Database design - google app engine

Question

I am working with google app engine and using the low leval java api to access Big Table. I\'m building a SAAS application with 4 layers:

• Client web browser

Answer 1

You specified two specific "views" your website needs to provide:

1. Scheduling an appointment. Your current scheme should work just fine for this - you'll just need to do the first query you mentioned.

2. Overall view of operations. I'm not really sure what this entails, but if you need to do the string of four queries you mentioned above to get this, then your design could use some improvement. Details below.

Four datastore queries in and of itself isn't necessarily overboard. The problem in your case is that two of the queries are expensive and probably even impossible. I'll go through each query:

1. Getting a list of appointments - no problem. This query will be able to scan an index to efficiently retrieve the appointments in the date range you specify.

2. Get all line items for each of appointment from #1 - this is a problem. This query requires that you do an IN query. IN queries are transformed into N sub-queries behind the scenes - so you'll end up with one query per appointment key from #1! These will be executed in parallel so that isn't so bad. The main problem is that IN queries are limited to only a small list of values (up to just 30 values). If you have more than 30 appointment keys returned by #1 then this query will fail to execute!

3. Get all invoices referenced by line items - no problem. You are correct that this query is cheap because you can simply fetch all of the relevant invoices directly by key. (Note: this query is still synchronous - I don't think asynchronous was the word you were looking for).

4. Get all payments for all invoices returned by #3 - this is a problem. Like #2, this query will be an IN query and will fail if #3 returns even a moderate number of invoices which you need to fetch payments for.

If the number of items returned by #1 and #3 are small enough, then GAE will almost certainly be able to do this within the allowed limits. And that should be good enough for your personal needs - it sounds like you mostly need it to work, and don't need to it to scale to huge numbers of users (it won't).

Suggestions for improvement:

• Denormalization! Try storing the keys for Line_Item, Invoice, and Payment entities relevant to a given appointment in lists on the appointment itself. Then you can eliminate your IN queries. Make sure these new ListProperty are not indexed to avoid problems with exploding indices

Other less specific ideas for improvement:

• Depending on what your "overall view of operations" is going to show, you might be able to split up the retrieval of all this information. For example, perhaps you start by showing a list of appointments, and then when the manager wants more information about a particular appointment you go ahead and fetch the information relevant to that appointment. You could even do this via AJAX if you this interaction to take place on a single page.
• Memcache is your friend - use it to cache the results of datastore queries (or even higher level results) so that you don't have to recompute it from scratch on every access.