evoq

%%% @doc 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.
%%%
%%% == v1 limitations ==
%%%
%%%   - context_filter() only supports any_of / all_of. Compound
%%%     and_ / or_ filters are supported by the backend's
%%%     conditional-append check but the runtime's read path doesn't
%%%     yet translate them; flat filters only.
%%%   - 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.
%%%
%%% @end
-module(evoq_decision).

-export_type([context_filter/0]).

%% Tag-filter for the consistency context. Per-event semantics:
%%   any_of(Tags)     - event has ANY of the given tags
%%   all_of(Tags)     - event has ALL of the given tags
%%   and_(Filters)    - event satisfies ALL sub-filters
%%   or_(Filters)     - event satisfies AT LEAST ONE sub-filter
%%
%% Mirrors reckon_gater_types:tag_filter() exactly. The runtime
%% read path supports compound filters as of evoq 1.18.1 (see
%% evoq_decision_runtime).
-type context_filter() ::
      {any_of, [binary()]}
    | {all_of, [binary()]}
    | {and_, [context_filter()]}
    | {or_,  [context_filter()]}.

%% Define the tag-filter context this decision queries.
-callback context(Command :: map()) -> context_filter().

%% Pure decision: given context events + command, return events to
%% append OR an error.
-callback decide(ContextEvents :: [map()], Command :: map()) ->
      {ok, [Event :: map()]}
    | {error, Reason :: term()}.

%% Maximum retries on context_changed conflicts. Default 3.
-callback retry_budget() -> non_neg_integer().
-optional_callbacks([retry_budget/0]).