CheckpointerBackend

A checkpoint store's contribution to CheckpointerConfigLoader.open (design 4.54): how a CheckpointerSettings block becomes an open CheckpointStore.

llm4s.agent.checkpointer.kind names the store an application runs on, and the loader hands the backend registered for that kind its own block, llm4s.agent.checkpointer.<kind>, flattened to strings. llm4s-agent builds the memory store itself; every other store module registers its backend, so choosing a store is adding its dependency and setting kind, with no code:

Implementations must be a plain class with a public no-arg constructor, '''not''' a Scala object: java.util.ServiceLoader instantiates the named class. Declare it in META-INF/services/org.llm4s.agent.graph.spi.CheckpointerBackend, and ship the block's defaults and its ${?VAR} bindings in the module's own reference.conf. A backend can also be registered explicitly with CheckpointerBackends.of or CheckpointerBackends.withBackend, which is the answer for a shaded jar whose services files did not survive.

final class MyStoreBackend extends CheckpointerBackend:
 val kind = "mystore"
 def open(settings: CheckpointerSettings): Result[CheckpointStore] =
   for
     url     <- settings.string("url")
     timeout <- settings.duration("callTimeout", 5.seconds)
     store   <- MyStore.open(url, timeout)
   yield CheckpointStore(kind, store, () => store.close())