Futuristic stairs
Futuristic stairs (source: Pixabay)

Google’s SRE teams have some basic principles and best practices for building successful monitoring and alerting systems. This chapter offers guidelines for what issues should interrupt a human via a page, and how to deal with issues that aren’t serious enough to trigger a page.


There’s no uniformly shared vocabulary for discussing all topics related to monitoring. Even within Google, usage of the following terms varies, but the most common interpretations are listed here.


Collecting, processing, aggregating, and displaying real-time quantitative data about a system, such as query counts and types, error counts and types, processing times, and server lifetimes.

White-box monitoring

Monitoring based on metrics exposed by the internals of the system, including logs, interfaces like the Java Virtual Machine Profiling Interface, or an HTTP handler that emits internal statistics.

Black-box monitoring

Testing externally visible behavior as a user would see it.


An application (usually web-based) that provides a summary view of a service’s core metrics. A dashboard may have filters, selectors, and so on, but is prebuilt to expose the metrics most important to its users. The dashboard might also display team information such as ticket queue length, a list of high-priority bugs, the current on-call engineer for a given area of responsibility, or recent pushes.


A notification intended to be read by a human and that is pushed to a system such as a bug or ticket queue, an email alias, or a pager. Respectively, these alerts are classified as tickets, email alerts,1 and pages.

Root cause

A defect in a software or human system that, if repaired, instills confidence that this event won’t happen again in the same way. A given incident might have multiple root causes: for example, perhaps it was caused by a combination of insufficient process automation, software that crashed on bogus input, and insufficient testing of the script used to generate the configuration. Each of these factors might stand alone as a root cause, and each should be repaired.

Node and machine

Used interchangeably to indicate a single instance of a running kernel in either a physical server, virtual machine, or container. There might be multiple services worth monitoring on a single machine. The services may either be:

  • Related to each other: for example, a caching server and a web server
  • Unrelated services sharing hardware: for example, a code repository and a master for a configuration system like Puppet or Chef

Any change to a service’s running software or its configuration.

Why Monitor?

There are many reasons to monitor a system, including:

Analyzing long-term trends

How big is my database and how fast is it growing? How quickly is my daily-active user count growing?

Comparing over time or experiment groups

Are queries faster with Acme Bucket of Bytes 2.72 versus Ajax DB 3.14? How much better is my memcache hit rate with an extra node? Is my site slower than it was last week?


Something is broken, and somebody needs to fix it right now! Or, something might break soon, so somebody should look soon.

Building dashboards

Dashboards should answer basic questions about your service, and normally include some form of the four golden signals (discussed later in this chapter).

Conducting ad hoc retrospective analysis (i.e., debugging)

Our latency just shot up; what else happened around the same time?

System monitoring is also helpful in supplying raw input into business analytics and in facilitating analysis of security breaches. Because this book focuses on the engineering domains in which SRE has particular expertise, we won’t discuss these applications of monitoring here.

Monitoring and alerting enables a system to tell us when it’s broken, or perhaps to tell us what’s about to break. When the system isn’t able to automatically fix itself, we want a human to investigate the alert, determine if there’s a real problem at hand, mitigate the problem, and determine the root cause of the problem. Unless you’re performing security auditing on very narrowly scoped components of a system, you should never trigger an alert simply because "something seems a bit weird."

Paging a human is a quite expensive use of an employee’s time. If an employee is at work, a page interrupts their workflow. If the employee is at home, a page interrupts their personal time, and perhaps even their sleep. When pages occur too frequently, employees second-guess, skim, or even ignore incoming alerts, sometimes even ignoring a "real" page that’s masked by the noise. Outages can be prolonged because other noise interferes with a rapid diagnosis and fix. Effective alerting systems have good signal and very low noise.

Setting Reasonable Expectations for Monitoring

Monitoring a complex application is a significant engineering endeavor in and of itself. Even with substantial existing infrastructure for instrumentation, collection, display, and alerting in place, a Google SRE team with 10–12 members typically has one or sometimes two members whose primary assignment is to build and maintain monitoring systems for their service. This number has decreased over time as we generalize and centralize common monitoring infrastructure, but every SRE team typically has at least one “monitoring person.” (That being said, while it can be fun to have access to traffic graph dashboards and the like, SRE teams carefully avoid any situation that requires someone to “stare at a screen to watch for problems.”)

