where can I get realtime ticker data for robots? I am writing a trading bot and concerned about rate limits.

I need realtime notifications for change in price either like a PUSH api service.

Bitfinex appears to be the only major exchange that offers PUSH notifications, so I am not concerned with Bitfinex. But all other major exchanges such as Kraken are of concern.

How can I get realtime ticker data for the major bitcoin exchanges?
Poloniex has a PUSH API, check their API FAQ here: https://poloniex.com/support/api/

The other API requests they have said to have 6 request/sec rate limitation, which in my opinion is still pretty close to real-time if you update the Ticker this way.

Kraken does not provide PUSH API, from their FAQ it looks like a normal user (Tier 2) has a limitation on ticker of 1 request per 3 seconds (0.33 request/sec), but will be blocked for 15 minutes when exceeded.

Coinbase has only a PULL API with a 10,000 request per hour (2.7 request/sec) based on their Developer guide, although they operate a separate business called GDAX which has a WebSocket based real-time API.