Russia lost out to French competitor Rafale in a competition to build a new fighter for India, so this lucrative contract could be tempting for the Russians. On the other hand, Russia has been out-flanked before in weapons deals with China, and the two nations have never enjoyed what might be considered a secure or stable relationship, so there are matters of security to consider in sharing such advanced technology.
For their part, China would essentially be admitting that their advanced 5th-generation J-20 project was not going as well as planned. Which means of course that the Su-35 deal would fill an important role in their military capabilities while at the same time creating a loss of face for China and their own domestic capabilities. It has been suggested that China may be trying to acquire the Flankers in order to pirate the power-plant tech, a long time weakness in their aerospace industry and possibly a stalling point in making the J-20 a practical production model.
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Should Russia Sell Su-35 to China?