Google Site Reliability Engineering (SRE)
Overview
Site Reliability Engineering (SRE) is Google's approach to running production systems. It applies software engineering principles to operations, treating reliability as a feature that can be measured, budgeted, and engineered.
References
- Book: "Site Reliability Engineering: How Google Runs Production Systems" (O'Reilly, 2016)
- Workbook: "The Site Reliability Workbook" (O'Reilly, 2018)
- Online: https://sre.google/
Core Philosophy
"Hope is not a strategy."
"SRE is what happens when you ask a software engineer to design an operations function."
"Reliability is the most important feature."
SRE balances the tension between development velocity and system reliability using measurable objectives and error budgets.
Key Concepts
The Service Level Hierarchy
SLI (Service Level Indicator)
↓ Quantitative measure of service
↓ Example: "Request latency < 100ms"
SLO (Service Level Objective)
↓ Target value for SLI
↓ Example: "99.9% of requests < 100ms"
SLA (Service Level Agreement)
↓ Contract with consequences
↓ Example: "If SLO missed, credits issued"
Error Budget = 100% - SLO
Example: 99.9% SLO = 0.1% error budget = 43 minutes/month downtimeError Budget Philosophy
Error Budget Remaining?
│
┌───┴───┐
│ │
YES NO
│ │
↓ ↓
Ship new Focus on
features reliabilityDesign Principles
- Embrace Risk: 100% reliability is wrong target; it's too expensive.
- Error Budgets: Explicit budget for unreliability enables velocity.
- Eliminate Toil: Automate repetitive operational work.
- Simplicity: Simple systems are more reliable.
When Implementing
Always
- Define SLIs before launching a service
- Set SLOs based on user needs, not engineering pride
- Track error budget consumption
- Measure and reduce toil
- Conduct blameless postmortems
- Automate incident response where possible
Never
- Set SLOs at 100% (it's impossible and wrong)
- Ignore SLO violations
- Blame individuals for outages
- Accept toil as "just how things are"
- Skip postmortems for "small" incidents
Prefer
- Automation over manual processes
- Gradual rollouts over big-bang deploys
- Monitoring over hoping
- Documentation over tribal knowledge
- Proactive work over reactive firefighting
Implementation Patterns
Defining SLIs and SLOs
# slo_definitions.py
# Define service level indicators and objectives
from dataclasses import dataclass
from enum import Enum
from typing import Optional
class SLIType(Enum):
AVAILABILITY = "availability"
LATENCY = "latency"
THROUGHPUT = "throughput"
ERROR_RATE = "error_rate"
FRESHNESS = "freshness"
@dataclass
class SLI:
"""Service Level Indicator - what we measure"""
name: str
type: SLIType
description: str
good_event_query: str # Events that are "good"
total_event_query: str # All events
def calculate(self, good_count: int, total_count: int) -> float:
if total_count == 0:
return 1.0
return good_count / total_count
@dataclass
class SLO:
"""Service Level Objective - our target"""
sli: SLI
target: float # e.g., 0.999 for 99.9%
window_days: int # Rolling window
@property
def error_budget(self) -> float:
"""How much unreliability we can tolerate"""
return 1.0 - self.target
def budget_remaining(self, current_sli: float) -> float:
"""What percentage of error budget remains"""
errors_used = 1.0 - current_sli
if self.error_budget == 0:
return 0.0
return max(0, 1.0 - (errors_used / self.error_budget))
# Example SLO definitions
availability_sli = SLI(
name="api_availability",
type=SLIType.AVAILABILITY,
description="Proportion of successful API requests",
good_event_query="http_status < 500",
total_event_query="all requests"
)
availability_slo = SLO(
sli=availability_sli,
target=0.999, # 99.9% availability
window_days=30 # Rolling 30-day window
)
# 99.9% over 30 days = 43 minutes of allowed downtimeError Budget Tracking
# error_budget.py
# Track and alert on error budget consumption
import time
from dataclasses import dataclass
from typing import List
from datetime import datetime, timedelta
@dataclass
class ErrorBudgetTracker:
slo: 'SLO'
window_seconds: int
def __init__(self, slo: 'SLO'):
self.slo = slo
self.window_seconds = slo.window_days * 24 * 60 * 60
self.events: List[tuple] = [] # (timestamp, is_good)
def record_event(self, is_good: bool):
"""Record an event"""
now = time.time()
self.events.append((now, is_good))
self._prune_old_events(now)
def _prune_old_events(self, now: float):
"""Remove events outside window"""
cutoff = now - self.window_seconds
self.events = [(t, g) for t, g in self.events if t > cutoff]
def current_sli(self) -> float:
"""Calculate current SLI value"""
if not self.events:
return 1.0
good = sum(1 for _, is_good in self.events if is_good)
return good / len(self.events)
def budget_remaining_percent(self) -> float:
"""Percentage of error budget remaining"""
return self.slo.budget_remaining(self.current_sli()) * 100
def time_until_budget_exhausted(self) -> Optional[timedelta]:
"""Estimate when budget will be exhausted at current burn rate"""
remaining = self.budget_remaining_percent()
if remaining <= 0:
return timedelta(0)
# Calculate burn rate (budget consumed per hour)
# This is simplified - real implementation needs more data
return None # Requires historical burn rate
def should_freeze_deployments(self) -> bool:
"""Should we stop deploying new features?"""
