Handling EC2 Description Rate Limiting In Boto3 Lambda? - amazon-web-services

Question

I'm creating a Lambda function with the intent of backing up my EC2 instances with their snapshots. However, I noticed reading the boto documentation the call to ec2.describe_instances is rate limited with MaxResults/NextToken. How can I combine the two of these to safely iterate through the list 50 at a time? Below is my work in progress:
import boto3
import datetime
import time
ec2 = boto3.client('ec2')
def lambda_handler(event, context):
try:
print("Creating snapshots on " + str(datetime.datetime.today()) + ".")
maxResults = 50
schedulers = ec2.describe_instances(Filters=[{'Name':'tag:GL-sub-purpose', 'Values':[Schedule]}], MaxResults=maxResults)
nextToken = schedulers['NextToken']
totalSchedulers = len(schedulers)
while totalSchedulers == maxResults:
schedulers = ec2.describe_instances(Filters=[{'Name':'tag:GL-sub-purpose', 'Values':[Schedule]}], MaxResults=maxResults, NextToken=nextToken)
nextToken = result['NextToken']
totalSchedulers = len(schedulers)
print("Performing backup on " + str(len(schedulers)) + " schedules.")
successful = []
failed = []
for s in schedulers:
#[...] More operations here, done 50 at a time.
I'm not really sure if I'm using the MaxResults/NextToken parameters correctly or efficiently here. Is this the best way to achieve my desired result/am I on the right track?

Answer 1

Just iterate through until NextToken is not returned. Here is a sample code to iterate through a batch of instances. Change it to suit your needs.
import boto3
ec2 = boto3.client('ec2')
insts = ec2.describe_instances(MaxResults=50)
while True:
#
# Process Instances (insts)
#
if 'NextToken' not in insts: break
next_token = insts['NextToken']
insts = ec2.describe_instances(MaxResults=50, NextToken=next_token)