SLI
In simple words Service Level Indicators are the metrics that represent how the reliability is perceived by the consumers of the service. They are normalized to be a number between 0 and 100 using this formula:
$SLI = \frac{Good}{Valid} \times 100$
You can read more about the definition of Good and Valid .

Common SLIs include latency, availability, yield, durability, correctness, etc.

🎉Tip: Use Templates

Title

Description

Type
SLIs can either be either: event-based or time-based .

Comparing SLI Types

Event-Based Time-Based

Use When consumers perceive reliability by events When consumers perceive reliability by time

Counts Good events Good timeslices

Advantage More accurately adjust to the amount of load More forgiving towards the negative impact of failed events

Formula $\frac{Good events}{Valid events} \times 100$ $\frac{Good timeslices}{All timeslices} \times 100$

Comparing SLI Types
	Event-Based	Time-Based
Use	When consumers perceive reliability by events	When consumers perceive reliability by time
Counts	Good events	Good timeslices
Advantage	More accurately adjust to the amount of load	More forgiving towards the negative impact of failed events
Formula	$\frac{Good events}{Valid events} \times 100$	$\frac{Good timeslices}{All timeslices} \times 100$

Event-Based

Time-Based

Timeslice: {{ timeslice }}
Time-based SLIs aggregate metric data over a timeslice to mark it as success or failure.

This can also reduce the resolution of the data. For example, probing an endpoint every 60 seconds to see if it is available, assumes that the endpoint is available for the entire 60 seconds.

Another common example is to compare the average of data points with a desired valud. Averages hide the spikes and valleys in the data. It is better to use percentiles instead.

Another example is percentiles. When calculating the 99th percentile of the latency every 5 minutes, the aggregation window is 5 x 60 = 300 seconds.

Typical timeslice lengths

Timeslice Seconds

{{ p.title }} {{ p.seconds }}

Typical timeslice lengths
Timeslice	Seconds
{{ p.title }}	{{ p.seconds }}

Valid events
How do consumers perceive reliability of your service? What kind of events are important to the service consumers?

You probably don't want to count all the. This is an opportunity to narrow down the scope of the optimization and what triggers an alert.

For simplicity, sometimes total is used instead of valid. But there is a difference .

While Service level indicator guides the optimization, the definition of valid scopes that optimization for two reasons:

Focus the optimization effort

Clarify responsibility and control

Note: use a plural form of the event name so that the UI reads more fluently.

Common events

Event Use case

{{ p.eventUnit }} {{ p.useCase }}

Common events
Event	Use case
{{ p.eventUnit }}	{{ p.useCase }}

Good {{ sloWindow.eventUnitNorm }}

Metric name
What are good {{ sloWindow.eventUnitNorm }} from consumer's perspective? How do good {{ sloWindow.eventUnitNorm }} look like?

What is the metric that you can measure to identify the good {{ sloWindow.eventUnitNorm }} from all the valid {{ sloWindow.eventUnitNorm }}?

Metric Unit
This is the unit of the metric ( `{{ metricName }}` ) not to be confused with the unit of the events ( `{{ sloWindow.eventUnitNorm }}` ).

Show boundaries

What metric values define a good {{ sloWindow.eventUnitNorm }}?

The actual thresholds are part of the SLO definition. This allows Multi-Tierd SLOs where different expectations have different commitments.

Lower Bound
Good values can be bound.

The actual Lower Threshold (`$LT`) is part of the SLO definition.

There seems to be a comparator in the definition of metric already: {{ metricName }} .

Having another bound makes the formula harder to read. Please use only one way to set the boundaries.

Upper Bound
Good values can be bound.

The actual Upper Threshold (`$UT`) is part of the SLO definition.

There seems to be a comparator in the definition of metric already: {{ metricName }} .

Having another bound makes the formula harder to read. Please use only one way to set the boundaries.

Service Level Formula
The formula for calculating SLI for the given SLO window is the percentage of good per valid.

Depending on whether the SLI is time-based or event-based, the formula calculates the percentage of bad time or bad events.
$SLI = \frac{Good {{ sloWindow.eventUnitNorm }}}{Valid {{ sloWindow.eventUnitNorm }}} \times 100$

SLO: {{ percL10n(slo) }}
Service Level Objective (SLO) is the target percentage of good {{ sloWindow.eventUnitNorm }} out of total {{ sloWindow.eventUnitNorm }} in {{ sloWindow }}.

Using the two sliders below you can fine tune the SLO to your needs. The first slider is for the integer part of the percentage ({{ percL10n(sloInt) }}). The second slider is for the fractional part of the percentage ({{ percL10n(sloFrac) }}).

