[erlang-questions] Mnesia: Whether it is good for my application! help?

[Magnus Henoch]

aman mangal <mangalaman93@REDACTED> writes:

> Hi everyone,
>
> This is my first post on this forum! I am new to erlang, just started using
> it a month ago or so. Since I have come across erlang, I have become such a
> big fan of it that I have started working on a product in erlang for a
> company.
>
> Coming to the point, here is my question. I want to make
> a distributive application. There are multiple processes running on a node.
> The processes are divided in logical groups. Each group serves a specific
> purpose. Each group has some data(few tables) shared among the processes
> inside the group but not outside the group. Now my question is that whether
> to use mnesia or not to store the data? If I don't use mnesia, I'll store
> the data as state in a gen server processes running in the group. In that
> case I don't know how will I will keep back up of the data for the case
> when any node goes down.

It sounds like Mnesia would be a good fit.  You would get persistence
for free by creating your tables with disc_copies.  The only potential
issue I see is table names: each Mnesia table must be named by an atom.
Are your process groups created dynamically?  If so, you'd need to come
up with a scheme to get the correct table names from within the
processes that need to access the tables.  You'll also need to use the
record_name option when creating the tables, as by default Mnesia
requires you to use records with the same name as the table.

> Also my database can grow as large as thousand tables each having hudred
> entries on a node. Is mnesia a good idea to use for such a large (I'm not
> sure whether it is large :P ) database? Is there any free alternative to
> mnesia? Please note that there is no inter node communication in order to
> get data except with the back up node!

Assuming each entry takes no more than a kilobyte, you're looking at
roughly 100 megabytes of data in total.  When using the disc_copies
table storage type, Mnesia will store the entire database in memory as
well as on disk, but I presume that you have enough RAM that 100
megabytes isn't a big problem.

Could you elaborate on your backup node setup?  If you want your data to
be distributed to protect against disk failure on the main node, then
Mnesia distribution is probably the way to go, but depending on your
requirements you might be able to get away with a simpler solution.

Hope this helps,
Magnus