Why it is preferred to use GCM for push notificati

I know, it's so. But I don't understand, why?

Why not simply send queries to server periodically? Sure, it may discharge battery and increase internet traffic. I understand it. But how usage of Google Cloud Messaging can eliminate this problems?

I have found an answer. But it isn't pretty clear for me.

Can anyone give me a clear explanation?

Let's say you have 50 applications on your phone that do not use GCM. Each app developer decides it is appropriate to poll their respective backend once a minute.

Since these are all separate applications, each call will likely not happen at the same time as another api call. The biggest kill to battery is when the radio within an android device has to turn back on after being off to make an API call, so multiple calls happening with blocks of time in between drains battery faster (read this article on the radio state machine to better understand why this is https://developer.android.com/training/efficient-downloads/efficient-network-access.html)

In addition, each application will be hitting a separate endpoint. Each time you make an API call, you have to go through the connect process for a given server. With batched api requests or HTTP 2.0, multiple calls going to the same server can be optimized by not having to re-do a handshake or the connect process.

Now imagine, all 50 applications used GCM. GCM will poll an endpoint at some regular time interval on behalf of all 50 apps. Let's say GCM polls once a minute to a server that all the respective apps' backends send their notifications to to send to a device. You have reduced 50 different oddly timed API calls that are likely turning on and off the battery to one api call. You will use less data for polling. You do not incur the cost of the connect step of an HTTP call to 50 different servers. In addition, google is using the same polling already in place checking for OS updates, so there is no additional network overhead from using GCM (this info is based on old docs What technology does GCM (Google Cloud Messaging) use?)

Also, see this explanation straight from the Android website in an article entitled "Minimizing the Effects of Regular Updates" (http://developer.android.com/training/efficient-downloads/regular_updates.html):

Every time your app polls your server to check if an update is required, you activate the wireless radio, drawing power unnecessarily, for up to 20 seconds on a typical 3G connection.

Google Cloud Messaging for Android (GCM) is a lightweight mechanism used to transmit data from a server to a particular app instance. Using GCM, your server can notify your app running on a particular device that there is new data available for it.

Compared to polling, where your app must regularly ping the server to query for new data, this event-driven model allows your app to create a new connection only when it knows there is data to download.

The result is a reduction in unnecessary connections, and a reduced latency for updated data within your application.

GCM is implemented using a persistent TCP/IP connection. While it's possible to implement your own push service, it's best practice to use GCM. This minimizes the number of persistent connections and allows the platform to optimize bandwidth and minimize the associated impact on battery life.