Typical SLO values

Informal Name SLO Value

{{ p.title }} {{percL10n(p.slo)}}

Typical SLO values
Informal Name	SLO Value
{{ p.title }}	{{percL10n(p.slo)}}

Note: Just be mindful of the price tag for this high service level objective!

Everyone wants the highest possible number but not everyone is willing to pay the price.

Note: this is an unusually low service level objective. Typically service level objective is above {{ percL10n(90) }} with some rare exceptions. Please check the Error budget for implications of your chosen SLO.

Fraction: {{ percL10n(sloFrac) }}
This slider allows fine tuning the SLO. It is mostly for convenience when deciding a reasonable error budget while keeping an eye on it.

{{ sloInt }}.XYZ Subtract Add

X

Y

Z

XYZ

Window: {{ windowDays }} days
The SLO window (also known as the compliance period) is the time period for which the SLO is calculated.

Essentially this adjusts the forgiveness of the SLO. For example if the window is 30 days, we are not concerned with any incidents and breaches of SLO that happened before that.

Smaller windows also help prevent the error budget from accumulating too much. For example, if the SLO is 99% for a time-based Availability SLI (uptime), the error budget allows 432 minutes of downtime per month. This amount can be consumed in multiple down times during the month or one chunk of long downtime. But the same SLO allows only 100 minutes of downtime per week.

It is usually 30 days or 4 weeks.

You can play with different ranges to see how a given SLO translates to different good {{ sloWindow.eventUnitNorm }} and how it impacts the error budget.

Typical compliance periods

Window Days Advantage

{{ p.title }} {{ p.days }} {{ p.useCase }}

Typical compliance periods
Window	Days	Advantage
{{ p.title }}	{{ p.days }}	{{ p.useCase }}

Thresholds {{ eventUnit }}
The thresholds are the values that put boundaries on the values of {{ metricName }} to define good {{ sloWindow.eventUnitNorm }}.

They are part of the SLO definition and allow for Multi-Tierd SLOs .

$LT (`{{ metricUnit }}`)
The lower threshold (LT) defines the minimum possible values for the {{ metricName }} (in `{{ metricUnit }}`) to indicate good {{ sloWindow.eventUnitNorm }}.

LT is part of the SLO definition and for example allows for Multi-Tierd SLOs .

$UT (`{{ metricUnit }}`)
The upper threshold (UT) defines the maximum possible values for the {{ metricName }} (in `{{ metricUnit }}`) to indicate good {{ sloWindow.eventUnitNorm }}.

UT is part of the SLO definition and for example allows for Multi-Tierd SLOs .

The upper threshold must be greater than the lower threshold.

Service Level Status
Service Level Status (SLS) is the percentage of good {{ sloWindow.eventUnitNorm }} in a given time.

SLS is the status of the Service Level and directly relates to SLO. Whenever SLS is below SLO, we have breached the SLO. In case of SLA, this may have severe consequences.

Error budget: {{ percL10n(errorBudgetPerc) }}
Error budget is one of the core ideas behind using SLI/SLOs to improve reliability. Instead of denying or forbidding errors, error budget allows the system to fail within a pre-defined limit.

The number one enemy of reliability is change. But we need change to be able to improve the system. Error budgets do exactly that. They provide a budget of error for the team to improve the system while keeping the consumers happy enough.

Error budget is the complement of SLO. It is the percentage of bad {{ sloWindow.eventUnitNorm }} that you can have before you violate the SLO.
$\begin{matrix} error_budget & = 100 - SLO \\ = {{ percL10n(100) }} - {{ percL10n(slo) }} = {{ percL10n(errorBudgetPerc) }} \end{matrix}$

Subtract Add

{{ numL10n(1) }}

{{ numL10n(10) }}

{{ numL10n(100) }}

{{ numL10n(1000) }}

{{ numL10n(10000) }}

Warning: The error budget is 0 based on your estimated number of valids {{ sloWindow.eventUnitNorm }}.

Expected number of `{{ eventUnit }}` in {{ sloWindow.humanTime }} Here you can enter the numbers for your expected load and see how many {{ eventUnit }} are allowed to fail during the SLO window while still being within the error budget.

Average cost of failed `{{ sloWindow.eventUnitNorm }}`
How much does a bad `{{ sloWindow.eventUnitNorm }}` cost the business or your team?

This cost will be used to put a tangible number on various windows and events. It might be hard to put a number on failures especially if some resilience patterns are part of the architecture.

