Marco Pracucci

Prometheus: understanding the delays on alerting

by Marco Pracucci Comments

Prometheus is a great monitoring and alerting system. At Spreaker (self promotion - we’re hiring!) we’re slowly migrating to Prometheus, and thus a correct understanding in how it works is very important.

Today, me and my colleague Rocco were experimenting on the delays introduced by our Prometheus setup. In particular, it was our intention to measure how much time it takes to get an event, occured on a monitored target, notified through our notification channels. This article sums up what we’ve learned.

Scraping, Evaluation and Alerting

Prometheus scrape metrics from monitored targets at regular intervals, defined by the scrape_interval (defaults to 1m). The scrape interval can be configured globally, and then overriden per job. Scraped metrics are then stored persistently on its local storage.

Prometheus has another loop, whose clock is independent from the scraping one, that evaluates alerting rules at a regular interval, defined by evaluation_interval (defaults to 1m). At each evaluation cycle, Prometheus runs the expression defined in each alerting rule and updates the alert state.

An alert can have the following states:

An alert transitions from a state to another only during the evaluation cycle. How the transition occurs, depends if the alert’s FOR clause has been set or not. From the doc:

The optional FOR clause causes Prometheus to wait for a certain duration between first encountering a new expression output vector element (like an instance with a high HTTP error rate) and counting an alert as firing for this element.

The Alert Lifecycle

Let’s do an example to better explain the lifecycle of an alert. We do have a simple alert that monitors the load 1m of a node, and fires when it’s higher than 20 for at least 1 minute.

  IF node_load1 > 20
  FOR 1m

Prometheus is configured to scrape metrics every 20 seconds, and the evaluation interval is 1 minute.

  scrape_interval: 20s
  evaluation_interval: 1m

Question: how much time does it take to fire NODE_LOAD_1M, once the avgload on the machine is higher than 20?

Answer: it takes a time between 1m and 20s + 1m + 1m. The upper bound is probably higher than you may expect when you set FOR 1m, but it actually makes much sense in the Prometheus architecture.

The reason of such worst-case timing is explained by the lifecycle of an alert. The following diagram shows the sequence of events over a timeline:


  1. The load of a node constantly changes, but it gets scraped by Prometheus every scrape_interval (ie. 20s)
  2. Alert rules are then evaluated against the scraped metrics every evaluation_interval (ie. 1m)
  3. When an alert rule expression is true (ie. node_load1 > 20), the alert switches to pending, in order to honor the FOR clause
  4. At the next evaluation cycles, if the alert expression is still true, once the FOR clause is honored the alert finally switches to firing and a notification is pushed to the alert manager

The Alert Manager

The Alert Manager is the last piece in the pipeline, before an alert notification gets dispatched. It receives in input the alerts once switched to firing or switched back to inactive, and then it dispatch notifications about the alert (please note that pending alerts are not forwarded to the Alert Manager).

The Alert Manager has several features, including grouping similar alerts together into a single notification. The grouping feature is particularly important to avoid bombing notification receivers when multiple alerts of the same type occur (ie. when the same alert condition occurring on multiple nodes, we may want to receive just one notification that groups all nodes together instead of a single notification per node).

The downside of grouping notifications is that it may introduce further delays.

The Alert Manager notifications grouping is basically controlled by the following settings:

group_by: [ 'a-label', 'another-label' ]
group_wait: 30s
group_interval: 5m

When a new alert is fired, it waits for group_wait time until the notification gets dispatched. This waiting time is necessary to eventually group further upcoming alerts that match the same group_by condition (in other terms, two alerts are grouped together if they have the same exact values for the group_by labels).

This means that if group_wait is set to 30s, the alert manager will buffer the alert notification for 30 seconds, waiting for other potential notifications to be appended to the buffered notification. Once the group_wait time expires, the notification is finally sent.

Great! The notification is finally sent.

But what happens if at the next evaluation cycle further alerts of the same group get fired? They will not wait for group_wait before getting dispatched, but they will wait for group_interval.

# How long to wait before sending notification about new alerts that are
# in are added to a group of alerts for which an initial notification
# has already been sent. (Usually ~5min or more.)
group_interval: 5m

The group_interval setting controls how long to wait before dispatching further notifications of the same group, and the time interval is calculated starting from the last notification sent.


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