Tracks event reads so appends can enforce optimistic concurrency.
Each call to read_events/2 records the query and the highest position
seen. When append_event/2 or append_events/2 is later called, the
DCB store checks that no new events matching any recorded query have been
appended since those positions. If they have, the append fails with
{:error, :append_condition_failed} and Counterpoint.CommandRunner
retries the entire command from scratch.
You typically import Counterpoint.ReadAppender inside a command module
to bring read_events/2, append_event/2, and append_events/2 into
scope without qualifying them.
Example
import Counterpoint.ReadAppender
alias Counterpoint.Query
def run(%__MODULE__{order_id: id}, ra) do
{events, ra} = read_events(ra, Query.new() |> Query.add_item(tags: ["order_id:#{id}"]))
# ... validate ...
append_event(ra, %OrderPlaced{order_id: id})
endSummary
Functions
Serialize and append a single event, guarded by all positions recorded so far.
Append multiple pre-serialized events atomically, guarded by recorded positions.
Create a new ReadAppender bound to the given store.
Read events matching query and record the read position for concurrency checks.
Types
Functions
Serialize and append a single event, guarded by all positions recorded so far.
Returns {:ok, _} on success or {:error, :append_condition_failed} on a
concurrent-write conflict.
Append multiple pre-serialized events atomically, guarded by recorded positions.
Prefer append_event/2 for single events. Use this when you need to emit
several events in one atomic write.
Create a new ReadAppender bound to the given store.
Read events matching query and record the read position for concurrency checks.
Returns {envelopes, updated_ra}. Always use the returned updated_ra for
subsequent reads and appends — it carries the accumulated read positions.