The syncing is done on the app level: when a new document is inserted into the db that needs to be indexed a background job is launched that insert the document

The syncing is done on the app level: when a new document is inserted into the db that needs to be indexed a background job is launched that insert the document in Solr. This shouldn't be too hard manage if you can write post-save handlers in your model code. The average document is about 1KB and has about 30 attributes (but only 2-3 of those attributes needs to be index).