What application scenarios should mongdb be used?

Mysql and redis are currently very good and skilled. in what scenarios should I discard these tools and use mongo to store data? In addition: Currently, we are exploring and storing data of the document type. the style of the document content must be saved without compromising the retrieval capability of the retrieval tool on the content, and the performance is also high... mysql and redis are currently very good and skilled. in what scenarios should I discard these tools and use mongo to store data?

In addition:
Currently, we are exploring and storing the data of the document type. the style of the document content must be saved without compromising the retrieval capability of the retrieval tool on the content, and the performance has to go through, I need a set of good solutions. I certainly checked the database features and comparisons in advance, so the answer is not simply to compare the database features, my actual problems may be based on actual practice and experience.

I have made millions of APP backgrounds with daily orders. Mysql and Redis can be supported by MySQL and Redis in general. What are the reasons or scenarios that will force me or recommend me to use mongo, mongo flexible data type storage. Currently, I only think that it is better to use mongo for log storage. I am a newbie in article-type website development and need experience guidance. thank you first

Reply content:

Mysql and redis are currently very good and skilled. in what scenarios should I discard these tools and use mongo to store data?

In addition:
Currently, we are exploring and storing the data of the document type. the style of the document content must be saved without compromising the retrieval capability of the retrieval tool on the content, and the performance has to go through, I need a set of good solutions. I certainly checked the database features and comparisons in advance, so the answer is not simply to compare the database features, my actual problems may be based on actual practice and experience.

I have made millions of APP backgrounds with daily orders. Mysql and Redis can be supported by MySQL and Redis in general. What are the reasons or scenarios that will force me or recommend me to use mongo, mongo flexible data type storage. Currently, I only think that it is better to use mongo for log storage. I am a newbie in article-type website development and need experience guidance. thank you first

It is very useful to list the appropriate use cases one by one...

In turn, we think about what scenarios are not suitable for mangodb?

We have a personal user Center:
A user can have many friends, and his or her friends can have many comments. they can have many replies and many likes.

Using mongodb for modeling is a large json, where we have a user, and there is an array of friends in the user, an array of messages, an array of replies, and a group of likes, the complete information is stored in these arrays and is obtained at one query!

However, there is a problem
Because the friend information in the friend array is actually the user information, storing the complete information causes data redundancy. this problem can also be solved, that is, storing the id in the array, check the data in the corresponding data source based on the id,

However, mongodb does not have a join operation, so it is necessary to manually perform Association work in the code layer ..

In this case, mongodb is not suitable for storing data that has a relationship, relationship, and relationship.

Why shouldn't you use MongoDB?
Why You shoshould Never Use MongoDB