Tutorials PostgreSQL Tutorial

Pgpool-II — Complete Guide

Pgpool-II — Complete Guide: free step-by-step lesson with examples, common mistakes, and interview tips — part of PostgreSQL Tutorial on Toolliyo Academy.

8 min read Updated 8/21/2026

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PostgreSQL Tutorial · Lesson 65 of 100

Pgpool-II

SQL ✓ → Advanced

Advanced · 2 — Production · ~10 min · PostgreSQL — Replication & High Availability

What is this?

Pgpool-II is middleware between apps and PostgreSQL — connection pooling, read/write splitting, query cache (limited), and automatic failover coordination with watchdog.

Why should you care?

PostgresVerse Java monolith opens 800 threads — Pgpool pools to 100 real postgres connections and sends SELECTs to standby.

See it live — copy this example

Run in pgAdmin or psql.

# pgpool.conf excerpt (conceptual)
load_balance_mode = on
backend_hostname0 = 'primary.postgresverse.local'
backend_weight0 = 1
backend_flag0 = 'ALLOW_TO_FAILOVER'
backend_hostname1 = 'standby.postgresverse.local'
backend_weight1 = 1
backend_flag1 = 'DISALLOW_TO_FAILOVER'

Run Example »

This lesson uses terminal or setup steps. Run commands on your computer — the live editor appears on coding lessons.

What happened?

load_balance_mode sends read queries to backends by weight.
Primary handles writes; standby serves balanced reads.
Flags control failover participation.

Practice next

Install Pgpool-II in lab stack (Docker compose doc).
Point psql to Pgpool port 9999 not 5432.
Run SELECT — may hit standby; INSERT goes primary.
Compare latency psql direct vs via Pgpool.
Disable load_balance and route all reads to primary for A/B test.

Remember

Pgpool is proxy + pool + optional LB. Not a replacement for PgBouncer-only pooling — overlaps exist. Watchdog handles virtual IP failover scenarios.

PostgresVerse legacy app pool

800-connection Tomcat app fronted by Pgpool; DB max_connections stays 120.

Outcome: OOM on postgres avoided during marketing push.

Interview prep for this lesson

Practice these questions aloud after reading—each links to a full structured answer.

Junior Detailed

Explain SQL queries in the context of PostgreSQL.

Short answer: Interviewers want a crisp definition, a practical example from your projects, and awareness of trade-offs—not textbook dumps. Explain a bit more How to structure your answer (60–90 seconds) Define SQL queri…

What are common mistakes teams make with Schema design when using PostgreSQL?

Short answer: Interviewers want a crisp definition, a practical example from your projects, and awareness of trade-offs—not textbook dumps. Explain a bit more How to structure your answer (60–90 seconds) Define Schema de…

Senior Detailed

How would you debug a production issue related to Transactions in a PostgreSQL application?

Short answer: Interviewers want a crisp definition, a practical example from your projects, and awareness of trade-offs—not textbook dumps. Explain a bit more How to structure your answer (60–90 seconds) Define Transacti…

Compare two approaches to Indexing—when would you choose each?

Short answer: Interviewers want a crisp definition, a practical example from your projects, and awareness of trade-offs—not textbook dumps. Explain a bit more How to structure your answer (60–90 seconds) Define Indexing…

Junior Detailed

Describe a real-world scenario where Normalization mattered in a PostgreSQL project.

Short answer: Interviewers want a crisp definition, a practical example from your projects, and awareness of trade-offs—not textbook dumps. Explain a bit more How to structure your answer (60–90 seconds) Define Normaliza…

All course Q&A Practice exams

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