The US Naval Institute on the news site featured a commentary by Daniel Trombly that takes a bottom up approach to gleaning Russia’s intentions in Syria and they are not nearly as mysterious nor as dire as others have claimed.  Other than the showcase of Russia’s new capabilities for sea launched cruise missiles, the Russians have not introduced any capabilities that the Syrians didn’t already possess, and from that we can determine that Russia’s aims are limited, merely restoring capabilities that the Assad regime had that was lost due to attrition during the years of civil war.

From this you can surmise that Russia is doing what Russia normally does as a matter of foreign policy, supporting its allies.  Russia (Soviet Union too) has historically prided itself on its steadfast support of allies as opposed to the fickle support allies in the Third World often receive from Western democracies.  It is in the nature of autocracies, that they make more reliable friends when interests are aligned.