feat: GuLambdaFunction should use JSON logging by default

[NovemberTang]

What does this change?

I think JSON log formatting is a more sensible default value.

There are two log formats in AWS Lambda, Text, and JSON. Text is the default (possibly for historical reasons?), but has disadvantages, such as not being as easily machine-readable.

Using Text means that any changes made to the ApplicationLogLevel will be flat out ignored (defaulting to DEBUG as far as I can tell). Application log filtering only works when the log format is JSON.

I've verified that Lambdas using this configuration are still compatible with Central ELK. Repocop has been using JSON logging for several weeks without issue.

How to test

• Verify default behaviour of lambdas has changed
• Verify that providing the Text value results in the expected snapshot

How can we measure success?

Teams will be able to filter their application logs by severity level without having to do as much googling/asking around as I had to

Have we considered potential risks?

This will cause snapshot changes to everyone using a GuLambdaFunction or its descendants. This will be slightly disruptive, but no work is required beyond updating the snapshot tests.

To maintain the previous behaviour, users should add the following to their lambda props

logFormat: "Text"

Checklist

• I have listed any breaking changes, along with a migration path ¹
• I have updated the documentation as required for the described changes ²

Footnotes

1. Consider whether this is something that will mean changes to projects that have already been migrated, or to the CDK CLI tool. If changes are required, consider adding a checklist here and/or linking to related PRs. ↩

2. If you are adding a new construct or pattern, has new documentation been added? If you are amending defaults or changing behaviour, are the existing docs still valid? ↩

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[akash1810]

Just to double check - does https://github.com/guardian/cloudwatch-logs-management apply the log markers to JSON format logs?

[NovemberTang]

Just to double check - does https://github.com/guardian/cloudwatch-logs-management apply the log markers to JSON format logs?

It seems like it to me. Here's a link to the repocop logs, which have been using JSON formatting for months.