return self.budget_remaining_percent() < 10 # Less than 10% remaining
# Alert policies based on error budget
def create_error_budget_alerts(tracker: ErrorBudgetTracker):
"""Create tiered alerts for error budget consumption"""
remaining = tracker.budget_remaining_percent()
if remaining < 0:
return "CRITICAL: Error budget exhausted! Focus 100% on reliability."
elif remaining < 10:
return "WARNING: Error budget nearly exhausted. Freeze deployments."
elif remaining < 25:
return "CAUTION: Error budget below 25%. Review recent changes."
elif remaining < 50:
return "INFO: Error budget at 50%. Monitor closely."
else:
return "OK: Error budget healthy. Safe to ship features."Toil Measurement and Elimination
# toil_tracker.py
# Measure and track operational toil
from dataclasses import dataclass
from enum import Enum
from typing import List, Dict
from datetime import datetime, timedelta
class ToilCategory(Enum):
MANUAL = "manual" # Could be automated
REPETITIVE = "repetitive" # Done frequently
TACTICAL = "tactical" # Reactive, not strategic
NO_VALUE = "no_value" # Doesn't improve service
SCALES_LINEARLY = "scales" # Grows with service size
@dataclass
class ToilTask:
name: str
categories: List[ToilCategory]
time_spent_minutes: int
frequency_per_week: float
automation_possible: bool
automation_effort_days: float
@property
def weekly_toil_hours(self) -> float:
return (self.time_spent_minutes * self.frequency_per_week) / 60
@property
def automation_roi_weeks(self) -> float:
"""Weeks until automation pays off"""
if not self.automation_possible:
return float('inf')
effort_hours = self.automation_effort_days * 8
return effort_hours / self.weekly_toil_hours
@dataclass
class ToilBudget:
"""SRE teams should spend <50% time on toil"""
team_size: int
hours_per_week: int = 40
max_toil_percent: float = 0.50
@property
def max_toil_hours_per_week(self) -> float:
return self.team_size * self.hours_per_week * self.max_toil_percent
def is_over_budget(self, current_toil_hours: float) -> bool:
return current_toil_hours > self.max_toil_hours_per_week
class ToilTracker:
def __init__(self, budget: ToilBudget):
self.budget = budget
self.tasks: Dict[str, ToilTask] = {}
def add_task(self, task: ToilTask):
self.tasks[task.name] = task
def total_weekly_toil(self) -> float:
return sum(t.weekly_toil_hours for t in self.tasks.values())
def toil_percent(self) -> float:
max_hours = self.budget.team_size * self.budget.hours_per_week
return (self.total_weekly_toil() / max_hours) * 100
def automation_priorities(self) -> List[ToilTask]:
"""Rank tasks by automation ROI"""
automatable = [t for t in self.tasks.values() if t.automation_possible]
return sorted(automatable, key=lambda t: t.automation_roi_weeks)
def report(self) -> str:
report = []
report.append(f"Total weekly toil: {self.total_weekly_toil():.1f} hours")
report.append(f"Toil percentage: {self.toil_percent():.1f}%")
report.append(f"Budget: {self.budget.max_toil_percent * 100}%")
report.append(f"Status: {'OVER' if self.toil_percent() > 50 else 'OK'}")
report.append("\nTop automation targets:")
for task in self.automation_priorities()[:5]:
report.append(f" - {task.name}: {task.automation_roi_weeks:.1f} weeks to ROI")
return "\n".join(report)Blameless Postmortem Template
# Postmortem: [Incident Title]
**Date**: YYYY-MM-DD
**Authors**: [Names]
**Status**: Draft | In Review | Complete
**Severity**: P0 | P1 | P2 | P3
## Summary
[2-3 sentences describing what happened, impact, and resolution]
