Overall Program Flow

AwsLimitChecker provides the full and only public interface to this project; it’s used by the awslimitchecker command line script (entry point to runner) and should be the only portion directly used by external code.

Each AWS Service is represented by a subclass of the _AwsService abstract base class; these Service Classes are responsible for knowing which limits exist for the service they represent, what the default values for these limits are, querying current limits from the service’s API (if supported), and how to check the current usage via the AWS API (boto3). When the Service Classes are instantiated, they build a dict of all of their limits, correlating a string key (the “limit name”) with an AwsLimit object. The Service Class constructors must not make any network connections; connections are created lazily as needed and stored as a class attribute. This allows us to inspect the services, limits and default limit values without ever connecting to AWS (this is also used to generate the Supported Limits documentation automatically).

All calls to boto3 client (“low-level”) methods that return a dict response that can include ‘NextToken’ or another pagination marker, should be called through paginate_dict() with the appropriate parameters.

When AwsLimitChecker is instantiated, it imports services which in turn creates instances of all* classes and adds them to a dict mapping the string Service Name to the Service Class instance. These instances are used for all interaction with the services.

So, once an instance of AwsLimitChecker is created, we should have instant access to the services and limits without any connection to AWS. This is utilized by the --list-services and --list-defaults options for the command line client.

Trusted Advisor

When AwsLimitChecker is initialized, it also initializes an instance of TrustedAdvisor. In get_limits(), find_usage() and check_thresholds(), when called with use_ta == True (the default), update_limits() is called on the TrustedAdvisor instance.

update_limits() polls Trusted Advisor data from the Support API via _poll(); this will retrieve the limits for all “flaggedResources” items in the Service Limits Trusted Advisor check result for the current AWS account. It then calls _update_services(), passing in the Trusted Advisor check results and the dict of _AwsService objects it was called with (from AwsLimitChecker).

_update_services() iterates over the Services in the Trusted Advisor check result and attempts to find a matching _AwsService (by string service name) in the dict passed in from AwsLimitChecker. If a match is found, it iterates over all limits for that service in the TA result and attempts to call the Service’s _set_ta_limit() method. If a matching Service is not found, or if _set_ta_limit raises a ValueError (matching Limit not found for that Service), an error is logged.

When AwsLimitChecker initializes TrustedAdvisor, it passes in the dictionary of all services and limits. At initialization time, TrustedAdvisor iterates all services and limits, and builds a new dictionary mapping the limit objects by the return values of their ta_service_name() and ta_limit_name() properties. This allows limits to override the Trusted Advisor service and limit name that their data comes from. In the default case, their service and limit names will be used as they are set in the awslimitchecker code, and limits which have matching Trusted Advisor data will be automatically populated.

In the TrustedAdvisor class’s _poll() method, _get_refreshed_check_result() is used to retrieve the check result data from Trusted Advisor. This method also implements the check refresh logic. See the comments in the source code for the specific logic. There are three methods of refreshing checks (refresh modes), which are controlled by the ta_refresh_mode parameter to TrustedAdvisor:

  • If ta_refresh_mode is the string “wait”, the check will be refreshed and awslimitchecker will poll for the refresh result every 30 seconds, waiting for the refresh to complete (or until ta_refresh_timeout seconds have elapsed). This is exposed via the CLI as the --ta-refresh-wait option.
  • If ta_refresh_mode is an integer, it will operate like the “wait” mode above, but only if the current result data for the check is more than ta_refresh_mode seconds old. This is exposed via the CLI as the --ta-refresh-older option.
  • If ta_refresh_mode is the string “trigger”, the check will be refreshed and the program will continue on immediately, without waiting for the refresh to complete; this will almost certainly result in stale check results in the current run. However, this may be useful if you desire to keep awslimitchecker runs short, and run it on a regular schedule (i.e. if you run awslimitchecker every 6 hours, and are OK with Trusted Advisor check data being 6 hours old). This is exposed via the CLI as the --ta-refresh-trigger option.

Additionally, TrustedAdvisor has a ta_refresh_timeout parameter. If this is set to a non-None value (an integer), refreshes of the check will time out after that number of seconds. If a timeout occurs, a message will be logged at error level, but the program will continue running (most likely using the old result data). This parameter is exposed via the CLI as the --ta-refresh-timeout option.

Important: It may take 30 to 60 minutes for the Service Limits check to refresh on large accounts. Please be aware of this when enabling the refresh options.

Using the check refresh options will require the trustedadvisor:RefreshCheck IAM permission.

For use via Python, these same parameters (ta_refresh_mode and ta_refresh_timeout) are exposed as parameters on the AwsLimitChecker constructor.

Service Quotas service

Unless use of Serivce Quotas is disabled with the --skip-quotas command line option or by passing skip_quotas=False to the AwsLimitChecker constructor, awslimitchecker will retrieve all relevant data from the Service Quotas service. In the AwsLimitChecker constructor (so long as skip_quotas is True), an instance of the ServiceQuotasClient class is constructed, passing in our boto3 connection keyword arguments for the current region. This client class instance is then passed to the constructor of every Service class (_AwsService subclass) when the class is created, via the quotas_client argument. Each _AwsService class stores this as the _quotas_client instance variable.

As the AwsLimitChecker class iterates over all (configured) services in its get_limits(), find_usage(), and check_thresholds() methods, it will call the service class’s _update_service_quotas() method after calling update_limits() and the service class’s _update_limits_from_api() method (if present), and before the actual operation of getting limits, finding usage, or checking thresholds.

The _AwsService._update_service_quotas() method will iterate through all limits (AwsLimit) for the service and call the get_quota_value() method for each. Assuming it returns a non-None result, that result will be passed to the limit’s _set_quotas_limit() method for later use in get_limit().

When retrieving values from Service Quotas, the ServiceCode is taken from the _AwsService.quotas_service_code attribute on the Service class. If that is set to None, Service Quotas will not be consulted for that service. The ServiceCode can also be overridden on a per-limit basis via the quotas_service_code argument to the AwsLimit constructor. The QuotaName used by each limit defaults to the limit name itself (AwsLimit instance variable name) but can be overridden with the quota_name argument to the AwsLimit constructor.

Note that quota names are stored and compared in lower case.

Service API Limit Information

Some services provide API calls to retrieve at least some of the current limits, such as the DescribeAccountAttributes API calls for RDS and EC2. Services that support such calls should make them in a _update_limits_from_api() method, which will be automatically called from get_limits(). The _update_limits_from_api() method should make the API call, and then update all relevant limits via the AwsLimit class’s _set_api_limit() method.

Limit Value Precedence

The value used for a limit is the first match in the following list:

  1. Limit Override (set at runtime)
  2. API Limit
  3. Service Quotas
  4. Trusted Advisor
  5. Hard-coded default

Threshold Overrides

For more information on overriding thresholds, see Python Usage / Setting a Threshold Override as well as the documentation for AwsLimitChecker.check_thresholds() and AwsLimitChecker.set_threshold_override().