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Difference between revisions of "Mnesia Table Fragmentation"

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               mnesia:select(Cont)
 
               mnesia:select(Cont)
 
             end,
 
             end,
   Result2 = mnesia:activity(transaction, AddFun, [], mnesia_frag)
+
   Result2 = mnesia:activity(transaction, SelFun2, [], mnesia_frag)
  
 
   Result2 -> '$end_of_table' | {[Objects], Cont} | transaction abort
 
   Result2 -> '$end_of_table' | {[Objects], Cont} | transaction abort
 +
 +
== List of fragmented tables ==
 +
 +
To obtain the list of tables, use mnesia:table_info/2 with the option frag_names:
 +
  mnesia:activity(async_dirty, mnesia:table_info/2, [store, frag_names], mnesia_frag).
  
  
 
That's it! Now you know how to write a basic Mnesia fragmented tables program.
 
That's it! Now you know how to write a basic Mnesia fragmented tables program.
 
[[Category:HowTo]]
 
[[Category:HowTo]]

Revision as of 03:04, 13 August 2010

Contents

Overview

This HOWTO describes how you make Fragmented Mnesia tables and how to use them.

Requirements

Let's say I have to make a book library index application. There's a table I use to record all the available library books. The record structure is as below. Due to the high volume of data, I want this table to be fragmented in a single Erlang node. If you want to make this fragmented table distributed, you may refer to http://www.trapexit.org/Distributing_a_Mnesia_schema tutorial on making distributed table. All the rest of the work related to table framentation remains same.

Sample Fragmented Table

I need this table to be disk_copies. Other modes also operate the same way.

-record(book_info, {isbn, name, author, keywords, category, description}).

Start an Erlang node

Our example node foo@example has the default disc storage path set to the directory Mnesia.foo@example in the current directory.

 erl -sname foo

The directory can be overridden by using -mnesia dir '"/path/of/your/preference"' ' when starting the node.

Let create a disk based schema by running,

 mnesia:create_schema([node()]). 
 

Create the Fragmented table with 20 table fragments

In this example all 20 fragments are in the same Erlang/Mnesia node. Also the fragments are disc_copies.

 mnesia:create_table(book_info,
   [{frag_properties, [{node_pool, [node()]}, {n_fragments, 20}, {n_disc_copies, 1}]},
   {index, [name, keywords, category]},
   {attributes, record_info(fields, book_info)}]),
 

Data operations

In order to be able to access a record in a fragmented table, Mnesia must determine to which fragment the actual record belongs. This is done by the mnesia_frag module, which implements the mnesia_access callback behaviour. Wrap standard Mnesia operation functions inside the function and pass to mnesia:activity/4 with callback module mnesia_frag.

Add records

Create a function which calls mnesia:write/3.

 AddFun = fun() ->
           mnesia:write(book_info, Record, write)
          end

Now call that function inside mnesia:activity/4 as below.

 ok = mnesia:activity(transaction, AddFun, [], mnesia_frag)
 

Notice that I have used the activity access context as "transaction". Transaction makes sure that the operation is all successfull or all fail (atomic). AccessContext I can use are,

 {transaction, Retries} 
 sync_transaction 
 {sync_transaction, Retries} 
 async_dirty 
 sync_dirty 
 ets 

For example if you want to do above activity in dirty mode, you can write,

 ok = mnesia:activity(async_dirty, AddFun, [], mnesia_frag)
 
 Refer to mnesia:activity/4 documentation for more info.
 

Select records with limit

As an example let's select books by Author "steve" with 10 books limit. Remember 10 is not a hard limit. Create a function with mnesia:select/4 function.

 MatchHead = #book_info{author = "steve", _ = '_'},
 Guard = [],
 Result = ['$_'],
 MatchSpec = [{MatchHead, Guard, Result}],
 SelFun = fun() ->
           mnesia:select(book_info, MatchSpec, 10, read)
          end,

Now call that function inside mnesia:activity/4 as below.

 Result = mnesia:activity(transaction, AddFun, [], mnesia_frag)
 Result -> '$end_of_table' | {[Objects], Cont} | transaction abort
 

In a fragmented table, if it returns {[Objects], Cont} and the number of objects returned is less than the number of objects expected (10), you need to recursively run mnesia:select/1 with the return Cont (continuation) until you get the expected number of results or '$end_of_table'.

 SelFun2 = fun() ->
             mnesia:select(Cont)
           end,
 Result2 = mnesia:activity(transaction, SelFun2, [], mnesia_frag)
 Result2 -> '$end_of_table' | {[Objects], Cont} | transaction abort

List of fragmented tables

To obtain the list of tables, use mnesia:table_info/2 with the option frag_names:

 mnesia:activity(async_dirty, mnesia:table_info/2, [store, frag_names], mnesia_frag).


That's it! Now you know how to write a basic Mnesia fragmented tables program.