## Impact
- **Duration**: X hours Y minutes
- **Users affected**: N users / X% of traffic
- **Revenue impact**: $X (if applicable)
- **Error budget consumed**: X%
## Timeline (all times UTC)
| Time | Event |
|------|-------|
| HH:MM | First alert fired |
| HH:MM | On-call engaged |
| HH:MM | Root cause identified |
| HH:MM | Mitigation applied |
| HH:MM | Service fully recovered |
## Root Cause
[Technical explanation of what caused the incident]
## Resolution
[What was done to resolve the incident]
## Detection
- How was the incident detected?
- Could we have detected it sooner?
- What monitoring would have helped?
## Lessons Learned
### What went well
- [Things that worked]
### What went wrong
- [Things that didn't work]
### Where we got lucky
- [Things that could have made it worse]
## Action Items
| Action | Type | Owner | Due Date | Status |
|--------|------|-------|----------|--------|
| Add monitoring for X | Detect | @name | YYYY-MM-DD | TODO |
| Implement circuit breaker | Mitigate | @name | YYYY-MM-DD | TODO |
| Update runbook | Process | @name | YYYY-MM-DD | TODO |
## Supporting Information
- Relevant logs, graphs, or documentation
- Links to related incidentsOn-Call Rotation Best Practices
# oncall.py
# On-call rotation management
from dataclasses import dataclass
from datetime import datetime, timedelta
from typing import List, Optional
@dataclass
class OnCallShift:
engineer: str
start: datetime
end: datetime
@property
def duration_hours(self) -> float:
return (self.end - self.start).total_seconds() / 3600
@dataclass
class OnCallPolicy:
"""Google SRE on-call best practices"""
# Shift structure
max_shift_hours: int = 12 # No more than 12 hours
min_time_between_shifts: int = 12 # At least 12 hours rest
max_incidents_per_shift: int = 2 # Escalate if exceeded
# Team structure
min_team_size: int = 8 # For sustainable rotation
secondary_oncall: bool = True # Always have backup
# Compensation
time_off_per_incident: float = 0.5 # Hours of comp time
def validate_shift(self, shift: OnCallShift,
previous_shifts: List[OnCallShift]) -> List[str]:
"""Check shift against policy"""
violations = []
if shift.duration_hours > self.max_shift_hours:
violations.append(
f"Shift too long: {shift.duration_hours}h > {self.max_shift_hours}h"
)
# Check rest time
for prev in previous_shifts:
if prev.engineer == shift.engineer:
gap = (shift.start - prev.end).total_seconds() / 3600
if gap < self.min_time_between_shifts:
violations.append(
f"Insufficient rest: {gap}h < {self.min_time_between_shifts}h"
)
return violations
def calculate_comp_time(self, incidents_handled: int) -> float:
"""Calculate compensation time for incidents"""
return incidents_handled * self.time_off_per_incidentMental Model
Google SRE asks:
- What's the SLO? Reliability target based on user needs
- What's the error budget? How much unreliability can we afford?
- Is this toil? Manual, repetitive, automatable, no lasting value?
- What does the postmortem say? Learn from failures, don't blame
- Can we ship this safely? Gradual rollout with monitoring
Signature SRE Moves
- Error budgets to balance reliability and velocity
- SLI/SLO/SLA hierarchy for clear targets
- Toil tracking and elimination
- Blameless postmortems
- On-call that doesn't burn out engineers
- Automation as first response to toil