Understanding Prometheus - Metrics, Data Types, and Querying

An example of Prometheus data:

http_request_total{method="GET", endpoint="/contact-us", status="200"} 1 2 3 4 5 
http_request_total{method="POST", endpoint="/auth", status="400"} 1 6 8 10 15

Key Concepts

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• Metric: Quantity measurement (e.g.: http_request_total)
• Metric label: Metadata for the measurement (e.g.: method="GET")
• Sample: Data point at a certain time (e.g.: 5) - float64
• Series: Unique combination of metric labels (e.g.: http_request_total{method="GET", endpoint="/contact-us", status="200"} and http_request_total{method="POST", endpoint="/auth", status="400"})
• Time series: Samples over time (e.g.: 1 2 3 4 5)

Data Types

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• Instant vector: http_request_total{method="GET"}
• Range vector: http_request_total{method="GET"}[5m]
• Scalar: numbers

Metric Types

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Prometheus supports four metric types:

1. Gauge: Values can go up and down (e.g.: logged_users)
2. Counter: Values can only increase (e.g.: http_request_total)
3. Histogram: Provides <metric_name>_bucket, <metric_name>_sum, <metric_name>_count. Use histogram_quantile() for server-side quantile calculation (e.g.: http_request_duration_seconds)
4. Summary: Similar to Histogram, but quantiles are calculated client-side (application). Thus, it cannot be further aggregated.

promql

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Operator Precedence

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Prometheus supports a range of binary operators with different precedence levels. From highest to lowest precedence:

1. ^
2. *, /, %, atan2
3. +, -
4. ==, !=, <=, <, >=, >
5. and, unless
6. or

Reference

Modifiers

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• @ 1609746000 - pretend the query time is 1609746000
• offset 5m - pretend the query time is 5 minutes ago

Have to use right after the select (before any function call)

Vector Matching

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• Vector scalar:

  • Example: http_request_total / 2

• Vector Vector:

  • Types of matching:
    • One-to-One
    • One-to-Many
    • Many-to-One
  • Matches vectors using labels by default
  • Customize matching key with ignore() or in()
  • Use group_right() or group_left() for many side
  • Use group_left(labels) to bring labels from one to many side

Example:

method_code:http_errors:rate5m{method="get", code="500"}  24
method_code:http_errors:rate5m{method="get", code="404"}  30
method_code:http_errors:rate5m{method="put", code="501"}  3
method_code:http_errors:rate5m{method="post", code="500"} 6
method_code:http_errors:rate5m{method="post", code="404"} 21

method:http_requests:rate5m{method="get", foo="bar"}  600
method:http_requests:rate5m{method="del", foo="bar1"}  34
method:http_requests:rate5m{method="post", foo="bar2"} 12

method_code:http_errors:rate5m{code="500"} / ignoring(code) group_left(foo) method:http_requests:rate5m

{method="get", code="500", foo="bar"} 0.04 
{method="get", code="404", foo="bar"} 0.05 
{method="post", code="500", foo="bar2"} 0.05 
{method="post", code="404", foo="bar2"} 0.175

If no group_left(foo), foo=”bar” will gone

Reference

Common Prometheus Functions

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• changes(): Number of changes over time
• time(): Current timestamp
• timestamp(): Timestamp of the sample
• Derivative and Rate:
  • deriv(): gauge;
  • rate(), irate(): counter
• Delta and Increase:
  • delta(), idelta(): gauge
  • increase(): counter
• irate() vs rate():
  • irate(): (last - first datapoint)/time range
  • rate(): (projected end - start time datapoint)/time range
• Aggregration:
  • <aggregation>: sum, count, max, min, avg, etc: Aggregates across dimensions (group by labels)
  • <aggregation>_over_time(): Aggregates across time (group by time)

Examples:

sum(http_request_total)

Result:

{} 9

sum_over_time(http_request_total{method="GET"}[5m])

Result:

{method="GET", endpoint="/contact-us", status="200"} 10 # 1+2+3+4+5
{method="POST", endpoint="/auth", status="400"} 25 #1+6+8+10+15

Reference

Prometheus Client Library Usage

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1. Instrumentation
2. Writing exporters
3. Pushing metrics to Pushgateway

Gist reference

Storage

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• Not recommended to use NFS for storage: reference for storage

Agent Mode

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• Disables query, alert, and recording rule functions
• Scrapes metrics from target and remotely writes to other instances
• Reference

Service Discovery

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1. Static: Define target servers in the config file
2. *_sd_config: Use built-in configurations (e.g.: EC2, Kubernetes, file)
3. Custom: Use file_sd_config. Update the file periodically.

Each scrape config can have:

• interval
• timeout
• proxy
• metrics_path

Relabeling

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1. relabel_configs: Modify scrape parameters before scraping (e.g.: Blackbox exporter)
2. metrics_relabel_configs: Modify data collected after scraping (e.g.: remove unwanted metrics)

Alerting in Prometheus

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• Evaluates rules, fires alerts, routes to destination
• Does not handle notifications
• Routes by matching rules with labels
• Labels: alert identity
• Annotations: longer-form description
• Annotations support templating with go lang syntax
• Reference labels in annotations can be done by {{ $labels.foo }}

Alertmanager

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Silencing alerts use cases:

• Provisioning new servers
• Decommissioning servers
• Maintenance

Inhibiting:

• Stop a group of alerts when another alert is triggered
• Example: Cluster down alert inhibits memory or disk check alerts