diff --git a/src/machinery.erl b/src/machinery.erl
index 24dcc69..3c62261 100644
--- a/src/machinery.erl
+++ b/src/machinery.erl
@@ -25,15 +25,15 @@
 -type aux_state(T)    :: T.
 
 -type event_cursor()  :: undefined | event_id().
--type limit()         :: undefined | pos_integer().
+-type limit()         :: undefined | non_neg_integer().
 -type direction()     :: forward | backward.
 -type range()         :: {event_cursor(), limit(), direction()}.
 -type signal(T)       :: {init, args(T)} | timeout.
--type machine(T)      :: #{
+-type machine(E, A)   :: #{
     namespace         := namespace(),
     id                := id(),
-    history           := history(T),
-    aux_state         := aux_state(_)
+    history           := history(E),
+    aux_state         := aux_state(A)
     %% TODO
     %% history_range ?
     %% timer ?
@@ -44,7 +44,7 @@
 -export_type([range/0]).
 -export_type([args/1]).
 -export_type([response/1]).
--export_type([machine/1]).
+-export_type([machine/2]).
 
 -type modopts(O) :: module() | {module(), O}.
 
@@ -76,15 +76,15 @@
 -export([dispatch_call/4]).
 
 %% Behaviour definition
--type seconds() :: pos_integer().
+-type seconds() :: non_neg_integer().
 -type timer()   ::
     {timeout, seconds()} |
     {deadline, timestamp()}.
 
--type result(T) :: #{
-    events    => [event_body(T)],
+-type result(E, A) :: #{
+    events    => [event_body(E)],
     action    => action() | [action()],
-    aux_state => aux_state(_)
+    aux_state => aux_state(A)
 }.
 
 -type action() ::
@@ -93,17 +93,19 @@
     continue             |
     remove.
 
--export_type([result/1]).
+-export_type([timestamp/0]).
+-export_type([seconds/0]).
+-export_type([result/2]).
 -export_type([action/0]).
 
--callback init(args(_), machine(T), handler_args(_), handler_opts(_)) ->
-    result(T).
+-callback init(args(_), machine(E, A), handler_args(_), handler_opts(_)) ->
+    result(E, A).
 
--callback process_timeout(machine(T), handler_args(_), handler_opts(_)) ->
-    result(T).
+-callback process_timeout(machine(E, A), handler_args(_), handler_opts(_)) ->
+    result(E, A).
 
--callback process_call(args(_), machine(T), handler_args(_), handler_opts(_)) ->
-    {response(_), result(T)}.
+-callback process_call(args(_), machine(E, A), handler_args(_), handler_opts(_)) ->
+    {response(_), result(E, A)}.
 
 %% API
 
@@ -125,26 +127,26 @@ call(NS, ID, Range, Args, Backend) ->
     machinery_backend:call(Module, NS, ID, Range, Args, Opts).
 
 -spec get(namespace(), id(), backend(_)) ->
-    {ok, machine(_)} | {error, notfound}.
+    {ok, machine(_, _)} | {error, notfound}.
 get(NS, ID, Backend) ->
     get(NS, ID, {undefined, undefined, forward}, Backend).
 
 -spec get(namespace(), id(), range(), backend(_)) ->
-    {ok, machine(_)} | {error, notfound}.
+    {ok, machine(_, _)} | {error, notfound}.
 get(NS, ID, Range, Backend) ->
     {Module, Opts} = machinery_utils:get_backend(Backend),
     machinery_backend:get(Module, NS, ID, Range, Opts).
 
 %% Internal API
 
--spec dispatch_signal(signal(_), machine(T), logic_handler(_), handler_opts(_)) ->
-    result(T).
+-spec dispatch_signal(signal(_), machine(E, A), logic_handler(_), handler_opts(_)) ->
+    result(E, A).
 dispatch_signal({init, Args}, Machine, {Handler, HandlerArgs}, Opts) ->
     Handler:init(Args, Machine, HandlerArgs, Opts);
 dispatch_signal(timeout, Machine, {Handler, HandlerArgs}, Opts) ->
     Handler:process_timeout(Machine, HandlerArgs, Opts).
 
