[erlang-questions] Couple of questions about mnesia locking

Ryan Lepidi <>
Sat Apr 4 22:16:20 CEST 2009


I meant to say unnecessary*

On Sat, Apr 4, 2009 at 4:14 PM, Ryan Lepidi <> wrote:

> I know a write lock would eventually be acquired, but aren't locks acquired
> AS the functions in the transaction are called? Like wouldn't it do this:
>
> NodeA reads balance of 5; gets read lock on record
> NodeB reads balance of 5; gets read lock on record
> NodeA writes balance of 10; gets write lock on record
> NodeB writes balance of 15; gets write lock on record
> ^ Obviously this would be bad
>
> Or does mnesia read the entire transaction first and then apply all of the
> locks before doing any of the actions? I don't think this is the case
> because then functions such as mnesia:wread would be necessary.
>
>
> On Sat, Apr 4, 2009 at 3:59 PM, Hynek Vychodil <>wrote:
>
>> Write operation will require aquiring write lock, obviously.
>>
>> On Sat, Apr 4, 2009 at 7:54 PM, ryeguy <> wrote:
>>
>>> Hmm..well what if we had a transaction where we add to a bank account
>>> balance. You obviously read the balance then increment it.
>>>
>>> If NodeA and NodeB both have replicas of the table containing the
>>> balance, couldn't this happen:
>>>
>>> NodeA reads balance of 5
>>> NodeB reads balance of 5
>>> NodeA writes balance of 10
>>> NodeB writes balance of 15
>>>
>>> Since there is no read lock on each node? I'm assuming it would work
>>> like this because, if I understand correctly, the locks are acquired
>>> AS the transaction processes, and not the second the transaction fun
>>> is executed, right?
>>>
>>> In this situation, is the solution to simply just grab a write lock on
>>> the record when reading it?
>>> This poses another question: how do you acquire a write lock on a
>>> record when you do an index read?
>>>
>>> On Apr 4, 7:31 am, Richard Andrews <> wrote:
>>> > > If I wanted to check if a username is in use before registering an
>>> > > account like this:
>>> >
>>> > > F=fun()->
>>> > > case is_username_available(User) of    %% does a mnesia:read to see
>>> if
>>> > > there is a record with that username
>>> > >     false -> throw(username_in_use);
>>> > >     true -> mnesia:write(User)
>>> > > end,
>>> >
>>> > > mnesia:transaction(F).
>>> >
>>> > > Don't I have a potential race condition here? Since there is no
>>> record
>>> > > to lock because it doesn't exist, how can mnesia guarantee another
>>> > > process isn't also going to write that same Username to the database?
>>> > > If I'm doing this wrong, what's the right way? A table lock (eww)?
>>> >
>>> > You could use a gatekeeper - a singleton process on only one node which
>>> is allowed to create usernames. Creators make a request to the gatekeeper
>>> which serialises the requests and therefore prevents the race. The problem
>>> then shifts to process registration consistency and takeover in the case of
>>> the gatekeeper failure.
>>> >
>>> > Sharding/splitting the gatekeeper responsibility by eg. username first
>>> letter would quarantine the damage from a gatekeeper failure.
>>> >
>>> > > My second question is regarding a quote from the mnesia manual:
>>> > > "Write locks are normally acquired on all nodes where a replica of
>>> the
>>> > > table resides (and is active). Read locks are acquired on one node
>>> > > (the local one if a local replica exists). "
>>> >
>>> > > What does it mean read locks are acquired on one node? What would
>>> > > happen when a table is distributed? Wouldn't that defeat the purpose?
>>> >
>>> > A read lock on any node prevents a write lock from being acquired on
>>> that item.
>>> > A write lock must be acquired on *all* nodes so it is sufficient to
>>> take a read lock on any one node to block a write lock from being acquired.
>>> >
>>> > --
>>> >   Rich
>>> >
>>> >       Enjoy a safer web experience. Upgrade to the new Internet
>>> Explorer 8 optimised for Yahoo!7. Get it now.
>>> > _______________________________________________
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>>> > ://
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>>
>>
>>
>> --
>> --Hynek (Pichi) Vychodil
>>
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>
>
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