Mnesia: unexpectedly getting aborted, cyclic transactions

问题

I have a 5 processes that insert/update the same 3 records in a mnesia table. Each of these processes does its insert/updates within a single transaction.

I have 5 other process that read these very same 3 records, also within a single transaction.

Unless I lock the entire table as part of the multi-record transaction, I get an {aborted, {cyclic, node....}} error. My intuition is that my use-case is ordinary and should not, in of itself, result in in an aborted transaction. Can someone help me with my bone-headed thinking? All I am doing is inserting (or reading) multiple rows in a cache (mnesia table) in one transaction.

Inserting the 3 records looks like this

insert_keylist(Keys) ->  
    F = fun() -> insert_iter(Keys) end,  
    transactional_execute(F).

insert_iter([]) ->
    ok;
insert_iter([{Key, Value} | KVs]) ->
   insert(Key, Value),
   insert_iter(Kvs).

insert(Key, Value) ->
  F = 
      fun() ->
        case sc_store:lookup(Key) of
            {ok, _Value}       -> sc_store:replace(Key, Value);
            {error, not_found} -> sc_store:insert(Key,Value)
        end
      end,
  transactional_execute(F).    


transactional_execute(F) ->
    case mnesia:is_transaction() of
       true -> F();
       false ->
           try mnesia:sync_transaction(F) of
               {atomic, Result}   -> Result;
               {aborted, Reason}  -> {aborted, Reason}
           catch
                ErrorCl:Error     -> {error, {ErrorCl, Error}}
        end
     end.

sc_store:replace, insert and lookup are is as follows:

replace(Key, Value) ->
    try
       case mnesia:wread({key_to_value, Key}) of
          [#key_to_value{} = Rec] -> 
            mnesia:write(Rec#key_to_value{value = Value});
          []                       -> 
        {error, not_found}
        end
    catch
       _Err:Reason ->
         {error, Reason}
    end.

insert(Key, Value, Type, Scope, TTL, TTLMessage, Ref) ->
   try
      NowDT = calendar:now_to_datetime(erlang:now()),
      ok = mnesia:write(#key_to_value{key = Key, 
                type = Type,
                scope = Scope,
                value = Value,
                create_time_dt = NowDT,
                ttl_secs = TTL,
                ttl_message = TTLMessage,
                ref = Ref})
   catch
      _Error:Reason ->
        {error, Reason}
   end.

lookup(Key) ->
   try 
      case mnesia:read(key_to_value, Key) of
         [#key_to_value{type = Type, scope = Scope, value = Value}] -> 
            {ok, {Value, Type, Scope}};
         []                       -> 
            {error, not_found}
      end
   catch
      _Err:Reason -> {error, Reason}
   end.

回答1:

Actually, turns out the problem was using try/catch around mnesia operations within a transaction. See here for more.

回答2:

Mnesia is right !!
According to your code, the function insert_keylist/1 calls the function transactional_execute/1 with a fun. Inside the function: transactional_execute/1, you ask mnesia wether its already in a transaction, and if its true, you execute the fun F. However, looking closely in fun F, you see that it calls the function: transactional_execute/1 all over again through the iterative function: insert_iter/1. So, this creates a loop of transactions. Hence your problem is arising because a process is executing the top most function: insert_keylist/1 in a transaction which is creating a nested loop of transactions which never get executed, only, they keep asking mnesia wether they are in a transaction and it accepts yet again they keep asking and asking and looping without ever getting executed !!

Before we modify your code, we have not seen whats in the functions: sc_store:replace/2 and sc_store:insert/1. I will assume that these functions are again NOT creating transactions within themselves!! I will assume that they are directly using mnesia functions (That must be executed within a transaction), such as : mnesia:write/1, mnesia:read/1,mnesia:write/3 e.t.c. Otherwise if you were creating another transaction in these functions, surely this would be a mess of mnesia transactions!! Now lets correct the code as shown below:

%% This function will always be sure to be in a 
%% mnesia transaction. So no need to call
%% transactional execute

insert_iter([])-> ok;
insert_iter([{Key, Value} | KVs]) ->
   case sc_store:lookup(Key) of
        {ok, _Value} -> sc_store:replace(Key, Value);
        {error, not_found} -> sc_store:insert(Key,Value)
    end,
    insert_iter(Kvs).

That's the only change required, assuming that the functions sc_store:insert/1 and sc_store:replace/2 access mnesia write and read functions directly without creating other transactions also.

来源：https://stackoverflow.com/questions/8053957/mnesia-unexpectedly-getting-aborted-cyclic-transactions

标签

erlang

mnesia