Feasible Solutions

Middleware solution with Salesforce Development
You can use an integration software called Jitterbit to connect webadmin and Salesforce. Jitterbit will identify transactions on the webadmin’s end and migrate the data to Salesforce. Inturn, Salesforce codes take care of all the transaction scenarios like creating contact, updating lead statuses, creating opportunity etc. This is one of the highly recommended solutions as it adapts much easier to higher volumes or any changes in the future.
Integration using Custom coding
Coding in Salesforce to establish a connection with the web admin database is possible. Salesforce will continuously query the webadmin database to identify new transactions or updates to maintain Salesforce on a real-time basis. Although it costs lesser than a middleware, the constant maintenance and the lack of adaption to changes add to the cons.
Integration with SnowMirror
SnowMirror is a data replication tool. You can download filtered data ,which results in smaller mirror database and lower load on the ServiceNow instance. Use of encoded query to filter the rows is also available. For e.g. synchronize incidents only for the last one year. It is almost effortless to prepare a filter in ServiceNow and copy paste it to SnowMirror.
Schedule your Synchronizations
Synchronizations could be triggered manually anytime you want. However, a better option would be to schedule synchronization. Configurable scheduler allows you to define individual execution plans for any synchronization. You can specify an interval between executions or use a scriptable CRON extension. For e.g.: synchronize static data once a day and frequently changing data every five minutes.
Speed is important
ServiceNow customers process millions of records every day. SnowMirror was designed with focus on performance. SnowMirror performance has been tested on heavy ServiceNow instances and it is able to synchronize millions of records in less than an hour. SnowMirror has no or very low impact on ServiceNow instance performance. The impact is much smaller than live reporting (or any live integration) over a data straight in ServiceNow.