In general, Google has trended toward simpler and faster monitoring systems, with better tools for post hoc analysis. We avoid "magic" systems that try to learn thresholds or automatically detect causality. Rules that detect unexpected changes in end-user request rates are one counterexample; while these rules are still kept as simple as possible, they give a very quick detection of a very simple, specific, severe anomaly. Other uses of monitoring data such as capacity planning and traffic prediction can tolerate more fragility, and thus, more complexity. Observational experiments conducted over a very long time horizon (months or years) with a low sampling rate (hours or days) can also often tolerate more fragility because occasional missed samples won’t hide a long-running trend.

Google SRE has experienced only limited success with complex dependency hierarchies. We seldom use rules such as, "If I know the database is slow, alert for a slow database; otherwise, alert for the website being generally slow." Dependency-reliant rules usually pertain to very stable parts of our system, such as our system for draining user traffic away from a datacenter. For example, "If a datacenter is drained, then don’t alert me on its latency" is one common datacenter alerting rule. Few teams at Google maintain complex dependency hierarchies because our infrastructure has a steady rate of continuous refactoring.

Some of the ideas described in this chapter are still aspirational: there is always room to move more rapidly from symptom to root cause(s), especially in ever-changing systems. So while this chapter sets out some goals for monitoring systems, and some ways to achieve these goals, it’s important that monitoring systems—especially the critical path from the onset of a production problem, through a page to a human, through basic triage and deep debugging—be kept simple and comprehensible by everyone on the team.

Similarly, to keep noise low and signal high, the elements of your monitoring system that direct to a pager need to be very simple and robust. Rules that generate alerts for humans should be simple to understand and represent a clear failure.

Symptoms Versus Causes

Your monitoring system should address two questions: what’s broken, and why?

The "what’s broken" indicates the symptom; the "why" indicates a (possibly intermediate) cause. Table 1-1 lists some hypothetical symptoms and corresponding causes.

Table 1-1. Example symptoms and causes
Symptom Cause

I’m serving HTTP 500s or 404s

Database servers are refusing connections

My responses are slow

CPUs are overloaded by a bogosort, or an Ethernet cable is crimped under a rack, visible as partial packet loss

Users in Antarctica aren’t receiving animated cat GIFs

Your Content Distribution Network hates scientists and felines, and thus blacklisted some client IPs

Private content is world-readable

A new software push caused ACLs to be forgotten and allowed all requests

"What" versus "why" is one of the most important distinctions in writing good monitoring with maximum signal and minimum noise.

Black-Box Versus White-Box

We combine heavy use of white-box monitoring with modest but critical uses of black-box monitoring. The simplest way to think about black-box monitoring versus white-box monitoring is that black-box monitoring is symptom-oriented and represents active—not predicted—problems: "The system isn’t working correctly, right now." White-box monitoring depends on the ability to inspect the innards of the system, such as logs or HTTP endpoints, with instrumentation. White-box monitoring therefore allows detection of imminent problems, failures masked by retries, and so forth.

Note that in a multilayered system, one person’s symptom is another person’s cause. For example, suppose that a database’s performance is slow. Slow database reads are a symptom for the database SRE who detects them. However, for the frontend SRE observing a slow website, the same slow database reads are a cause. Therefore, white-box monitoring is sometimes symptom-oriented, and sometimes cause-oriented, depending on just how informative your white-box is.

When collecting telemetry for debugging, white-box monitoring is essential. If web servers seem slow on database-heavy requests, you need to know both how fast the web server perceives the database to be, and how fast the database believes itself to be. Otherwise, you can’t distinguish an actually slow database server from a network problem between your web server and your database.

For paging, black-box monitoring has the key benefit of forcing discipline to only nag a human when a problem is both already ongoing and contributing to real symptoms. On the other hand, for not-yet-occurring but imminent problems, black-box monitoring is fairly useless.

1Sometimes known as "alert spam," as they are rarely read or acted on.

Article image: Futuristic stairs (source: Pixabay).