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What is boto3?

Boto3 is the python SDK for interacting with the AWS api. It makes it easy to use the python programming language to manipulate AWS resources and automation infrastructure.

What are boto3 waiters and how do I use them?

A number of requests in AWS using boto3 are not instant. Common examples are deploying a new server or RDS instance. For some long running requests, we are ok to initiate the request and then check for completion at some later time. But in many cases, we want to wait for the request to complete before we move on to the subsequent parts of the script that may rely on a long running process to have been completed. One example would be a script that might copy an AMI to another account by sharing all the snapshots. After sharing the snapshots to the other account, you would need to wait for the local snapshot copies to complete before registering the AMI in the receiving account. Luckily a snapshot completed waiter already exists, and here’s what that waiter would look like in Python:

As far as the default configuration for the waiters and how long they wait, you can view the information in the boto3 docs on waiters, but it’s 600 seconds in most cases. Each one is configurable to be as short or long as you’d like.

Writing your own custom waiters.

As you can see, using boto3 waiters is an easy way to setup a loop that will wait for completion without having to write the code yourself. But how do you find out if a specific waiter exists? The easiest way is to explore the particular boto3 client on the docs page and check out the list of waiters at the bottom. Let’s walk through the anatomy of a boto3 waiter. The waiter is actually instantiated in botocore and then abstracted to boto3. So looking at the code there we can derive what’s needed to generate our own waiter:

  1. Waiter Name
  2. Waiter Config
    1. Delay
    2. Max Attempts
    3. Operation
    4. Acceptors
  3. Waiter Model

The first step is to name your custom waiter. You’ll want it to be something descriptive and, in our example, it will be “CertificateIssued”. This will be a waiter that waits for an ACM certificate to be issued (Note there is already a CertificateValidated waiter, but this is only to showcase the creation of the waiter). Next we pick out the configuration for the waiter which boils down to 4 parts. Delay is the amount of time it will take between tests in seconds. Max Attempts is how many attempts it will try before it fails. Operation is the boto3 client operation that you’re using to get the result your testing. In our example, we’re calling “DescribeCertificate”. Acceptors is how we test the result of the Operation call. Acceptors are probably the most complicated portion of the configuration. They determine how to match the response and what result to return. Acceptors have 4 parts: Matcher, Expected, Argument, State.

  • State: This is what the acceptor will return based on the result of the matcher function.
  • Expected: This is the expected response that you want from the matcher to return this result.
  • Argument: This is the argument sent to the matcher function to determine if the result is expected.
  • Matcher: Matchers come in 5 flavors. Path, PathAll, PathAny, Status, and Error. The Status and Error matchers will effectively check the status of the HTTP response and check for an error respectively. They return failure states and short circuit the waiter so you don’t have to wait until the end of the time period when the command has already failed. The Path matcher will match the Argument to a single expected result. In our example, if you run DescribeCertificate you would get back a “Certificate.Status” as a result. Taking that as an argument, the desired expected result would be “ISSUED”. Notice that if the expected result is “PENDING_VALIDATION” we set the state to “retry” so it will continue to keep trying for the result we want. The PathAny/PathAll matchers work with operations that return a python list result. PathAny will match if any item in the list matches, PathAll will match if all the items in the list match.

Once we have the configuration complete, we feed this into the Waiter Model call the “create_waiter_with_client” request. Now our custom waiter is ready to wait. If you need more examples of waiters and how they are configured, check out the botocore github and poke through the various services. If they have waiters configured, they will be in a file called waiters-2.json. Here’s the finished code for our customer waiter.

And that’s it. Custom waiters will allow you to create automation in a series without having to build redundant code of complicated loops. Have questions about writing custom waiters? Contact us

-Coin Graham, Principal Cloud Consultant

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