evoq_decision behaviour (evoq v1.23.0)

DCB Decision behaviour (Dynamic Consistency Boundary).

A Decision is a write-side construct that sits alongside the aggregate. Where an aggregate locks on its own stream's version, a Decision locks on the absence of new events matching a tag-filter context query. Decisions are for cross-cutting checks that don't fit the per-entity Dossier shape: uniqueness, allocation against shared resources, idempotency keys, rate limits.

Callbacks

Required: context(Command :: map()) -> context_filter() Describe the tag-filter context this decision queries. The runtime reads matching events, then passes them to decide/2.

decide(ContextEvents :: [map()], Command :: map()) -> {ok, [Event :: map()]} | {error, Reason :: term()} Pure decision function. Given the current context and the command, return the events to append OR an error. The runtime appends conditionally via the configured event-store adapter.

Optional: retry_budget() -> non_neg_integer() Maximum retries on context_changed conflicts. Default 3.

CCC payload conditions

The context query can scope on opaque event-data fields, not just tags/types: {payload_match, Key, Value} and {payload_hash_match, Keys, Values}. The store must declare the matching payload index; otherwise the decision fails loudly with {error, {payload_index_unavailable, Filter}}. See the context_filter() type docs.

limitations

- The runtime considers only events from the DCB pseudo-stream (the binary "_dcb"). Mixed-mode use cases (aggregate streams + DCB sharing tags) are not supported; use evoq_aggregate if the consistency boundary is per-aggregate, evoq_decision if it's cross-cutting via DCB.

Summary

Types

context_filter/0

Callbacks

apply_context_event/2

boundary_key/1

context/1

decide/2

init_decision_model/0

retry_budget/0

Types

context_filter/0

-type context_filter() ::
          {any_of, [binary()]} |
          {all_of, [binary()]} |
          {event_type, binary()} |
          {payload_match, Key :: binary(), Value :: binary()} |
          {payload_hash_match, Keys :: [binary()], Values :: [binary()]} |
          {and_, [context_filter()]} |
          {or_, [context_filter()]}.

Callbacks

apply_context_event/2

(optional)

-callback apply_context_event(Model :: term(), Event :: map()) -> Model :: term().

boundary_key/1

(optional)

-callback boundary_key(Command :: map()) -> binary() | undefined.

context/1

-callback context(Command :: map()) -> context_filter().

decide/2

-callback decide(ContextEvents :: [map()], Command :: map()) ->
                    {ok, [Event :: map()]} | {error, Reason :: term()}.

init_decision_model/0

(optional)

-callback init_decision_model() -> Model :: term().

retry_budget/0

(optional)

-callback retry_budget() -> non_neg_integer().