What would be best approach to integrate with extenal system(Legacy)?  For an example,  Contact information should be in sync with both salesforce and our external system. Whenever "Contact Record" is created/updated/deleted in salesforce, it should be sent to external system and vice versa?  

What would be best approach to integrate with extenal system(Legacy)?  For an example,  Contact information should be in sync with both salesforce and our external system. Whenever "Contact Record" is created/updated/deleted in salesforce, it should be sent to external system and vice versa?