How to keep an ordered list on the App Engine DataStore, in a way each entity knows its position on the list?

I have a ranking of people, based on points they earn. I need to tell each participant their position on the game for example, John (1) - 170 points, Mary (2) - 160 points, Sarah (3) - 110 points

So John's the first one, Mary's the seconds and Sarah's the third. Now if Mary wins 20 more points, she'll be the first and john will be the second.

I'm trying to avoid having to run a task on cron to list and recalculate everybody's position.

My first try was to maintain a separate set of entities (PersonRank) so I wouldn't run into transaction problems, this rank would have the same key name, so I could db.get() by key. This entity would have the person's calculated rank, so when a Person receives points, I'd have to check if the next Person on the line has fewer points than me, and exchange places with me so that's true. The problem is that Sarah, on the example, may have won 100 points, and is now number one. On the previous algorithm, I'd have to "walk" among a lot of entities, which means a lot of DataStore gets and puts (updating each involved Entity to the new position).

My next guess is maybe 开发者_高级运维some kind of linked list with ReferenceProperties, maybe using the key names to denote the position.

Any clues about how to implement this ?

Much more complex than I though, but hopefully some guys at Google already implemented this solution - http://googleappengine.blogspot.com/2009/01/google-code-jams-ranking-library.html

As you self-answered 3 years ago, Google implemented this solution, but they did that for Python exclusively. There is unofficial solution for java, someone re-writted Python solution and decided to share it: http://toolongdidntread.com/google-app-engine/using-googles-datastore-to-implement-a-ranked-scoreboard/

People say it works fine, I did not use it yet though. I might decide to use it in next few months so I could write more about it then. Be aware that both Java and Python solutions can deal with 3 requests/sec only (!).

More reading here: https://cloud.google.com/developers/articles/fast-and-reliable-ranking-in-datastore/

Basically, it says Google upgraded the code so it could deal with 300 requests/sec, and they said if you buy their premium-level support, they could help you with that, but they will not share the ready-to-go solution with the mere mortals. Also, all the code and references are in Python, but thats pretty obvious.

There is still no better solution than those mentioned here, at least I'm not aware of any.