## Core Concepts

To facilitate the configuration and use of Prometheus, we will briefly introduce the data model, metric types, and concepts such as instance and job in Prometheus.

### Data Model

The data stored in Prometheus is time series, which is uniquely identified by the name of the metric and a series of labels (key-value pairs). Different labels represent different time series.

- **Metric Name:** This name should be semantic and is generally used to represent the functionality of the metric. For example, http_requests_total represents the total number of HTTP requests. The metric name consists of ASCII characters, numbers, underscores, and colons, and must satisfy the regular expression [a-zA-Z_:][a-zA-Z0-9_:]*.

- **Labels:** Provides different dimensions for the same time series. For example, http_requests_total{method="Get"} represents all HTTP requests of the Get request. When method="post", it is a new metric. The keys in the label are made up of ASCII characters, numbers, and underscores, and must satisfy the regular expression [a-zA-Z_:][a-zA-Z0-9_:]*.

- **Sample:** The actual time series, each series includes a float64 value and a millisecond-level timestamp.

- **Format:** Such as http_requests_total{method="POST",endpoint="/api/tracks"}.

### Metric Types

The Prometheus client library mainly provides four main types of metrics, which are as follows:

1. Counter

A kind of cumulative metric, typical applications such as: the number of requests, the number of tasks completed, the number of errors occurred, etc. For example, query http_requests_total{method="get", job="kubernetes-nodes", handler="prometheus"} returns 8, query again after 10 seconds, then return 14.

2. Gauge

A regular metric, typical applications such as: temperature, number of running goroutines. For example: go_goroutines{instance="10.9.81.55", job="kubernetes-nodes"} returns 147, and 124 after 10 seconds.

3. Histogram

Can be understood as a histogram, typical applications such as: request duration, response size. It can sample, group and statistic the observation results. For example, when querying http_request_duration_microseconds_sum{job="kubernetes-nodes", handler="prometheus"}, the result is as follows:

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4. Summary

Similar to Histogram, typical applications such as: request duration, response size. It provides count and sum functions of observable values. Provide percentile function, that is, trace results can be divided by percentage.

### instance & job

**Instance:** A single scrape target, usually corresponding to a process.

**Jobs:** A group of the same type of instances

For example, a job of the api-server can include 4 instances:

- job: api-server

  - instance 1: 1.2.3.4:5670
  - instance 2: 1.2.3.4:5671
  - instance 3: 1.2.3.4:5672
  - instance 4: 1.2.3.4:5673

When scraping targets, Prometheus will automatically attach some labels to this scrape time series to better distinguish, such as: instance, job.