Optimizing Stream Design
Interview Phase (Optional)
When to Interview: Skip if the user has specified: expected event frequency, stream lifetime, and growth patterns. Interview when stream length concerns exist but growth estimates are unclear.
Interview Strategy: Establish growth expectations and performance requirements before recommending snapshotting. Most snapshotting proposals stem from poor stream boundary design, not genuine volume — surface the estimates first to distinguish real performance concerns from design problems.
Critical Questions
- Growth Estimates (Impact: Determines if snapshotting is needed or if stream design should change)
- Question: "Estimate events: (A) Per entity per day, (B) Lifetime total, (C) Growth over years. Example: 5-10 events/order, 1-10 million orders/year?" - Why it matters: Growth estimates reveal if streams will genuinely be too long or if design is wrong - Follow-up triggers: If estimates exceed 300 events per stream lifetime → ask "Is the stream identity correct? Could this stream be split by a narrower business entity?"
- Performance SLAs (Impact: Determines acceptable latency and snapshotting decisions)
- Question: "Performance requirements? (A) <100ms read latency, (B) <1s acceptable, (C) Eventual consistency OK?" - Why it matters: Strict SLAs might need snapshotting; loose SLAs often don't - Follow-up triggers: If (A) → ask "Which commands specifically need sub-100ms replay? Are those commands reading from a read model or replaying the stream directly?"
Interview Flow
Conditional Entry:
If user has provided:
- Event frequency estimate (per entity per day or per transaction)
- AND stream lifetime estimate (months or years)
- AND read latency SLA (or confirmation that eventual consistency is acceptable)
Then: Skip interview, proceed directly to stream analysis
Else: Conduct interviewPhase 1: Growth Estimation (Question 1)
- Establish per-entity event volume
- Project lifetime stream length using the estimation formula
- Determine whether redesign or snapshotting analysis is warranted
Phase 2: SLA Requirements (Question 2)
- Identify read latency requirements per command
- Determine whether read models or direct stream replay satisfies the SLA
- Establish whether snapshotting is justified by SLA alone
Capturing Interview Findings
Append findings to the project's event modeling file:
File: .trogonai/interviews/[project-name]/EVENTMODELING.md
Use Write tool to add/update this section:
## Optimizing Stream Design (eventmodeling-optimizing-stream-design)
### Growth Estimates
[From Q1: Events per entity per day, lifetime total, annual growth]
### Performance SLAs
[From Q2: Latency requirements per command or view]
### Optimization Decisions
- Streams requiring redesign: [list or "None"]
- Streams where snapshotting is justified: [list or "None"]
- Streams within acceptable bounds: [list]Update Interview Trail:
| Optimization | eventmodeling-optimizing-stream-design | Done | Stream growth estimates, SLA review, snapshotting decisions |Stream Design Optimization
Purpose: Optimize event stream design by validating stream boundaries, estimating growth, and making snapshotting decisions based on design quality—not just size.
Applies To: Any domain - e-commerce, banking, SaaS, marketplace, healthcare, etc.
When to Use:
- After defining event streams in domain analysis
- When concerned about stream length or performance
- Before implementing to validate stream design
- During performance planning to determine snapshotting strategy
- When redesigning streams for scalability
What It Does:
- Analyzes event stream design for proper event organization
- Estimates stream growth over time
- Identifies when snapshotting is genuinely needed vs. design issue
- Recommends optimal stream identity boundaries
- Balances performance optimization against complexity
- Provides snapshotting strategy without over-engineering
Core Principle: Design First, Snapshot Second
Golden Rule:
If you find yourself needing to snapshot because the stream is too long, first ask: "Is my stream identity wrong?" Usually, the answer is yes.
Snapshotting is a performance optimization, not a design problem. Good stream design (proper identity boundaries) often eliminates the need for snapshotting entirely.
Stream Design Analysis Framework
1. Estimate Stream Growth
Formula:
Estimated Stream Length (total events/instance) =
Events Per Aggregate Instance Per Year (events/instance/year)
× Lifetime of Instance (years)
× Annual Growth Factor (dimensionless year-over-year multiplier ≥ 1.0)Quick Examples:
E-commerce Order: 8 events/year × 1.5 year lifetime = 8-16 events → NOT NEEDED
Banking Account: 100-200 events/year × 10 years = 1000-2000 events → CONSIDER AT 1000+
Order Processing: 100+ events/year × 5 years = 300-500+ events → PROBABLY NEEDED
SaaS User: 12-60 events/year × 5 years = 60-300 events → RARELY NEEDED
2. Identify Stream Length Categories
| Length | Status | Action | Snapshotting |
|---|---|---|---|
| < 50 events | IDEAL | Keep as-is | NOT NEEDED |
| 50-100 events | GOOD | Monitor growth | NOT NEEDED |
| 100-300 events | ACCEPTABLE | Review boundary | CONSIDER if replayed |
| 300-1000 events | LONG | REDESIGN first | Only last resort |
| 1000+ events | CRITICAL | REDESIGN required | Won't help |
Quick Decision Matrix
| Stream Length | Read Pattern | Frequency | Action |
|---|---|---|---|
| < 50 | Any | Any | IDEAL - Keep as-is |
| 50-100 | Any | Any | Good - Monitor |
| 100-300 | From Model | Any | OK - No snapshot |
| 100-300 | Stream Replay | Low | OK - Monitor |
| 100-300 | Stream Replay | High | REDESIGN |
| 300-1000 | From Model | Any | OK - No snapshot |
| 300-1000 | Stream Replay | Any | REDESIGN |
| 1000+ | Any | Any | CRITICAL - REDESIGN |
Reference Files
Aggregate Boundary Design: See patterns.md for:
- 5 aggregate patterns (single entity, composite, collections, event logs, historical)
- Stream size decision tree
- Red flags that indicate redesign needed
- Tips for optimal stream design
Snapshotting Strategy: See snapshotting.md for:
- Criteria for when snapshotting is truly needed
- Context-based decision thresholds
- Snapshot frequency, versioning, and cleanup strategies
- Cost-benefit analysis
Domain-Specific Guidance: See domain-patterns.md for:
- E-commerce patterns (orders, carts, accounts)
- Banking patterns (accounts, transactions, loans)
- SaaS patterns (subscriptions, workspaces, data collections)
- Implementation checklist
Key Insights
Why Snapshotting Usually Isn't the Answer
Before implementing snapshotting, ask:
1. Can I split this aggregate into smaller ones?
→ YES: Do that instead. Simpler, better design.
2. Can I reduce event granularity?
→ YES: Batch events or create coarser state changes.
3. Am I using a read model for this aggregate?
→ NO: Create a read model (cached projection).
Stream size becomes irrelevant.
4. Have I measured actual replay latency?
→ NO: Measure first. Most systems exceed expectations.
If ANY of these is YES, do that before snapshotting.
Only after exhausting design improvements, consider snapshots.The Snapshotting Trade-off
Snapshotting Complexity ≈ 2-3x Complexity of Better Design
Before snapshot: 50 lines of code, simple, testable
With snapshots: 150+ lines, versioning, recovery logic, testing matrix
Better to redesign and keep streams < 300 events.Quality Checklist
- Each stream is identified by a business entity identity (e.g.,
orderId), not a category or type - No stream grows unboundedly without a design reason — event frequency and stream lifetime estimated
- Streams under 1000 events require no snapshotting justification
- If snapshotting is proposed, all simpler alternatives (split stream, shorter lifetime) have been eliminated first
- Command handler state is reconstructed from stream events — no persistent state stored outside the stream
- Each stream can be independently versioned and replayed without affecting other streams