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Database Setup

Goals

The Principle of least privilege1 is followed by creating explicit GRANT commands for the owner, app, and app_readonly users.

The configuration is based on My GOTO Postgres Configuration for Web Services.2 One of the other goals besides minimizing access, is to prevent accidental table drops.

Since the schema rideshare is created, the public schema is not needed and is removed.

For psql commands, use a DATABASE_URL environment variable that's set in your terminal.

The connection string connects to the Rideshare database, using the owner user. The value of DATABASE_URL is a connection string, with the format format protocol://role:password@host:port/databasename.

Configuring Host Based Authentication (HBA)

You may want to configure Host Based Authentication (HBA)3.

Do that by editing your pg_hba.conf file. Changes in pg_hba.conf can be applied by reloading PostgreSQL.

Reloading your PostgreSQL configuration

To reload your configuration, run: pg_ctl reload in your terminal. If you run into the following message, we'll get that addressed.

pg_ctl: no database directory specified and environment variable PGDATA unset
Try "pg_ctl --help" for more information.

This command assumes PGDATA is set and points to the data directory for your PostgreSQL installation.

Run echo $PGDATA to see the value. How do you set it if it's empty? Run the following commands in your terminal:

# Look at the value
psql -U postgres -c 'SHOW data_directory'

# Assign the value to PGDATA
export PGDATA="$(psql -U postgres \
  -c 'SHOW data_directory' \
  --tuples-only | sed 's/^[ \t]*//')"
echo "Set PGDATA: $PGDATA"

With PGDATA set, run pg_ctl reload again. Once PostgreSQL config reloads, you're all set.

Docker

Reset everything:

sh reset_docker_instances.sh

Tear down docker:

sh teardown_docker.sh

Slow Clients

Replace config/database.yml (or just the "slow clients" section)

cp config/database-slow-clients.sample.yml config/database.yml

With that in place, create a model:

class SlowClientModel < ApplicationRecord
  self.establish_connection :slow_clients
end

Run query code that takes 5 seconds, and verify that it's canceled in the normal configuration.

The "slow client" configuration allows it since it has a higher statement timeout configured.

Trip.connection.execute("SELECT PG_SLEEP(5)")
SlowClientModel.connection.execute("SELECT PG_SLEEP(5)").first

pg_cron

Scheduling maintenance with the PostgreSQL pg_cron extension

  • The extension is created using the postgres superuser
  • The superuser grants usage privileges to the owner role, for the cron schema
  • Now the owner user can schedule their own jobs, for objects they own
psql -U postgres -d rideshare_development;

CREATE EXTENSION pg_cron;

GRANT USAGE ON SCHEMA cron TO owner;

Run a job:

SELECT cron.schedule(
  'rideshare trips manual vacuum',
  '10 * * * *',
  'VACUUM (ANALYZE) rideshare.trips'
);

View the jobs:

SELECT * FROM cron.job;

View job runs:

SELECT * FROM cron.job_run_details;

Screenshot of PgHero Scheduled Jobs

active-record-doctor

Run the tool from your terminal:

bundle exec rake active_record_doctor:

database_consistency

Run the tool from your terminal:

database_consistency

rails-pg-extras

Specify a custom schema for table_cache_hit

bin/rails runner \
  'RailsPgExtras.table_cache_hit(args: { schema: "rideshare" })'

Or for version >= 5.3.1, set a schema using an environment variable:

export PG_EXTRAS_SCHEMA=rideshare

For example, now we can search for unused indexes, and make sure that indexes that are in databases within the specified schema (rideshare) are examined

bin/rails pg_extras:unused_indexes
bin/rails pg_extras:diagnose

rails_best_practices

bin/rails_best_practices .

Fake data

bin/rails data_generators:generate_all

bin/rails data_generators:drivers

bin/rails data_generators:trips_and_requests

PgBouncer Prepared Statements

  • Run brew services and confirm PgBouncer is running on port 6432
  • Set DATABASE_URL to be port 6432
  • Disable Query Logs in config/application.rb (currently incompatible)
  • Restart PgBouncer to clear out the prepared statements

Run the following script to observe how prepared statements are populated:

sh pgbouncer_prepared_statements_check.sh

Footnotes

  1. https://en.wikipedia.org/wiki/Principle_of_least_privilege ↩

  2. https://tightlycoupled.io/my-goto-postgres-configuration-for-web-services/ ↩

  3. https://www.postgresql.org/docs/current/auth-pg-hba-conf.html ↩