There are many ways to make the failures cheaper. In a future article, we will discuss all patterns of reliability and how to make errors cheap. In the mean time check out the following techniques:

Fallback

Failover

Currency
You can set the currency to see how much it costs to violate the SLO. If you can't put a currency on the errors, feel free to get creative.

Typical Currencies
Abbreviation Description

{{ p.currency }} {{ p.description }}

Typical Currencies
Abbreviation	Description
{{ p.currency }}	{{ p.description }}

Alerting
What is the point of setting SLI/SLO if we are not going to take application when the SLO is violated?

Alerting on error budgets enable us to be on top of the reliability of our system. When using service levels, the alert triggers on the rate of consuming the error budget.

When setting an alert, the burn rate decides how quickly the alert reacts to errors.

Too fast and it will lead to false positives (alerting unnecessarily) and alert fatigue (too many alerts).

Too slow and the error burget will be burned before you know it.

Google SRE Workbook goes through 6 alerting strategies based on SLOs .

Burn rate: {{ burnRate }}x
Burn rate is the rate at which the error budget is consumed. It is the ratio of the error budget to the SLO window.

A burn rate of means that the error budget will be consumed during the SLO window (accepted).

A burn rate of means that the error budget will be consumed in half the SLO window. This is not acceptable because at this rate, the SLO will be violated before the end of the SLO window.

You have selected a burn rate of {{ burnRate }}x. This means the error budget ({{ errorBudget.eventCountL10n }} failed {{ errorBudget.eventUnitNorm }}) will be consumed in {{ errorBudgetBurn.humanTime }} instead of being spread across {{ sloWindow.humanTime }}.

If the error budget continues to burn at this rate throughout the SLO window, there will be {{ sloWindowBudgetBurn }}.

Google SRE Workbook goes through 6 alerting strategies and recommends :

Burn Rate Error Budget Long-Window Short-Window Action

14.4x {{ percL10n(2) }} Consumed 1 hour 5 minutes Page

6x {{ percL10n(5) }} Consumed 6 hours 30 minutes Page

1x {{ percL10n(10) }} Consumed 3 days 6 hours Ticket

Note: The above values for Long-Window and Short-Window are based on a 1-month SLO window. You can see your actual values in the comments below Long-Window and Short-Window.

Burn Rate	Error Budget	Long-Window	Short-Window	Action
14.4x	{{ percL10n(2) }} Consumed	1 hour	5 minutes	Page
6x	{{ percL10n(5) }} Consumed	6 hours	30 minutes	Page
1x	{{ percL10n(10) }} Consumed	3 days	6 hours	Ticket

Long-Window Long-window alert is the "normal" alert. The reason it is called "long" is to distinguish it from the "short-window" alert which is primarily used to reduce false positives and improve the alert reset time.

After Consuming ≥ {{ percL10n(longWindowPerc) }}
We don't want to wait for the entire error budget to be consumed before alerting! It will be too late to take action.

Therefore the alert should trigger before a significant portion of the error budget is consumed.

Based on your setup, the alert will trigger after we have consumed {{ percL10n(longWindowPerc) }} of the entire time allotted for the error budget (or SLO compliance window) which is {{ sloWindow.humanTime }}.
$TTTrigger = {{ percL10n(longWindowPerc) }} \times {{ sloWindow.humanSec }} = {{ alertLongWindow.humanSec }}$
Which is {{ alertLongWindow.humanTime }}.

Assuming that the entire error budget was available at the begining of the incident, the maximum time available to respond before the entire error budget is exhausted is:
${TTRespond}_{max} = {{ errorBudgetBurn.humanSec }} - {{ alertLongWindow.humanSec }} = {{ alertTTRWindow.humanSec }}$
Which is {{ alertTTRWindow.humanTime }}.

Remember that this is the best case scenario. In reality, you may have much less time if you don't want to consume the entire error budget for an incident. Also note that the burn rate can be higher than {{ burnRate }}x.

Warning: The alert is too "jumpy" and will trigger too often. This may lead to alert fatigue or even worse: ignoring the alerts.

Note: The time to resolve (TTR) is too short for a human to react. It is strongly recommended to automate the incident resolution instead of relying on human response to alerts.

Warning: Remember that the alert will trigger after {{ percL10n(longWindowPerc) }} of the error budget is consumed! That error budget is for {{ sloWindow.humanTime }}.

Based on your setting an alert burns {{ percL10n(longWindowPerc) }} just to trigger. Then it needs some time to resolve too.

How many alerts like this can you have in {{ sloWindow.humanTime }} before the entire error budget is consumed?

