CowDB

1> {ok, Pid} = cowdb:open("testing.db").
{ok,<0.35.0>}
2> cowdb:put(Pid, a, 1).
{ok, 1}
3> cowdb:get(Pid, a).
{ok,{a,1}}
4> cowdb:mget(Pid, [a, b]).
[{ok,{a,1}},not_found]
5> cowdb:put(Pid, b, 2).
{ok, 2}
6> cowdb:mget(Pid, [a, b]).
[{ok,{a,1}},{ok,{b,2}}]
7> cowdb:mget(Pid, [a, b, c, d]).
[{ok,{a,1}},{ok,{b,2}},not_found,not_found]
8> cowdb:transact(Pid, [
    {add, c, 2},
    {remove, b},
    {fn, fun(Db) ->
                {ok, {a, V}} = cowdb:get(Db, a),
                [{add, d, V}] end}]).
{ok, 3}
9> cowdb:mget(Pid, [a, b, c, d]).
[{ok,{a,1}},not_found,{ok,{c,2}},{ok,{d,1}}]
10> cowdb:fold(Pid, fun(Got, Acc) -> {ok, [Got | Acc]} end, []).
{ok,{[],[3]},[{d,1},{c,2},{a,1}]}