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Google (s GOOG) is about to take a big step towards connecting the web with the apps on your phone. The company is scheduled to roll out in-app search, including app content in your mobile search results, by mid-November. In-app search is for now limited to about a dozen launch partners — but eventually, it could be key to transforming the Android ecosystem, while at the same time cementing Google’s role as a gatekeeper.

Google first announced in-app search when it introduced Android 4.4 aka Kit Kat two weeks ago, but the feature will be included in Google’s Search app and thus available on previous Android versions as well. Here’s how it works:

Let’s say you’re looking to rent an apartment, and you’re searching for open listings via Google. One of the first links is going to be a result from Trulia — and if you have Trulia’s Android app installed, it’s going to give you the option to open that specific result in Trulia’s app, where you can then take action on the content, including exploring the apartment’s neighborhood on a map or and contacting a real estate agent.

Google is launching this program with twelve high-profile partners, including Expedia, Moviefone, Trulia, OpenTable and Etsy, and it’s easy to see why app publishers would like this kind of experience. “Our Android app experience will always be better than our mobile (web) experience,” explained Trulia’s director of mobile Steven Yarger during an interview last week, adding that it’s just easier to incorporate zoomable maps and other content in an app setting.

Google executives have also said that they eventually also want to offer buttons to install an app if you don’t have it on your phone yet, which could help publishers grow their install base. The bigger question is: What’s in it for Google?

Making apps searchable could help Google sell more ads

A lot, actually. In order to index in-app search results, developers need to make their apps searchable and enable so-called deep linking to content within their apps. Google’s Webmaster Central blog recently explained it this way:

“Just like it crawls and indexes websites, Googlebot can now index content in your Android app. Webmasters will be able to indicate which app content you’d like Google to index in the same way you do for webpages today — through your existing Sitemap file and through Webmaster Tools.”

Some developers have already prepared their apps for this in the past. Trulia, for example, has used deep links to allow its users to directly jump from a real estate listing in its email newsletter to that very same listing in its mobile app.

Most apps have however been content silos, insular and without any inbound connections from the outside world. The promise of increased usage — and possibly even additional installs — could prompt more developers to change that, and incorporate deep linking within their own apps and make their app content searchable.

Once Google indexes all of that app content, it could easily offer publishers ways to monetize their apps through contextual advertising. It’s also theoretically possible that Google could charge publishers for sponsored links to app installs, even though the company hasn’t said that it intends to do so.

A way to surface in-app content — and keep Samsung in check

There’s also an upside for the future of Android. More interconnected apps could benefit the platform as a whole, and enable additional functionality down the road. Search really is just a first point for Google to surface app content, but one could easily imagine that Google Now could be next.

What if the service, which is meant to respond to your daily needs without being prompted to do so, would offer you to reserve a room in a hotel near the airport as soon as it knows that your last flight out has been canceled? Or what if it gave you a link to the OpenTable app as soon as someone emailed you about going out for lunch at a certain restaurant? And once these data points show up in Google Now, it’s not such a stretch to also imagine them coming to other devices, including Google Glass or that rumored Google smartwatch.

However, the biggest upside could easily be that it firmly puts Google in control of the Android experience. Samsung in particular has been trying to win over developers, trying to lure them into its own app store with promises of a more curated experience as well as a huge Android device footprint. But there’s one thing Samsung doesn’t have: a search engine. Linking apps closely to search could help Google to tie developers to Google Play, and keep Samsung in check.

Image courtesy of Flickr user Skakerman.