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Event sourcing begins with a clear thesis: every state change in the domain is captured as an immutable event that can be stored, replayed, and inspected. In TypeScript, you can enforce this discipline by modeling events as discriminated unions and using exhaustive type guards to ensure all event kinds are handled correctly. Start by defining a canonical Event interface that includes a type field, a timestamp, and a payload that is strongly typed for each variant. This approach prevents accidental loss of information and makes the commit history self-describing. As the system evolves, new events should extend this union in a backward-compatible way, so older readers remain functional.

A well-typed event store complements the event definitions by preserving exact sequences without mutation. Use a durable, append-only log that records serialized events, ensuring each entry is immutable once written. In TypeScript, you can model the persisted representation with a generic, parameterized Message type that carries the serialized event along with a version and a checksum. This structure supports safe deserialization and validation at read time. Implement an event metadata layer to capture provenance, such as actor identity and source, which aids debugging and audit trails without polluting the core domain events.

The first practical step is to establish a strict contract between domain events and their readers. Create a sealed hierarchy where each event type is explicitly listed and cannot silently drift into an unsupported shape. Employ TypeScript’s literal types and discriminated unions to force exhaustive checks in downstream handlers. Pair each event with a corresponding payload schema validated at runtime using a library or a bespoke validator. The combination of compile-time guarantees and runtime validation catches misalignments early, preventing subtle bugs from propagating through the event stream. Over time, maintainers should update both the TypeScript types and the runtime validators in tandem to preserve alignment.

When replaying events to reconstruct state, deterministic behavior is essential. Design your domain aggregates to apply events in the exact order they were emitted and to respond to each event in a purely functional style. Avoid mutating input events or attempting to derive state from partial histories. Instead, design an apply method for each aggregate that takes an event and returns a new, updated instance. By keeping side effects out of the apply step and recording only domain events, you achieve reproducibility. Coupled with strict typing, this model makes it straightforward to test replay scenarios and prove that the current state is faithful to the historical narrative.

The write path is where type safety proves its value most directly. Implement a serializer that translates strongly typed events into a transport-safe wire format, with a clearly defined schema per event variant. Include an event envelope containing type, version, and metadata to facilitate backward compatibility. Validation should occur both at serialization and deserialization boundaries to guard against malformed data. In TypeScript, leverage generics to capture the event type throughout the write process, ensuring that only valid payload shapes pass through. This discipline reduces runtime surprises and makes it easier to diagnose issues when they arise, especially in distributed systems.

The read path must rehydrate state without ambiguity. When deserializing events, map the raw payload back into the exact event variant and reconstruct the aggregate’s past by folding events from the initial version to the present. Use a factory or registry that can instantiate the correct event class based on the type discriminator, throwing a precise error if an unknown event type is encountered. Maintain an immutable rehydration flow that never mutates an existing event stream; instead, it generates a new state snapshot for each replay. This approach provides strong guarantees about the integrity of the reconstructed domain and makes it easier to troubleshoot inconsistencies.

Embracing immutability in the domain requires careful modeling of commands and events. Separate the intent (command) from the record of what happened (event) so the system can validate the feasibility of a request before it becomes a fact. In TypeScript, define a Command type that carries the necessary data and a ValidateResult outcome. If validation passes, emit one or more events that reflect the actual changes. This separation keeps the system resilient to partial failures and helps ensure that only verifiable changes become part of the persisted history.

Another crucial pattern is event versioning. As business rules evolve, events may change shape, add fields, or rename properties. Introduce a non-breaking versioning strategy that tags events with a version and provides adapters to translate older versions to the current schema. Keep the canonical form immutable and preserve historical payloads exactly as emitted. When reading, apply the appropriate migration logic to each event version before applying it to the aggregate. This approach protects long-term compatibility and reduces the risk of data loss during evolution.

Observability in an event-sourced system means more than logging. Build a granular observability layer that records successful and failed replays, deserialization errors, and the health of the event store. Use structured telemetry to connect events to business outcomes, enabling analysts to query how particular events influenced state changes over time. In TypeScript, you can define a lightweight tracing schema that attaches contextual data to each event, such as correlation IDs and user segments. This data becomes invaluable when diagnosing production issues or auditing the system's behavior in complex workflows.

Governance ensures the event stream remains trustworthy as the system grows. Enforce access controls on who can publish or modify events and establish a clear policy for retention and archival. Keep a tamper-evident log by leveraging append-only storage and cryptographic hashing to detect any alteration of historical events. Regularly perform integrity checks that compare event histories against derived snapshots to confirm consistency. Document the evolution of the event types, validators, and migrations so new team members can quickly understand how the domain history was captured and preserved.

Start small with a minimal, well-typed event model and a lean event store, then gradually expand as needs arise. Define a single aggregate as a proof of concept, implement the full write-read cycle, and verify deterministic replay against a known state. Focus on precise error messages and predictable failure modes so developers can quickly identify why a particular event could not be applied or deserialized. As you scale, automation around code generation for event types and validators can help maintain consistency across services and teams, reducing manual drift and misalignment.

Finally, invest in testing that targets the guarantees your design provides. Create property-based tests to exercise all possible event sequences and validate that the emitted events, when replayed, yield the same aggregate state. Include regression tests that simulate schema changes and ensure migrations preserve historical semantics. Integrate tests with your continuous integration pipeline to catch incompatibilities early. By coupling rigorous typing, deterministic replay, and disciplined migration, you build an ecosystem where immutable domain changes are captured faithfully, audited comprehensively, and replayed with confidence.