Question

Looking for a simple example of retrieving 500 items from dynamodb minimizing the number of queries. I know there’s a “multiget” function that would let me break this up into chunks of 50 queries, but not sure how to do this.

I’m starting with a list of 500 keys. I’m then thinking of writing a function that takes this list of keys, breaks it up into “chunks,” retrieves the values, stitches them back together, and returns a dict of 500 key-value pairs.

Or is there a better way to do this?

As a corollary, how would I “sort” the items afterwards?

Answer 1

Depending on you scheme, There are 2 ways of efficiently retrieving your 500 items.

1 Items are under the same hash_key, using a range_key

• Use the query method with the hash_key
• you may ask to sort the range_keys A-Z or Z-A

2 Items are on "random" keys

• You said it: use the BatchGetItem method
• Good news: the limit is actually 100/request or 1MB max
• you will have to sort the results on the Python side.

On the practical side, since you use Python, I highly recommend the Boto library for low-level access or dynamodb-mapper library for higher level access (Disclaimer: I am one of the core dev of dynamodb-mapper).

Sadly, neither of these library provides an easy way to wrap the batch_get operation. On the contrary, there is a generator for scan and for query which ‘pretends’ you get all in a single query.

In order to get optimal results with the batch query, I recommend this workflow:

• submit a batch with all of your 500 items.
• store the results in your dicts
• re-submit with the UnprocessedKeys as many times as needed
• sort the results on the python side

Quick example

I assume you have created a table "MyTable" with a single hash_key

import boto

# Helper function. This is more or less the code
# I added to devolop branch
def resubmit(batch, prev):
    # Empty (re-use) the batch
    del batch[:]

    # The batch answer contains the list of
    # unprocessed keys grouped by tables
    if 'UnprocessedKeys' in prev:
        unprocessed = res['UnprocessedKeys']
    else:
        return None

    # Load the unprocessed keys
    for table_name, table_req in unprocessed.iteritems():
        table_keys = table_req['Keys']
        table = batch.layer2.get_table(table_name)

        keys = []
        for key in table_keys:
            h = key['HashKeyElement']
            r = None
            if 'RangeKeyElement' in key:
                r = key['RangeKeyElement']
            keys.append((h, r))

        attributes_to_get = None
        if 'AttributesToGet' in table_req:
            attributes_to_get = table_req['AttributesToGet']

        batch.add_batch(table, keys, attributes_to_get=attributes_to_get)

    return batch.submit()

# Main
db = boto.connect_dynamodb()
table = db.get_table('MyTable')
batch = db.new_batch_list()

keys = range (100) # Get items from 0 to 99

batch.add_batch(table, keys)

res = batch.submit()

while res:
    print res # Do some usefull work here
    res = resubmit(batch, res)

# The END

EDIT:

I’ve added a resubmit() function to BatchList in Boto develop branch. It greatly simplifies the worklow:

1. add all of your requested keys to BatchList
2. submit()
3. resubmit() as long as it does not return None.

this should be available in next release.