1
(1 row)

Install

cd /usr/lib/postgresql/10/lib
git clone git@github.com:samber/pg_cron.git
cd pg_cron
make install

Append shared_preload_libraries = 'pg_cron' to your postgresql.conf and restart PG.

psql --command 'CREATE EXTENSION pg_cron CASCADE'

$ pg_ctl restart

Demo

$ docker-compose build
$ docker-compose up -d

$ docker-compose exec postgres psql -U postgres --command 'CREATE EXTENSION pg_cron CASCADE'
NOTICE:  installing required extension "pgcrypto"
CREATE EXTENSION

$ docker-compose restart

$ docker-compose exec postgres psql -U postgres

psql (10.4 (Debian 10.4-2.pgdg90+1))
Type "help" for help.

postgres=# CREATE TABLE test (id UUID PRIMARY KEY DEFAULT gen_random_uuid(), a INT DEFAULT NULL);
CREATE TABLE

postgres=# SELECT * FROM pg_cron_create_task('*/10 * * * * *', 'INSERT INTO test (a) values (42);');

postgres=# SELECT COUNT(*) FROM test; \watch

Warning

• ⚠️ This is NOT production ready. And it will never be ;)
• Do NOT pay attention to code quality 🤮

Todo

• filling _cron_tasks_history with execution outputs and errors
• async task execution (today tasks are executed in sequence) - pg threading is quite tricky ⚠️
• 1 cron manager bgworker and a pool of bgworker
• safe memory management (cf @TODO + sed 's/malloc/palloc/g' + sed 's/free/pfree/g')

Contributing

1-line build command:

$ docker-compose build \
    && docker-compose down -v \
    && docker-compose up -d \
    && sleep 2 \
    && docker-compose exec postgres psql -U postgres --command 'CREATE EXTENSION pg_cron CASCADE' \
    && docker-compose stop
    && docker-compose up

$ docker-compose exec postgres psql -U postgres --command "SELECT * FROM pg_cron_create_task('* * * * * *', 'SELECT 1;');"

Useful resources for building extensions

• https://www.postgresql.org/docs/10/static/bgworker.html
• https://github.com/postgres/postgres/blob/master/src/test/modules/worker_spi/worker_spi.c
• https://www.postgresql.org/docs/9.4/static/xfunc-c.html#AEN55804
• http://big-elephants.com/2015-10/writing-postgres-extensions-part-i/
• PG headers: https://doxygen.postgresql.org/