Question

Making my way through the GAE documents.

I have a question I can’t find an obvious answer to. Given that transaction to an entity group is limited to 1/sec, how can you scale a request where say, 10,000 users all want to access a particular user’s page, at the same time?

Wouldn’t this give you 10,000 reads on the particular user’s entity group in 1/sec, thereby causing catastrophic system failure and unhappy users?

Or am I confused, and only writes get contentious.

Answer 1

AppEngine uses for transactions a optimistic concurrency control, meaning that they do not lock the data, but throw an exception when they detect that data is “dirty”. So, first transaction to change data is ok, the second gets the exception and must retry.

Given this, I assume that reads do not block if they are not part of transaction, even if some other transaction is in progress.

Also, to make transactions less of a bottleneck, one should carefully organize entity groups and make them as small as possible and also have them organized in such a way that there is as few contention (parallel requests) as possible. Meaning:

1. Have small entity graphs – do not put a lot of entities under common parent.
2. Try having user entity as a root parent. Users usually do not create parallel transactions (e.g. make multiple money transfers at the same time, etc..)