This post is written by Tarun Rai Madan, Principal Product Manager –  Serverless, and Rajesh Kumar Pandey, Principal Software Engineer, Serverless

Today, AWS is announcing new opt-in Amazon CloudWatch metrics for AWS Lambda Event Source Mappings that subscribe to Amazon Simple Queue Service (Amazon SQS), Amazon Kinesis, and Amazon DynamoDB event sources. These metrics include PolledEventCount, InvokedEventCount, FilteredOutEventCount, FailedInvokeEventCount, DeletedEventCount, DroppedEventCount, and OnFailureDestinationDeliveredEventCount. The new metrics enable customers to monitor the processing state of events read by Event Source Mappings (ESMs), and helps them diagnose processing issues.

Previously, customers found it challenging to monitor the processing state of events read by an ESM. An ESM is a resource that polls events from an event source and invokes a Lambda function. With the new metrics for ESMs, you can count events by their processing state, which includes events that were polled, invoked, filtered out, deleted, dropped, failed, or sent to on-failure destination.

Overview

Customers building modern event-driven applications use services like SQS, Kinesis, and DynamoDB as fundamental building blocks for developing decoupled architectures, and use a Lambda function as a consumer to benefit from its simplicity, auto-scaling and cost effectiveness. To subscribe to an event source, customers configure a Lambda Event Source Mapping (ESM). An ESM is a fully-managed Lambda resource that runs an event poller which polls, processes (e.g., filters and batches), and delivers the events to a Lambda function. Due to the processing that happens on an ESM, for example, filtering, batching, and delivery to on-failure destinations, events can end up in varying terminal states. As a result, some polled events may not invoke a Lambda function. Previously, the count of polled, filtered, invoked, deleted or dropped events was not visible to customers. This made it challenging for customers to diagnose processing issues with their ESM, resulting from faulty permissions, misconfiguration, or function errors.

What’s new

With today’s announcement, customers can opt-in to CloudWatch metrics to monitor the processing state of events that are read by an ESM configured with SQS, Kinesis and DynamoDB as event sources.

PolledEventCount metric counts the number of events read by an ESM from the event source.

InvokedEventCount metric counts the number of events that invoked your Lambda function. For an event that experiences function errors, this metric may increase the count multiple times for the same polled event, due to retries.

FilteredOutEventCount metric counts the number of events filtered out by your ESM, based on the Filter Criteria defined by you.

FailedInvokeEventCount metric counts the number of events that attempted to invoke a Lambda function, but encountered partial or complete failure.

DeletedEventCount metric counts the events that have been deleted from the SQS queue by Lambda upon successful processing.

DroppedEventCount metric counts the number of events dropped due to event expiry or exhaustion of retry attempts, for Kinesis and DynamoDB ESMs configured with MaximumRecordAgeInSeconds or MaximumRetryAttempts.

OnFailureDestinationDeliveredEventCount metric counts the events sent to an on-failure destination upon reaching the MaximumRecordAgeInSeconds or MaximumRetryAttempts, for ESMs configured with DestinationConfig.

How to use the new ESM metrics

Once an ESM is created and reaches enabled state, it continuously polls the event source for new events. You can monitor the PolledEventCount metric to catch issues with polling due to misconfigured or deleted event source, misconfigured or deleted Lambda function execution role, incorrect permissions, or throttles from the event source. This metric typically increases when there is an increase in traffic in the event source. You can observe the InvokedEventCount metric to catch issues with the Lambda function, and whether the events are properly invoking your Lambda function. In case of Lambda function errors, InvokedEventCount could be more than PolledEventCount due to retries. This metric would also increase when there is an increase in events processed by an ESM. For ESMs that have filter criteria configured, you can monitor the FilteredOutEventCount to count events that were not sent to a Lambda function because they were filtered out per the defined filter criteria.

You can monitor the FailedInvokeEventCount metric to observe the number of events that failed processing when Lambda service tried to invoke your Lambda function. Invocations can fail due to network configuration issues, incorrect permissions, or a deleted Lambda function, version, or alias. If your event source mapping has partial batch responses enabled, this metric includes any event with a non-empty BatchItemFailures in the response. If all events in a batch are successfully processed by your Lambda function, Lambda service emits a 0 value for this metric. You can use the DeletedEventCount metric to ensure that processed events have been successfully deleted from your SQS queue after being processed by the Lambda function. You can use the DroppedEventCount metric to identify issues with message backlogs or misconfigured event expiry rules. You can use the OnFailureDestinationDeliveredEventCount metric to monitor issues such as failed events caused by Lambda function invocation errors.