--spec dispatch_call(args(_), machine(T), logic_handler(_), handler_opts(_)) ->
-    {response(_), result(T)}.
+-spec dispatch_call(args(_), machine(E, A), logic_handler(_), handler_opts(_)) ->
+    {response(_), result(E, A)}.
 dispatch_call(Args, Machine, {Handler, HandlerArgs}, Opts) ->
     Handler:process_call(Args, Machine, HandlerArgs, Opts).
diff --git a/src/machinery_backend.erl b/src/machinery_backend.erl
index 951282d..e9e402a 100644
--- a/src/machinery_backend.erl
+++ b/src/machinery_backend.erl
@@ -23,7 +23,7 @@
     {ok, machinery:response(_)} | {error, notfound}.
 
 -callback get(namespace(), id(), range(), backend_opts()) ->
-    {ok, machinery:machine(_)} | {error, notfound}.
+    {ok, machinery:machine(_, _)} | {error, notfound}.
 
 %% API
 
@@ -40,6 +40,6 @@ call(Backend, Namespace, Id, Range, Args, Opts) ->
     Backend:call(Namespace, Id, Range, Args, Opts).
 
 -spec get(backend(), namespace(), id(), range(), backend_opts()) ->
-    {ok, machinery:machine(_)} | {error, notfound}.
+    {ok, machinery:machine(_, _)} | {error, notfound}.
 get(Backend, Namespace, Id, Range, Opts) ->
     Backend:get(Namespace, Id, Range, Opts).
diff --git a/src/machinery_machine_unique_tag.erl b/src/machinery_machine_unique_tag.erl
index 2108b5e..80a6b5f 100644
--- a/src/machinery_machine_unique_tag.erl
+++ b/src/machinery_machine_unique_tag.erl
@@ -70,9 +70,9 @@ construct_namespace(NS) ->
 
 %%
 
--type machine()      :: machinery:machine(ev()).
+-type machine()      :: machinery:machine(ev(), _).
 -type handler_opts() :: machinery:handler_opts(_).
--type result()       :: machinery:result(ev()).
+-type result()       :: machinery:result(ev(), _).
 -type response()     :: machinery:response(
     ok | {error, id()}
 ).
diff --git a/src/machinery_mg_backend.erl b/src/machinery_mg_backend.erl
index e53285d..fc3d4fd 100644
--- a/src/machinery_mg_backend.erl
+++ b/src/machinery_mg_backend.erl
@@ -13,7 +13,7 @@
 -type range()           :: machinery:range().
 -type args(T)           :: machinery:args(T).
 -type response(T)       :: machinery:response(T).
--type machine(T)        :: machinery:machine(T).
+-type machine(E, A)     :: machinery:machine(E, A).
 -type logic_handler(T)  :: machinery:logic_handler(T).
 
 -define(BACKEND_CORE_OPTS,
@@ -138,7 +138,7 @@ call(NS, ID, Range, Args, Opts) ->
     end.
 
 -spec get(namespace(), id(), range(), backend_opts()) ->
-    {ok, machine(_)} | {error, notfound}.
+    {ok, machine(_, _)} | {error, notfound}.
 get(NS, ID, Range, Opts) ->
     Client = get_client(Opts),
     Schema = get_schema(Opts),
@@ -304,7 +304,7 @@ marshal(action, V) ->
 marshal(timer, {timeout, V}) when V > 0 ->
     {timeout, marshal(integer, V)};
 
-marshal(deadline, {deadline, V}) ->
+marshal(timer, {deadline, V}) ->
     {deadline, marshal(timestamp, V)};
 
 marshal(namespace, V) ->