Lifespan Strategies
Mutability × Time: The Compound Risk series – Part 4/4
The solution isn't to eliminate long-lived instances entirely—that's neither practical nor necessary. Instead, we need strategies that minimize risk while preserving functionality. The key is matching instance lifespan to actual need, and when longer lifespans are unavoidable, implementing patterns that contain the damage.
The safest approach is ensuring instances emerge fully formed from their creation methods, eliminating the dangerous gap between birth and initialization. Value objects exemplify this—immutable, self-contained, and impossible to corrupt after creation. When you need more complex initialization, builder patterns can construct the complete state before producing the final instance, ensuring no half-initialized objects ever escape into the wild.
For cases where long-lived instances are genuinely necessary, immutability becomes your primary defense. Copy-on-write semantics allow apparent mutation while preserving safety—each "change" produces a new instance rather than corrupting the original. Event sourcing takes this further, treating state changes as immutable events that can be replayed to reconstruct any point in the instance's history. When true mutability is unavoidable, consider immutable snapshots at key points, creating restore points that can recover from corruption.
Every system has a "trust horizon"—the point beyond which you can no longer confidently predict instance behavior. For some systems, this might be milliseconds; for others, hours or days. The key is to measure and monitor this boundary. Implement defensive patterns: validate state before critical operations, add logging that tracks state evolution, and consider automatic instance recycling when certain thresholds are crossed. Make illegal states literally unrepresentable through type systems and careful API design.
We eventually refactored our agent status manager using the simplest possible approach: eliminate the state entirely. Instead of storing the agent ID as instance state, we moved it to method parameters. Rather than manager.set_id(id) followed by manager.save(status), we implemented manager.save(agent_id, status). This shifted responsibility to the caller—they now needed to track and pass the agent ID—but the adjustment proved straightforward. The manager shed its problematic mutable state, transforming from an instance that accumulated agent-specific data into a collection of functions that, while still performing side effects through database connections, no longer carried business logic state between calls.
The goal isn't perfection—it's containment. When instances must live long, give them clear boundaries, immutable cores, and observable behavior. When they must be mutable, limit their scope and monitor their evolution. Most importantly, make the lifespan decision consciously rather than accidentally, because in software architecture, accidents compound over time.
Best Practices Design Philosophy
Jun 14 2025