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RFC-042: Implement Read Replicas for Analytics
Status: Draft | In Review | Accepted | Rejected | Implemented Author: Alice (alice@example.com) Reviewers: Bob, Charlie, David Created: 2024-01-15 Updated: 2024-01-20 Target Date: Q1 2024
Summary
Add PostgreSQL read replicas to separate analytical queries from transactional workload, improving database performance and enabling new analytics features.
Problem Statement
Current Situation
Our PostgreSQL database serves both transactional (OLTP) and analytical (OLAP) workloads:
- 1000 writes/min (checkout, orders, inventory)
- 5000 reads/min (user browsing, search)
- 500 analytics queries/min (dashboards, reports)
Issues
- Performance degradation: Analytics queries slow down transactions
- Resource contention: Complex reports consume CPU/memory
- Blocking features: Can't add more dashboards without impacting users
- Peak hour problems: Analytics scheduled during business hours
Impact
- Checkout p95 latency: 800ms (target: <300ms)
- Database CPU: 75% average, 95% peak
- Customer complaints about slow pages
- Product team blocked on analytics features
Success Criteria
- Checkout latency <300ms p95
- Database CPU <50%
- Support 2x more analytics queries
- Zero impact on transactional performance
Proposed Solution
High-Level Design
┌─────────────┐ │ Primary │────────────────┐ │ (Write) │ │ └─────────────┘ │ ▼ ┌─────────────┐ │ Replica 1 │ │ (Read) │ └─────────────┘ ▼ ┌─────────────┐ │ Replica 2 │ │ (Analytics)│ └─────────────┘
Architecture
- Primary database: Handles all writes and critical reads
- Read Replica 1: Serves user-facing read queries
- Read Replica 2: Dedicated to analytics/reporting
Routing Strategy
const db = {
primary: primaryConnection,
read: replicaConnection,
analytics: analyticsConnection,
};
// Write
await db.primary.users.create(data);
// Critical read (always fresh)
await db.primary.users.findById(id);
// Non-critical read (can be slightly stale)
await db.read.products.search(query);
// Analytics
await db.analytics.orders.aggregate(pipeline);
Replication
- Type: Streaming replication
- Lag: <1 second for read replica, <5 seconds acceptable for analytics
- Monitoring: Alert if lag >5 seconds
Detailed Design
Database Configuration
# Primary
max_connections: 200
shared_buffers: 4GB
work_mem: 16MB
# Read Replica
max_connections: 100
shared_buffers: 8GB
work_mem: 32MB
# Analytics Replica
max_connections: 50
shared_buffers: 16GB
work_mem: 64MB
Connection Pooling
const pools = {
primary: new Pool({ max: 20, min: 5 }),
read: new Pool({ max: 50, min: 10 }),
analytics: new Pool({ max: 10, min: 2 }),
};
Query Classification
enum QueryType {
WRITE = "primary",
CRITICAL_READ = "primary",
READ = "read",
ANALYTICS = "analytics",
}
function route(queryType: QueryType) {
return pools[queryType];
}
Alternatives Considered
Alternative 1: Vertical Scaling
Approach: Upgrade to larger database instance
- Pros: Simple, no code changes
- Cons: Expensive ($500 → $2000/month), doesn't separate workloads, still hits limits
- Verdict: Rejected - doesn't solve isolation problem
Alternative 2: Separate Analytics Database
Approach: Copy data to dedicated analytics DB (e.g., ClickHouse)
- Pros: Optimal for analytics, no impact on primary
- Cons: Complex ETL pipeline, eventual consistency, high maintenance
- Verdict: Defer - consider for future if replicas insufficient
Alternative 3: Materialized Views
Approach: Pre-compute analytics results
- Pros: Fast queries, no replicas needed
- Cons: Limited to known queries, maintenance overhead
- Verdict: Complement to replicas, not replacement