The classification for available Lambda ESM metrics by event source is presented below:

CloudWatch metric	SQS	DynamoDB	Kinesis Data Stream
PolledEventCount	√	√	√
InvokedEventCount	√	√	√
FilteredOutEventCount	√	√	√
FailedInvokeEventCount	√	√	√
DeletedEventCount	√
DroppedEventCount		√	√
OnFailureDestinationDeliveredEventCount		√	√

Activating and testing the new ESM metrics

You can enable the new ESM metrics using AWS Lambda Console, AWS Command Line Interface (CLI), Lambda ESM API, AWS SDK, AWS CloudFormation, and AWS Serverless Application Model (SAM). The metrics will be published under the AWS/Lambda namespace and EventSourceMappingUUID dimension in the CloudWatch console. To learn more, see CloudWatch metrics for Lambda.

Using AWS CLI

To turn on the new metrics using AWS CLI, use the –metrics-config parameter.

aws lambda create-event-source-mapping \ --region <region-name> \ --function-name <function-name> \ --event-source-arn <event-source-arn> \ --metrics-config '{"Metrics": ["EventCount"]}'

Using AWS Lambda Console

To turn on the new metrics using AWS Lambda Console, click on “Enable metrics” while adding the trigger for your function.

Figure 1: Enabling ESM metrics in AWS Console

A typical scenario where the new ESM metrics can help with better observability is an ESM that uses event filtering. To test the ESM metrics, you can deploy a sample Lambda application with Kinesis as an event source using this serverless pattern, which uses event filtering with a certain criteria to control which events are sent to Lambda. Use this pattern for both the example scenarios; please follow the setup guidelines for this pattern and continue with testing for the scenarios. Running this sample project in your account may incur charges. See AWS Lambda pricing and Amazon Kinesis pricing.

Figure 2: Configuring Lambda function with Kinesis event source

Example scenario 1: ESM metrics with event filtering configured

The following diagram shows the results for the test scenario with Kinesis ESM, where the total polled events, filtered events, invoked events, and failed events are represented by PolledEventCount, FilteredOutEventCount, InvokedEventCount and FailedInvokeEventCount.

Figure 3: ESM metrics for scenario 1

Example Scenario 2: ESM metrics with event filtering and On-Failure Destination configured

Another common scenario is where you want to have visibility around the number of events delivered to Lambda function, events filtered, and additionally, the count of events routed to on-failure destination upon failure. To test this scenario, follow a setup similar to the one in scenario 1. Create or update the ESM with an on-failure destination, and set MaximumRetryCount to 1, as shown below.

aws lambda update-event-source-mapping \ --uuid <event-source-mapping-uuid> \ --maximum-retry-attempts 1 \ --filter-criteria '{"Filters": [{"Pattern": "{\"data\": { \"tire_pressure\": [ { \"numeric\": [ \"<\", 32 ] } ] } }"}]}' \ --destination-config '{"OnFailure": {"Destination": "<your_SQS_queue_ARN>"}}' \ --function-name <lambda-function-name>

Publish a sample payload which matches the FilterCriteria defined above. Also generate sample data with different “tire_pressure” < 32 to match the event and invoke the Lambda function.

Sample Data:

{ "time": "2021-11-09 13:32:04", "fleet_id": "fleet-452", "vehicle_id": "a42bb15c-43eb-11ec-81d3-0242ac130003", "lat": 47.616226213162406, "lon": -122.33989110734133, "speed": 43, "odometer": 43519, "tire_pressure": [41, 40, 31, 41], "weather_temp": 76, "weather_pressure": 1013, "weather_humidity": 66, "weather_wind_speed": 8, "weather_wind_dir": "ne" }

Once you have published these records to the stream, you should be able to see the CloudWatch metrics under AWS/Lambda namespace with the EventSourceMappingUUID dimension, as shown below. Note that if an event experiences a function error, InvokedEventCount may increase multiple times for the same polled event due to automatic retries.

Figure 4: ESM metrics for scenario 2

Available Now

The new ESM metrics are generally available in all commercial regions that Lambda service is available in. Support is also available through AWS Lambda partners like Datadog, Elastic, and Lumigo. The Lambda service sends these new metrics to CloudWatch at no additional cost to you. However, charges apply for CloudWatch metrics at standard CloudWatch metrics pricing for these opt-in metrics, in addition to your AWS Lambda pricing and event source pricing.

Conclusion

With these new CloudWatch metrics, you can gain visibility into the processing state of your events that are polled by Lambda Event Source Mapping (ESM) for queue-based or stream-based applications. The blog explains the new metrics PolledEventCount, InvokedEventCount, FilteredOutEventCount, FailedInvokeEventCount, DeletedEventCount, DroppedEventCount, and OnFailureDestinationDeliveredEventCount, and how to use them to troubleshoot event processing issues for Lambda functions. These metrics help you track the invocation requests sent to Lambda via an ESM, monitor any delays or issues in processing, and take corrective actions if required. To learn more about these metrics, visit Lambda developer guide.

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