Why China buys Su-35?

In January, China and Russia have signed an intergovernmental agreement which would be the first step towards a purchase contract for 24 Su-35 

Since November 2012, a rumor about an alleged preliminary agreement between Russia and China to negotiate the purchase of Sukhoi Su-35 fighters.

Russia confirmed and the date of this agreement in February 2013, but did not specify the number of aircraft involved. However, the newspaper The Hindu confirmed in an article dated March 8 that talks involving 24 aircraft, two times less than expected .

Why this downward revision from Beijing? The Su-35 will they serve as spare parts for the new Chinese aircraft? On this point, the Russians are confident and have argued that China could not copy the Su-35 engines. But speculations remain on the Saturn 117S * new engine fitted to the Su-35 and the Chinese could use to propel their stealth fighter J-20.

If this is the case, it would be a setback for India on two fronts: 1) sell China a modern fighter SU-30MKI as India, 2) build a rival against the project Indo-Russian stealth fighter PAK FA T-50.

* Engine Saturn 117S (AL-41F1A) is intended to propel the Sukhoi Su-35BM. It develops 142 kN of thrust with afterburner and 86.3 kN dry. It has a compressor 3% larger diameter (932 mm against 905 mm), high and low pressure turbines improved a digital control system completely new, and the possibility to include thrust deflectors Vector as the AL-31FP. This motor is provided for a service life of 4000 hours, with a life of 1000 hours between failures (MTBO). The first flight of this engine was conducted on a Su-35BM February 20, 2008.