Tradeoffs
What We're Optimizing For
- Performance isolation
- Cost efficiency
- Quick implementation
- Operational simplicity
What We're Sacrificing
- Slight data staleness (acceptable for analytics)
- Additional infrastructure complexity
- Higher operational costs
Risks & Mitigations
Risk 1: Replication Lag
Impact: Analytics sees stale data
Probability: Medium
Mitigation:
- Monitor lag continuously
- Alert if >5 seconds
- Document expected lag for users
Risk 2: Configuration Complexity
Impact: Routing errors, performance issues
Probability: Low
Mitigation:
- Comprehensive testing
- Gradual rollout
- Easy rollback mechanism
Risk 3: Cost Overrun
Impact: Budget exceeded
Probability: Low
Mitigation:
- Use smaller instance for analytics ($300/month)
- Monitor usage
- Right-size after 1 month
Rollout Plan
Phase 1: Setup (Week 1-2)
- Provision read replica 1
- Provision analytics replica 2
- Configure replication
- Verify lag <1 second
- Load testing
Phase 2: Read Replica (Week 3)
- Deploy routing logic
- Route 10% search queries to replica
- Monitor errors and latency
- Ramp to 100%
Phase 3: Analytics Migration (Week 4-5)
- Identify analytics queries
- Update dashboard queries to analytics replica
- Test reports
- Migrate all analytics
Phase 4: Validation (Week 6)
- Measure checkout latency improvement
- Verify CPU reduction
- User acceptance testing
- Mark as complete
Success Metrics
Primary Goals
- ✅ Checkout latency <300ms p95 (currently 800ms)
- ✅ Primary DB CPU <50% (currently 75%)
- ✅ Zero errors from replication lag
Secondary Goals
- Support 2x analytics queries
- Enable new dashboard features
- Team satisfaction survey >8/10
Cost Analysis
Component
Current
Proposed
Delta
Primary DB
$500/mo
$500/mo
$0
Read Replica
-
$500/mo
+$500
Analytics Replica
-
$300/mo
+$300
Total
$500/mo
$1,300/mo
+$800/mo
ROI: Better performance enables revenue growth; analytics unlocks product insights
Open Questions
- What's acceptable replication lag for analytics? (Proposed: <5 sec)
- How do we handle replica failure? (Proposed: Fallback to primary)
- Should we add more replicas later? (Proposed: Monitor and decide in Q2)
Timeline
- Week 1-2: Provisioning and setup
- Week 3: Read replica migration
- Week 4-5: Analytics migration
- Week 6: Validation
- Total: 6 weeks
Appendix
References
- PostgreSQL Replication Docs
- Cost Analysis Spreadsheet
- Load Test Results
Review History
- 2024-01-15: Initial draft (Alice)
- 2024-01-17: Added cost analysis (Bob)
- 2024-01-20: Addressed review comments
## RFC Process
### 1. Draft (1 week)
- Author writes RFC
- Include problem, solution, alternatives
- Share with team for early feedback
### 2. Review (1-2 weeks)
- Distribute to reviewers
- Collect comments
- Address feedback
- Iterate on design
### 3. Approval (1 week)
- Present to architecture review
- Resolve remaining concerns
- Vote: Accept/Reject
- Update status
### 4. Implementation
- Track progress
- Update RFC with learnings
- Mark as implemented
## Best Practices
1. **Clear problem**: Start with why
2. **Concrete solution**: Be specific
3. **Consider alternatives**: Show you explored options
4. **Honest tradeoffs**: Every choice has costs
5. **Measurable success**: Define done
6. **Risk mitigation**: Plan for failure
7. **Iterative**: Update based on feedback
## Output Checklist
- [ ] Problem statement
- [ ] Proposed solution with architecture
- [ ] 2+ alternatives considered
- [ ] Tradeoffs documented
- [ ] Risks with mitigations
- [ ] Rollout plan with phases
- [ ] Success metrics defined
- [ ] Cost analysis
- [ ] Timeline estimated
- [ ] Reviewers assigned