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Two Roads to Durable Agents: Replay vs. Snapshot — Eric Allam, CEO, Trigger.dev
Takeaway
Durable agents need either replay-style journaling (Temporal) or snapshot-style state capture; replay's determinism constraints make it awkward for LLM-driven workflows.
Summary
- Eric Allam (Trigger.dev) traces backend history from CGI (1993) to LAMP's shared-nothing/stateless compute + DB-state model that dominated 30 years.
- Workflow/durable-execution engines (Temporal-style) emerged ~10-15 years ago to handle multi-step side effects via the 'replay' model: wrap every side effect as a cached, deterministic step.
- Replay model's downsides: rigid code structure, deterministic-only outside steps, painful versioning of replay journals across deploys — increasingly painful for LLM agents.
- Argues for an alternative 'snapshot' approach to durability for agents that doesn't force the replay-journal structure on developers.
durable-executionagentstrigger-dev
Original description
Replay-based durability — wrapping every step in a journal, replaying on recovery, requiring deterministic code — is how everyone makes agents durable today. It works until it doesn't: the journal grows with every turn, the structure starts constraining how you write code, and an agent that needs to run for hours starts looking less like a transaction and more like a session. This talk separates the problem in two: context durability (the append-only log of everything the LLM saw, which already fits in a database) and execution durability (the files, memory, and subprocesses that live in the compute layer, which don't). The answer to the second half isn't a smarter log — it's OS-level snapshot and restore. Eric Allam walks through how Trigger.dev built this on Firecracker microVMs, getting snapshots down to 14 megabytes compressed with sub-second save and hundred-millisecond restore times, and why IBM mainframes in 1966 got there first. Speaker info: - https://x.com/maverickdotdev - https://www.linkedin.com/in/eric-allam/ - https://github.com/ericallam