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Better application logging on AWS Lambda

In this post I will cover what I consider the best practices for producing useful logs when deploying to AWS Lambda. I’ll explain how AWS Lambda Powertools, an Open-source client library, can help do a lot of the heavy lifting for you. This post is aimed at developers, DevOps engineers and anyone else involved in the production or operation of systems using Lambda.

I started my tech career as a build engineer; I guess we’d call this a junior Ops role now. It was my responsibility to build other people’s code, get it onto the servers and understand if it wasn’t working correctly. You can imagine that logs of every kind became integral to my working life. Ant, Maven, Hudson (now Jenkins), WebLogic, Tomcat, all produced logs and could be a source of information on issues affecting the applications I supported.

Fast forward to today and although the build and continuous integration tooling has changed over time, the biggest difference I’ve seen is with application logs. I used to deploy applications to two servers in production and one server in the dev and test environments. I used to know all the server ip addresses by heart and had scripts to help me with common tasks. Working on a single server most of the time made life simple. If I had a problem, connect to the machine over ssh, grep the application log for ERROR and start debugging. I can’t tell you how many times I went to the first production server, did the same, found nothing and scratched my head, only to then remember I needed to check the second machine. So what does this have to do with AWS Lambda?

Well as a user of Lambda I have no idea what server my code is running on or what is happening with my application logs. It turns out both of these are huge advantages. The placement of my code onto a server spread across multiple availability zones, load balanced automatically and health checked is all work I no longer have to do. Zipping log files, shipping them to a central location and managing storage is no longer a pain for me.

Lambda automatically integrates with Amazon CloudWatch for a fully managed logging solution which makes working with ephemeral execution environments easy. As an application developer all I have to do is produce my log message to standard out and Lambda does the rest. So in this new world what are the best practices when producing application logs?

Include Lambda context information. The function name, its memory allocation, its version is all useful information to help debug problems. Has a problem started after a new version has been released? Did the function get more expensive after a memory allocation change?

Include AWS Identifiers. As well as the AWS request ID, include the X-Ray trace id if you have tracing enabled. Help yourself join information for multiple sources by including all the IDs you’ll need. This is also true for IDs for external or integration systems.

Structure your logs in JSON. Take advantage of the powerful querying in CloudWatch Insights by structuring your logs into JSON.

Log requests and responses. Where the workload allows, log requests to and responses from your function. This isn’t always possible if you’re working with particularly big requests such as encode images.

Include business identifiers. If the log message is about a customer, include the customer ID. If the log message is about a transaction failure, include the transaction ID.

Runtime configurability. Have the ability to change your logging level and sampling without having to re-release your function code. The ability to enable and disable DEBUG logging when you have a problem is fantastic. Since CloudWatch charges are based on amount ingested as well as storage it is a good cost optimization do be able to control both easily.

This may all sound like a lot of additional work and it can be, but AWS Lambda Powertools supports all of the functionality I listed above and more. Powertools currently supports Java and Python, with more language support coming.

For examples of Java Lambda function handlers and the log messages they produce, you can read the AWS blog post.

AWS Specialist Solutions Architect for Serverless. Working to create the best Java developer experience on AWS Lambda.

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