Using MySQL and MongoDB together

Question

I have never used NoSQL before, generally the applications I write requires relations. However, I have encountered a something that I don't know how to go about. So far, I am only designing the database. For now, my main logic is in the MySQL Database. I have static content that I will be hosting through a CDN. However, I have dynamic content that will be updated but very rarely but will be read almost on every request - like phone number, email address, address, additional info. They will not be used for searching, however this data is unstructured. A user can have multiple email addresses, phone numbers, and addresses; and they would be needed for multiple tables. So, using relational database in this case fails my needs (I don't want to create an Entity-Attribute-Value Table for this) and since I know that it doesn't affect the logic - its only used as a "meta-data" I want to keep them in a JSON format. And after Google-ing for sometime, I found out that MongoDB stores "documents" in JSON which sounded like the perfect solution. However, I have one question regarding this. How do I connect these databases together? Do I need to just add a user_id or organization_id "column"/field for a document on create/update and do a "select" query (whatever is the equivalent in the MongoDB) to receive the meta data? Or is there a different way?

Answer 1

I'll present here my opinion. What you're trying to do here is called "polyglot persistence". If you introduce mongo, you'll have 2 architectures for storing your data, different in strength, api, design, what not and this has its price.

Mongo DB, is a great product, I've used it by myself with a great success, but you have to understand that it doesn't provide all the features you would expect from RDBMS like MySQL. For example, it totally lacks transactions. Moreover if you store in both MySQL and Mongo you'll have to care for data integrity by yourself (what happens if as a part of logical transaction mysql transaction succeeds, but mongo fails to store the data), there is no rollback... I believe you've got my point.

Yes, mongo really allows to query by various JSON parameters, in fact it features the whole query language, it resembles SQL to some extent, but its not really a "relational" query engine, because mongo is not a relational database, so you don't have JOINs for example. But you've said by yourself that you are not going search by these fields, so I kind of don't understand what benefit you would have from using mongo. Maybe this is only about terminology, but I'm confused with this statement a little.

Where mongo is really shines is when you have a lot of data (its a big data product after all), then you have funny stuff like replica-sets and sharding, but the question is whether you really need it? do you really have "big data" - really huge amount of objects to be stored?

As an alternative, I think maybe you can use a text column for storing the JSON "as is". I mean, you might have a column, storing the JSON. You even sometimes have "JSON" type as a native type in the database, I'm not sure whether MySQL supports it. In this case you even can do some operations on these jsons (like, append, partial update and so forth).

Of course the choice is yours, all I'm saying is that you should think whether you have more benefits while using 2 persistence engines, or will make your project more complicated.

Hope this helps