Sinks

See Sinks for what a sink is and the storage backends Chronicle ships with. This page covers Elixir-specific configuration: setting the default sink per client, running multiple clients with different sinks, and the Chronicle.Sinks.WellKnownSinkTypes reference.

Default sink

Unless configured otherwise, Chronicle writes read models into MongoDB. You do not need to set anything — the default is correct for most applications.

Changing the default sink

Pass :default_sink_type_id when starting Chronicle.Client:

{Chronicle.Client,
  connection_string: "chronicle://localhost:35000",
  event_store: "my-store",
  default_sink_type_id: :sql,
  read_models: [MyApp.ReadModels.Account],
  reducers: [MyApp.Reducers.AccountReducer]}

The option accepts an atom from Chronicle.Sinks.WellKnownSinkTypes or a raw string identifier for custom sink types:

Atom	String	Storage backend
:mongodb	"MongoDB"	MongoDB (default)
:sql	"SQL"	SQL database
:in_memory	"InMemory"	In-process memory only
:not_set	"NotSet"	No sink — read model is not persisted

The configured sink applies to all projections and reducers registered by that client instance. There is no per-read-model sink override at this time.

Multiple clients with different sinks

If your application needs some read models in MongoDB and others in SQL, start two separate Chronicle.Client instances with different names and assign the relevant read models to each:

children = [
  {Chronicle.Client,
   name: :mongo_client,
   connection_string: "chronicle://localhost:35000",
   event_store: "store",
   default_sink_type_id: :mongodb,
   read_models: [MyApp.ReadModels.AccountSummary]},

  {Chronicle.Client,
   name: :sql_client,
   connection_string: "chronicle://localhost:35000",
   event_store: "store",
   default_sink_type_id: :sql,
   read_models: [MyApp.ReadModels.AccountReport]}
]

Supervisor.start_link(children, strategy: :one_for_one)

Pass :client when querying to target the right instance:

{:ok, summary} = Chronicle.ReadModels.get(MyApp.ReadModels.AccountSummary, id, client: :mongo_client)
{:ok, report} = Chronicle.ReadModels.get(MyApp.ReadModels.AccountReport, id, client: :sql_client)

Reference — Chronicle.Sinks.WellKnownSinkTypes

Function	Returns
WellKnownSinkTypes.mongodb/0	"MongoDB"
WellKnownSinkTypes.sql/0	"SQL"
WellKnownSinkTypes.in_memory/0	"InMemory"
WellKnownSinkTypes.not_set/0	"NotSet"
WellKnownSinkTypes.resolve/1	Resolves an atom or passthrough string

resolve/1 is used internally by Chronicle.Client to normalise the :default_sink_type_id option. You rarely need to call it directly, but it is useful when building custom tooling or test helpers:

alias Chronicle.Sinks.WellKnownSinkTypes

WellKnownSinkTypes.resolve(:mongodb)    # => "MongoDB"
WellKnownSinkTypes.resolve(:sql)        # => "SQL"
WellKnownSinkTypes.resolve("Custom")    # => "Custom"