migrating bad system into our current system with tons of data
i inherited of a system when my company bought another company. this system is a mix of lamp and .net:

1 windows server asp.net that controls thirdparty validation used only for apis and webservice - (let's call it WIN)
8 lamp servers (web, reports, cron, repositories etc...) - (let's call it NEW)

our current env.:

14 lamp servers (web, mail, repositories, etc...) - (let's call it CURRENT)

the good news is, the NEW code is pretty straigt foward, few millions lines of code (which most are apis, third party) and i can convert it to the CURRENT system. the NEW and CURRENT use both cent os which will make the transision easy, except the WIN that i have no idea what to do now. now the bad news. the NEW system database schema is not good at all . it is not normalized and the queries are slow (database queries and the code too). so my first idea is to redesign them to a more normalize structure that match the CURRENT code. but i dont work, the tables from the NEW system are gigantic. the NEW system have 7 databases, over 10000 tables, the smallest table have over 100k rows and some tables have over 500 millions rows, one of the database has most of the tables with over 25 millions rows each. 

is it safe to migrate or should i keep both running, if i should migrate i want to know what will the safest solution for me in order to migrate the WIN and NEW system into my CURRENT system?

migrating bad system into our current system with tons of data
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i inherited of a system when my company bought another company. this system is a mix of lamp and .net:

1 windows server asp.net that controls thirdparty validation used only for apis and webservice - (let's call it WIN)
8 lamp servers (web, reports, cron, repositories etc...) - (let's call it NEW)

our current env.:

14 lamp servers (web, mail, repositories, etc...) - (let's call it CURRENT)

the good news is, the NEW code is pretty straigt foward, few millions lines of code (which most are apis, third party) and i can convert it to the CURRENT system. the NEW and CURRENT use both cent os which will make the transision easy, except the WIN that i have no idea what to do now. now the bad news. the NEW system database schema is not good at all . it is not normalized and the queries are slow (database queries and the code too). so my first idea is to redesign them to a more normalize structure that match the CURRENT code. but i dont work, the tables from the NEW system are gigantic. the NEW system have 7 databases, over 10000 tables, the smallest table have over 100k rows and some tables have over 500 millions rows, one of the database has most of the tables with over 25 millions rows each. 

is it safe to migrate or should i keep both running, if i should migrate i want to know what will the safest solution for me in order to migrate the WIN and NEW system into my CURRENT system?
