(CDK watch): skip synthesis if the change is only to an Asset

[skinny85]

Description

There is a possible performance optimization with cdk watch: if a given change event is for a file that belongs to an Asset, we can skip synthesizing the Cloud Assembly again (we know it will be the same).

Since synthesis is actually the bottleneck for many hotswap deployments (like for Lambda, or for StepFunctions), this might be a significant performance improvement for some use cases, especially in non-NodeJS languages.

Use Case

Hotswapping of resources where cdk synthesis is the performance bottleneck, especially in non-NodeJS languages.

Proposed Solution

The implementation of this would be quite complex. Basically, when we receive a change in cdk watch, we have to:

1. Check that it belongs to the files/directories of any Asset in any of the Stack(s) we are deploying with watch.
2. For each of those found Assets, we now have to search the template of the Stack(s) to determine which resources reference that Asset.
3. For each of those resources, we now have to send them to the hotswap detectors to verify whether they support hotswapping.
4. If all of them do, we can now perform hotswapping without running cdk synth before.

Other information

No response

Acknowledge

• I may be able to implement this feature request
• This feature might incur a breaking change

[skinny85]

If I have prepared a Proof of Concept of how this functionality would look like. Here's a demo showing it in action: https://youtu.be/mSYJJDdCQzA

The code for the above demo is here. It's very much PoC-level code, but I think gives a good start with how to think about this functionality.

I tried to include ToDos in the code where appropriate, but here's a list of things that need to be fixed before we can think of making this functionality part of the CDK:

• support old-style synthesis (in addition to new-style synthesis, which works now)
• think whether no staging of the Assets is really required
• support bundling in Assets (very important for things like NodeJsFunction)
• executables in Assets (a feature of new-style synthesis)
• support for Docker assets (in addition to the file assets that work now)
• this should probably run the "build" command (if provided) before deployment (this command was introduced in feat(cli): added build field to cdk.json #17176)

Unfortunately, I won't have time to work on this functionality in the near future, so if anyone wants to pick this up (feel free to use the above PoC code as a starting-off point), that would be fantastic!

[phlrnnr]

This would be an excellent feature enhancement!