CODEWITHSUNDEEP
sqlgoogle-app-enginecloudbigtable
jsg
jsg

Reputation: 143

GAE Cloud SQL and High Replication Datastore

With HRD and BigTable, you are forced to deal with eventual consistency for all queries that are not ancestor queries. Your code has to be robust enough to cope with the fact that the results may be stale.

With Google's launching of Cloud SQL, they put in a disclaimer: ( https://developers.google.com/cloud-sql/faq#hrapps ) 

"We recommend that you use Google Cloud SQL with 
High Replication App Engine applications. While you can use use 
Google Cloud SQL with applications that 
do not use high replication, doing so might impact performance."

What does this mean? Does this mean that there are the same eventual consistency issues using SQL with HRD? There is no concept of entity groups in SQL, however could this mean that particular SQL queries in particular circumstances deliver stale results?

This would mean that Google's implementation of the SQL atomic transactional contract would be broken and SQL would not function as users of relational databases would expect. If this is not the case, what are the concerns for having a master/slave or HRD model with SQL and why would Google give you the option of choosing a model with poorer performance?

Upvotes: 4

Views: 1775

Answers (2)

Rob
Rob

Reputation: 176

(from forum)

The Cloud SQL and Data store systems are independent. You can use one or both as you see fit for your app.

We recommend using HRD apps because that type of app will be colocated with Cloud SQL. Master slave apps are served from a different set of datacenters where cloud sql does not have presence. It will work but it will be slower.

Upvotes: 5

Vishal Biyani
Vishal Biyani

Reputation: 4407

Quotes from documentation:

"Google Cloud SQL is, simply put, a MySQL instance that lives in the cloud. It has all the capabilities and functionality of MySQL"

TO answer your question, the high replication/master-slave options for a relational DB are not to do with consistency but with other factors like latency at peak loads and availability for write when there is a planned maintenance. For a high replication datastore the latency is low even if load spikes, and they are available for write even while there is a maintenance planned. Check the comparison at http://code.google.com/appengine/docs/java/datastore/hr/

And second part of question as to why would google offer a master-slave option which is not full proof. Answer is so that people who don't need complete uptime and want to try out GAE can use it.

Upvotes: 0

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