When to use Ecto , when to use Mnesia

This is a good FAQ on Mnesia - any help?

11 Mnesia Database Questions

Mnesia is the somewhat odd name for the real-time, distributed database which comes with Erlang.

11.1 What is Mnesia good at?

Locking and Transactions

If you need to keep a database that will be used by multiple processes and/or nodes, using Mnesia means you don’t have to write your own access controls.

Distribution

Tables can be replicated at many nodes, both for efficiency (database lookup is a local activity) and robustness (redundancy means if one node goes down, other nodes still have copies of the data.)

Non-fully-normalised data

Unlike most database systems, records can contain data of arbitrary size and structure.

Monitoring

Monitoring - processes can subscribe to events which are sent when various operations on the data take place (update, delete, etc) The RDBMS package allows even more fine-grained control.

11.2 What is Mnesia not so good at?

Mnesia is primarily intended to be a memory-resident database. Some of its design tradeoffs reflect this.

Really large tables must be stored in a fragmented manner.

Then I think you can go with something like GitHub - meh/amnesia: Mnesia wrapper for Elixir. · GitHub. It’s querying syntax is pretty nice. But I would consider Ecto + Postgres an essential, well supported and documented dependency.

Ecto gives you some handy stuff, combined with the speed, reliability and robustness of PostgreSQL. In itself, it can drive architecture adoption if the benefits are large, and believe me, they are.

It is not ~that~ coupled with Phoenix. You can still access the database directly, via PSQL or another client connected directed to it or via the Postgrex driver that Phoenix uses under the hood. PostgreSQL also has a lot of languages that can be used inside of it, such as Python, TCL, JavaScript and C.

Mnesia, on the other hand, is shipped with Erlang and can be used without additional installation. That is also tough to beat. I have experience using the Bolt database with Go, and embedded databases have their advantages in deployment and, sometimes, speed.

And what do you think about using PostgreSQL as persistent storage and Mnesia as cache for dictionaries (which are not supposed to be changed in runtime)?
Or it will be too complex?

Thank you very much for your replies!

You have convinced me to use PostgreSQL with Ecto right now, because it is probably easier to start with and maintain, and having Mnesia’s distributiveness or the extra speed of an in-memory database is not of the most importance for this application (at least at this point in time).

What would be cool, though, is if there would be an Ecto<->Mnesia adapter at some point, because being completely independent from external services remains very compelling.

I also recommend just sticking with Postgres, at least for now.

What would be cool, though, is if there would be an Ecto<->Mnesia adapter at some point, because being completely independent from external services remains very compelling.

Looks like someone already started writing an adapter but it’s not ready for production yet: GitHub - webengineer-max/mnesia_ecto: Ecto adapter for Mnesia · GitHub.

I can wholeheartedly recommend trying to write your own adapter as a weekend project or something - I learned a lot about Ecto’s internals (and appreciated it’s design) by doing so.

Shameless plug: you can look at my experiments at github_ecto or ets_ecto. A bare minimum adapter that does something could be this: lib/ets_ecto.ex (from an early commit in that project)

If you are using Mnesia only as a cache, why not use ETS?

Actualy, I haven’t use nor Mnesia, nor ETS, yet!

I think for It’s better for Erlang/Elixir community to use Mnesia. It has few limitations. I used PostgreSQL in very large and complex projects (Red9.ir) and it has it’s own limitations. Number of connections, parallel processing, slow indexes on very large tables, distribution problems and sometimes strange disconnections due to network problems are few to mention. The main reason not to use Mnesia in lack of good libraries for Elixir. Amnesia is cool but not complete and lacks good documentation.

Isn’t one of the downsides of Mnesia that it doesn’t handle split brain?