What is the Difference Between a Gauge and a Counter?

Gauges and counters are two core metric types in Prometheus. They serve different purposes and are used to track different kinds of data.

1. Counter

A counter is a metric that only increases over time (or resets to zero). It is used for tracking cumulative values.

Characteristics:

• Monotonically increasing.
• Resets to zero on application restart.
• Cannot decrease.

Common use cases:

• Counting the number of HTTP requests.
• Tracking the total number of errors or failed operations.
• Measuring completed tasks.

Example:

from prometheus_client import Counter

# Define a counter for HTTP requests
http_requests = Counter('http_requests_total', 'Total number of HTTP requests')

# Increment the counter
http_requests.inc()  # Increases by 1
http_requests.inc(3)  # Increases by 3

PromQL Example:

• Total number of requests:

  http_requests_total

• Rate of increase:

  rate(http_requests_total[5m])

2. Gauge

A gauge is a metric that can increase or decrease over time. It represents a snapshot of a value at a specific moment.

Characteristics:

• Can go up or down.
• Useful for measuring current states or values.
• Tracks instantaneous values.

Common use cases:

• Monitoring CPU or memory usage.
• Measuring the size of a queue.
• Tracking the number of active connections.

Example:

from prometheus_client import Gauge

# Define a gauge for memory usage
memory_usage = Gauge('memory_usage_bytes', 'Current memory usage in bytes')

# Set the gauge to a specific value
memory_usage.set(500)

# Increment or decrement the gauge
memory_usage.inc(100)  # Increases by 100
memory_usage.dec(50)   # Decreases by 50

PromQL Example:

• Current memory usage:

  memory_usage_bytes

• Average memory usage over 5 minutes:

  avg_over_time(memory_usage_bytes[5m])

Key differences at a glance

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