China has many pieces of obsolete weapons which need modernization, and fast.
Buying some Russian hardware is one way to alleviate the problem.
It is also a win-win for both from the political and economic angles.
After all, SinoRussian cooperation，especially in the field of arms trade，is the last the US wants to see happening。
Economicwise，purchasing Russian weapons is the only way for now to maintain and correct a trade relationship that's increasingly in China's favour。
Russia should not be a mere resource and raw material supplier to China。
Russia should also strive to export to China manufactured goods。
Selling arms is a start。
Exactly, trade surplus is all and fun, but if you have nothing but trade surpluses with the guy, sooner and later he is going to run out of money and the trade stops.
Let's face it, Russians sucked at light industry even in the Soviet Union era.
After the collapse of Soviet Union, its heavy industry is also nothing to write home about.
Its electronics can't compete with China in the 80s, let alone now.
It doesn't have food to export.
The petroleum is nice, but it has to go through a rather long pipe and it is about the only thing besides arms that Russian can sell to China.
Even in arms sale it is a tough choice:
Chinese infantry weapons are cheaper, better and well supplied.
In armored vehicles, the China actually export lighter ones to Russia and Type 99 and Type 96 give better performance than T-90 at lower cost.
For surface ships, the ones left by Soviet Union have been rusting and after losing Ukraine, Russia's ability to manufacture new vessels really took a hit.
This left some attack submarines and aircraft as potential sales target. In past, Chinese navy has been mostly defensive, thus the addition of attack submarines are welcome. Aircraft, on the other hand, it is much more complicated. China has already begun to retire 3rd gen (Chinese standard 2nd gen) aircraft such as J7. T-50 is nowhere near completion. Not to mention T-50's stealth capacity is simply terrible comparing to J-20. This leaves 4th and 4.5 gen aircraft.
Now, while I would like to wait for more concrete news on the Su-35 deal, in my personal opinion, there are some benefit for these purchase besides throwing Russia some money.
1. Su-35 and J-11 share a common parent. The technological comparison may be worthwhile.
2. Shenyang aircraft corporation's production capacity maybe overloaded right now with the production of J-15, J-11 and testing of J-21/F-60.
Su-35 is a proven design and has been used by Russians themselves for a while. So there is low risk of Russians pulling a Gorshkov on us.
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that's the best part of your comment. I like it 
