Application hosted on AWS to access multiple external DB's

This inquiry isn't absolutely identified with application improvement, however very from a structural perspective. Suppose an application must be facilitated on AWS, which will require correspondence from various outside outsider held databases (not obligatory to be on AWS architect , could be their very own server farm or a colocation) - possibly one customer would utilize MySQL, another MSSQL, or Oracle and so forth. Along these lines, the information held in these outside databases could be changing every now and again and all the time the application ought to recover the refreshed information.

One way I could consider is to utilize another RDBMS DB (from our side) particularly to store every one of these information from outside DB's and to keep our DB in a state of harmony constantly. Be that as it may, I don't know whether this is a decent method to accomplish this. Or on the other hand do I have to utilize any information warehousing instrument like Redshift to store every one of the information adjusted progressively and afterward utilize DW information in our application?

As indicated by the draftsmen here, what might be a decent answer for accomplishing this, predominantly with different outside DB availability and matching up?