Q: What does determinism mean in the context of a workflow engine?
A: Determinism means that a system always produces the same result given the same initial parameters and inputs. The outcome is fully determined by cause and effect, with no randomness or uncertainty.
Many popular workflow engines like GitHub Actions use a purely declarative model (YAML) where all workflow state is embedded right in the definition.
If you write your workflow in a general‑purpose language instead, the engine must capture and then recreate every bit of state deterministically so it can replay the execution reliably (for example, after a crash or restart).
Deterministic execution therefore requires that all workflow state, including child executions' parameters and their results, be persisted in an execution log.
Q: What is the difference between a workflow and an activity?
A: A workflow is your deterministic, side‑effect‑free orchestration logic. An activity is a unit of work that does produce side effects like executing one or more HTTP requests. Activities must be idempotent(retriable) so the runtime can safely retry them on failure. The workflow invokes activities, waits for their results, and then continues its logic based on those results.
Q: What happens when the system crashes during operation?
A: After a crash (or restart/update), the Obelisk runtime:
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Restarts each in-progress workflow from scratch with the same initial parameters.
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Replays each completed activity’s recorded result from the execution log.
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Retries any activity whose result wasn’t yet persisted when the crash occurred - another reason why activities must be idempotent.
Q: How to do a cleanup after an activity permanently failed?
A: Obelisk doesn’t yet offer built‑in saga support, so you handle compensation logic directly in your workflow. In practice, you wrap activities in a try/catch (or equivalent) block: if an activity exhausts its retries and fails, invoke the compensating activity to roll back or clean up any side effects and continue or exit the workflow function.