Warning: Long alert Window is too short at this burn rate ({{ burnRate }}x) which may lead to alert fatigue.

Error: Division by zero! Long alert Window is too short for enough valid {{ sloWindow.eventUnitNorm }} to be counted.

Alert Policy
This is a pseudo-code for trigerring alerts based on the SLI metric in relation to the desired SLO ( `{{ percL10n(slo) }}` ). You need to translate it to your observability and/or alerting tool.


                             ≤ 
                            
                                
                                    {{ percL10n(slo) }}


                            

                            &&
                            

                            
                            
                                
                                     ≤ 
                                    
                                        
                                            {{ percL10n(slo) }}

Use Short-Window Alert

The purpose of the short-window alert is to reduce false alerts.

It checks a shorter lookback window (hence the name) to make sure that the burn rate is still high before triggering the alert. This reduces false positives where an alert is triggered for a temporary high burn rate.

The Short-Window alert reduces false positives at the expence of making the alerting setup more complex.

Short/Long ratio: 1/{{ shortWindowDivider }}
The Short-Window is usually 1/12th of the Long-Window (per Google SRE Workbook recommendation). But you can play with different dividers to see how they impact the detection time of the alert.

Long-Window alert triggers after consuming {{ longWindowPerc }}% of the total error budget. Therefore, the Short-Window alert triggers after consuming:
${Lookbcack}_{Short} = \frac{{Lookback}_{Long}}{{{ shortWindowDivider }}} = \frac{{{ percL10n(longWindowPerc) }}}{{{ shortWindowDivider }}} = {{ percL10n(alertShortWindowPerc) }}$
Converted to time based on the current window ({{ sloWindow.humanTime }}):
${Lookbcack}_{Short} = {{ percL10n(alertShortWindowPerc) }} \times {{ sloWindow.humanSec }} = {{ alertShortWindow.humanSec }}$
This means the alert will trigger only if we are still burning the error budget at least at the {{ burnRate }}x burn rate in the past {{ alertShortWindow.humanTime }} .

Warning: Short alert Window is too short at this burn rate ({{ burnRate }}x) which may lead to alert fatigue.

Error: Short alert Window is too short for enough valid events to be counted.

	Subtract	Add
{{ numL10n(1) }}
{{ numL10n(10) }}
{{ numL10n(100) }}
{{ numL10n(1000) }}
{{ numL10n(10000) }}

Title

Description

Good {{ sloWindow.eventUnitNorm }}

Metric name What are good {{ sloWindow.eventUnitNorm }} from consumer's perspective? How do good {{ sloWindow.eventUnitNorm }} look like? What is the metric that you can measure to identify the good {{ sloWindow.eventUnitNorm }} from all the valid {{ sloWindow.eventUnitNorm }}?

Metric Unit This is the unit of the metric ( {{ metricName }} ) not to be confused with the unit of the events ( {{ sloWindow.eventUnitNorm }} ).

Lower Bound Good values can be bound. The actual Lower Threshold ($LT) is part of the SLO definition.

Upper Bound Good values can be bound. The actual Upper Threshold ($UT) is part of the SLO definition.

Thresholds {{ eventUnit }} The thresholds are the values that put boundaries on the values of {{ metricName }} to define good {{ sloWindow.eventUnitNorm }}. They are part of the SLO definition and allow for Multi-Tierd SLOs .

$LT ({{ metricUnit }}) The lower threshold (LT) defines the minimum possible values for the {{ metricName }} (in {{ metricUnit }}) to indicate good {{ sloWindow.eventUnitNorm }}. LT is part of the SLO definition and for example allows for Multi-Tierd SLOs .

$UT ({{ metricUnit }}) The upper threshold (UT) defines the maximum possible values for the {{ metricName }} (in {{ metricUnit }}) to indicate good {{ sloWindow.eventUnitNorm }}. UT is part of the SLO definition and for example allows for Multi-Tierd SLOs .

Service Level Status Service Level Status (SLS) is the percentage of good {{ sloWindow.eventUnitNorm }} in a given time. SLS is the status of the Service Level and directly relates to SLO. Whenever SLS is below SLO, we have breached the SLO. In case of SLA, this may have severe consequences.

Expected number of {{ eventUnit }} in {{ sloWindow.humanTime }} Here you can enter the numbers for your expected load and see how many {{ eventUnit }} are allowed to fail during the SLO window while still being within the error budget.

Currency You can set the currency to see how much it costs to violate the SLO. If you can't put a currency on the errors, feel free to get creative. Typical Currencies Abbreviation Description {{ p.currency }} {{ p.description }} Try!