[erlang-questions] Mnesia Fragmented Tables, del_table_copy

[Tessaro Alexej]

Ok, so I can remove the last fragment using change_table_copy(Tab,
del_frag), but what am I supposed to do if I loose one node (let's say
by physical disconnection) and the fragments in it are last replicas..

In my opinion, the on-top application logic should continue to work but
the data contained in the lost node fragments will not be
considered...right?

Well, for example, if I try to call (after loosing one node and his
fragments):

mnesia:activity(async_dirty, TableInfo, [Table, size], mnesia_frag)

where TableInfo is 

TableInfo = fun(TName, TInfo)	->
	mnesia:table_info(TName, TInfo)
end

I am looking for the whole table size (the sum of the table fragments
sizes), but the function aborts when considering the lost fragments. 
In my case I would like the mnesia table_info fun to return me the table
size anyway (simply adding 0 records to the table size accumulator when
considering not existing fragments).

But this is only a personal need in a personal application context,
where I prefer to maintain ram only copies with no replicas.  

On Thu, 2011-01-27 at 22:36 +0100, Håkan Mattsson wrote:
> A fragmented table in Mnesia is a big distributed hash table where each bucket
> is implemented as a "normal" Mnesia table. When you insert a record in the
> fragmented table, a hash function is used to compute which bucket (table
> fragment) to put the record in. The linear hashing algorithm that is used allows
> the hash table to shrink by deleting the last bucket (table fragment) and
> dynamically rehash the records in that bucket and move them to other buckets.
> The algorithm does however not cope with deletion of arbitrary buckets.
> 
> If you want to shrink the fragmented table (delete the last bucket) you need to
> use the function mnesia:change_table_frag(Tab, del_frag) as described in the
> user's guide: http://www.erlang.org/doc/apps/mnesia/Mnesia_chap5.html.
> 
> Each bucket (table fragment) can be replicated to several nodes. Replicas
> are deleted with the function mnesia:del_table_copy(Frag, Node) and it is
> not possible to use this function to delete the last replica.
> 
> In your case you had 4 fragments and you tried to delete the last replica
> of the second fragment. This is not allowed. But it would have been allowed
> to delete the fourth fragment (and move its records) by invoking
> 
>   mnesia:change_table_frag(constResourceData, del_frag)
> 
> /Håkan
> 
> PS.
>   The function mnesia:change_table_frag/2 is only mentioned in the user's
>   guide. It is not documented in the reference manual. Sorry for that.
> 
> On Thu, Jan 27, 2011 at 6:18 PM, Tessaro Alexej <atessaro@REDACTED> wrote:
> > Thank you for your reply.
> >
> > In my case the table is dynamically growing (by adding new fragments
> > when the existing ones reach some sort of threshold).
> >
> > Cause the contained data have no critical relevance (but real-time
> > reading and writing times constraints), I thought to maintain ram only
> > copies with no replicas.
> >
> > So, when a node disconnects from the cluster cloud, I would expect the
> > data it was containing to disappear from the cloud (and the requests
> > involving those records to fail).
> >
> > I expected mnesia_frag system to detect not running mnesia nodes, thus
> > not considering them when calling mnesia activity functions on the whole
> > table.
> >
> > Am I missing something?
> >
> > On Wed, 2011-01-26 at 14:45 -0200, Igor Ribeiro Sucupira wrote:
> >> I haven't looked at it in detail, but I know it doesn't make much
> >> sense to delete the second fragment (leaving no other copies of it) in
> >> a table that has 4 fragments. It would break your fragmented table
> >> (writing keys would not be possible in some cases).
> >>
> >> Best regards.
> >> Igor.
> >>
> >> On Wed, Jan 19, 2011 at 11:31 AM, Tessaro Alexej <atessaro@REDACTED> wrote:
> >> > Hello,
> >> >
> >> > I have started two communicating mnesia nodes and created one fragmented
> >> > table with fragments stored in RAM only (no other replicas) and schema
> >> > stored on disk on both nodes:
> >> >
> >> > node1 : constResourceData_frag3 (ram), constResourceData_frag4 (ram),
> >> > schema (disk)
> >> >
> >> > node2 : constResourceData (ram), constResourceData_frag2 (ram), schema
> >> > (disk)
> >> >
> >> > Now, from node1, I tried to call
> >> > mnesia:del_table_copy(constResourceData_frag2, node2)
> >> >
> >> > but it fails saying:
> >> >
> >> > node1> Mnesia(node1): Last replica deleted in table
> >> > constResourceData_frag2
> >> > node1>{aborted,{no_exists,constResourceData_frag2,frag_properties,
> >> >                    frag_hash}}
> >> > node1> Mnesia(node1): Transaction {tid,75,<0.371.0>} calling
> >> > #Fun<mnesia_schema.15.79614902> with [] failed:
> >> >  {aborted,{no_exists,constResourceData_frag2,frag_properties,frag_hash}}
> >> >
> >> > Looking at the mnesia_schema.erl source code, I found that the failure
> >> > seems to happen when it tries to delete the "whole_table" in
> >> > make_delete_table/2 and it runs the following check:
> >> >
> >> > %% Check if it is a base table
> >> > nesia_frag:lookup_frag_hash(Tab),
> >> >
> >> > Why is my attempt of removing the fragment failing?
> >> >
> >> > My erlang version is R12B.
> >> >
> >> > Thank you
> 
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-- 
Tessaro Alexej, Volunia Italia

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