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Mnesia Quickstart
Originally published on my WordPress blog on January 05, 2010.
Basic introduction on Mnesia
Mnesia is a RDBMS. But it belongs to the nosql category of RDBMS. Reason being the query language is not sql but Erlang. That makes it very easy to store and retrieve without having to go through Object Relational Mapping. So we can actually call Mnesia an object relational database.
Why and where would one want to Mnesia?
Erlang in general is used to program highly distributed and fault tolerant systems. Even though it has its roots in the telecommunication industry, it has proven useful in several other sites like ejabbered, Facebook chat etc. Mnesia is just a part of Erlang and is built with Erlang.
Hence it gives you configurable degree of Fault-tolerance (by means of replication).
Another important feature of mnesia is the Dirty read interface. It is possible to read, write and search Mnesia tables without protecting the operation inside a transaction.
Quickstart
The below steps should get you started on mnesia, if you are using it for the first time.
Required Software - Erlang
I use ubuntu as my OS. But that should not make it much different on any other OS.
Now lets start with some code. It is useful to install the table-viewer (erlang-tv).
The goal of this exercise is to create table called person, insert few records and read them.
create a file called Person.hrl
-record(person, {name, %% atomic, unique key
age, %% age
married_to, %% name of partner or undefined
children }).%% list of children
create a file called Person.erl
-module(person).
-include("<path_to_person.hrl>/person.hrl").
-export([init/0]).
-export([insert/0]).
-export([read/1]).
init() ->
mnesia:create_table(person,[{attributes,record_info(fields,person)}])
.
insert() ->
T = fun() ->
X = #person{name=john,
age=36,
married_to=ana,
children=[josh,kelly,samantha]
},
mnesia:write(X)
end,
mnesia:transaction(T)
.
read(Name) ->
R = fun() ->
mnesia:read(person,Name,write)
end,
mnesia:transaction(R)
Start command line erlang. Type in the below command from the directory which contains the above two files
erl mnesia dir .
The above command conveys that the current directory will be used to store Mnesia files.
Compile the person.erl
>c(person).
{ok,person}
Start mnesia
>mnesia:start().
Create person table.
>person:init().
Insert a record.
>person:insert().
Use table viewer to check if the record has been inserted.
>tv:start().
This will launch table view application. By default the table viewer shows the ETS tables. To look at the table we just created go to view menu and select Mnesia tables.
Read the record using Mnesia:read()
>person:read(klacke).
{atomic,[{person,john,36,ana,[josh,kelly,samantha]}]}
In my next post I will cover Mnesia queries as List Comprehension.