There are multiple options for storing data in your App Engine application:
|Google Cloud SQL||Relational (MySQL compatible)||Strongly consistent||Google offers two billing plans for Google Cloud SQL: packages and per use. More information is available in the Cloud SQL price sheet.|
|Google Cloud Storage||Files and their associated metadata (Cloud file storage)||Strongly consistent except when performing list operations that get a list of buckets or objects.||There are no charges associated with making calls to Google Cloud Storage.
However, any data stored in Google Cloud Storage is charged the usual
Google Cloud Storage data storage fees.
Cloud Storage prices are available on the Cloud Storage price sheet.
These options are not mutually exclusive. For example, the same application can store information in a Cloud SQL database, and also store files in a Cloud Storage bucket.
You can also choose from a variety of third-party databases to use with your applications such as Redis, MongoDB, PostgreSQL, Cassandra, and Hadoop. Any third-party database supported by your language can be used with your App Engine instance, provided the two can connect to each other.
Read more about connecting to third party databases.