[Next.js] Mark Cold Start When Deployed To Vercel

[souredoutlook]

Problem Statement

Customers would like to be able to differentiate events based on whether or not they are associated with a cold-start of a severless function.

The initial request specifically mentions Next.js when deployed to vercel.

Solution Brainstorm

Identifying whether or not the event context is from a cold-start using a tag may be very helpful.

Examples:

• alerting on % change for transaction duration only when considering a hot-start
• knowing whether an error is associated only with cold starts

cc @lforst

[lforst]

Generally, this would work by having a global variable (e.g. let visited) initialized to false. In our handler wrapper code, we would look at that variable and say if visited == false then coldStart else notColdStart. Right after we set visited to true.

I think this is a great idea but I still have my doubts about whether we can reliably do this from within the SDK:

• Per definition, the global must be truly global so all handlers really share this global variable
• What happens if one handler calls another one? The first one will examine coldStart the second one notColdStart.
• Maybe we attach the context to the req object so that the case above is handled?

Anyhow I think this would be a banger blog post.

[github-actions]

This issue has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you label it Status: Backlog or Status: In Progress, I will leave it alone ... forever!

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"A weed is but an unloved flower." ― Ella Wheeler Wilcox 🥀