Fix Pulsar Backlog

What's Actually Happening

Apache Pulsar topic backlog is growing. Messages are not being consumed.

The Error You'll See

```bash $ pulsar-admin topics stats persistent://public/default/mytopic

backlogSize: 1073741824 # 1GB backlog! subscriptions: {} ```

Why This Happens

1. 1.Consumer down - No active consumers
2. 2.Slow consumer - Cannot keep up with rate
3. 3.Subscription stuck - Subscription not advancing
4. 4.Consumer crash - Consumer keeps crashing

Step 1: Check Topic Stats

```bash # Check topic backlog: pulsar-admin topics stats persistent://public/default/mytopic

# Check subscriptions: pulsar-admin topics subscriptions persistent://public/default/mytopic ```

Step 2: Check Consumer Status

```bash # List consumers: pulsar-admin topics consumers persistent://public/default/mytopic

# Check consumer health: pulsar-admin topics stats persistent://public/default/mytopic | jq '.subscriptions[].consumers' ```

Step 3: Reset Subscription

```bash # Reset to latest: pulsar-admin topics reset-cursor persistent://public/default/mytopic -s mysub -t now

# Or clear backlog: pulsar-admin topics skip-all persistent://public/default/mytopic -s mysub ```

Step 4: Scale Consumers

# Increase consumer instances
# Check partition count matches consumer count

Pulsar Backlog Checklist

Verify the Fix

pulsar-admin topics stats persistent://public/default/mytopic
# Output: backlogSize: 0

Related Issues

• [Fix Kafka Consumer Lag](/articles/fix-kafka-consumer-lag)
• [Fix RabbitMQ Consumer Stuck](/articles/fix-rabbitmq-consumer-stuck)