Durability

The event stream is made durable behind the same by_* query API. The principle this establishes is the foundation for resume: the durable log is the source of truth, and the in-memory index is a rebuildable cache.

Opt-in disk_log persistence

soma_event_store gained opt-in persistence backed by disk_log. Calling start_link/1 with #{log => Path} opens a halt disk_log; append/2 writes the normalized event to the log and the in-memory index; and init/1 replays the log on boot to rebuild the index, tolerating a truncated tail.

%% Durable: events survive a BEAM restart by replaying the disk_log on boot.
{ok, Store} = soma_event_store:start_link(#{log => "/var/soma/events.log"}).

%% In-memory default, byte for byte the same behaviour minus persistence.
{ok, Store} = soma_event_store:start_link().

start_link/0 stays purely in-memory, so events survive a BEAM restart only on the persistent path.

Wiring it in production

soma_sup chooses the store’s backing through app env — application:get_env(soma_runtime, event_store_log, undefined). A path routes to start_link/1 (durable); leaving it unset routes to start_link/0 (the in-memory default for development and tests). The production release becomes durable simply by setting the env in sys.config.

# Setting the event_store_log env makes the release durable.
SOMA_EVENT_STORE_LOG=/var/soma/events.log