# §4. Execution model overview > Sema language specification — §4 Execution model overview. > Generated from `docs/LANGUAGE.md` §4. This is the normative specification; for a guided tour see the Language and Neurosymbolic sections. (Details in RUNTIME.md; stated here because construct semantics depend on it.) 1. **Event-sourced semantics.** Every nondeterministic boundary event — model call (prompt, seed, model hash, output), embedding, contract verdict, policy decision, monitor sample — is recorded in a hash-chained append-only event log. Replay against the log is the *definition* of deterministic reproduction; the same substrate powers the semantic debugger, `monitor` windows, self-healing context, and training-data export ([02](./research/02-aion-os.md); [12 §4.4](./research/12-syntax-dx.md); Gen traces, [PLDI 2019](https://dl.acm.org/doi/10.1145/3314221.3314642)). 2. **Parallel by default.** Independent dataflow branches execute concurrently under structured-concurrency scopes (§5.12); there is no GIL. Model calls are non-blocking effects; the scheduler batches them (continuous batching default, [06](./research/06-runtime-substrate.md)) — SGLang's co-design lesson: program structure is a cache/batch opportunity ([04 §2.1](./research/04-ai-native-languages.md)). 3. **Two-phase generation.** Constructs with distributional semantics use free reasoning → constrained emission, never whole-output token masking, which distorts distributions and reasoning ([CRANE](https://arxiv.org/abs/2502.09061); [Grammar-Aligned Decoding](https://arxiv.org/abs/2405.21047)). 4. **Serial decode budget is a semantic parameter.** A `simulate` call's expressive class and reliability scale with granted decode length ([Merrill & Sabharwal, ICLR 2024](https://openreview.net/pdf?id=CDmerQ37Zs)); budgets appear in signatures and are enforced by the scheduler with typed budget-exceeded errors. 5. **One incremental query engine.** Compiler, LSP, verification memos, and the semantic knowledge graph are one Salsa-style red-green database ([12 §4.1](./research/12-syntax-dx.md); [11](./research/11-semantic-memory.md)); contracts in signatures act as cache firewalls ([09 §3.2](./research/09-verification-testing.md)). ---