watch Option for SAM Build command

[jfuss]

Describe your idea/feature/enhancement

When using sam build to build functions that will later be invoked through sam local [invoke|start-lambda|start-api], you need to run sam build before each invoke. The reason for this requirement is because the build command places artifacts (code + dependencies + updated template) into a build folder. The sam local [invoke|start-lambda|start-api] use these built templates (this has updated locations to the built function code) by default but there is no way to update the built code without running sam build again.

Proposal

For the initial implementation of sam build, the --watch option was out of scope. The proposal is to add support for this option to all customers to enable building when source code changes.

An alternative could be to make the sam local suite of commands build the function during invoke. The main concern with this is the speed at which you can now invoke (other build enhancements might be a requirement for this, speed, incremental builds, etc).

Workaround

All of the sam local command allow 'reloading' of the function code. This means for a given template, if you update the contents of a functions Code/CodeUri, SAM CLI will mount this updated content. So the current option for making interactions easier is to have two terminal windows open, one that builds and one that you local invoke with.

[alxx]

To me this only happens using Ruby once I've added a gem into the equation, with my functions having Gemfiles in their respective directories. Then I'm forced to build using --use-container parameter, which takes a lot of time; simply running sam build doesn't work because the app doesn't find the gems.

If I don't use gems at all, then I have live reloading; but once I need gems, everything only works by rebuilding inside a container. And since that takes about a minute, it makes SAM pretty much unusable for me. I can't build every time I make a small change, it just takes too long.

[AmillerSC]

Is there any progress on this? Having to build every time for the most minor of code changes is not ideal.

[sliedig]

I'd like to see a slightly different take on this proposal. Rather than having a continual build function through --watch option, only build functions that have changed since the last build. I often find myself having to rebuild functions that haven't been modified, or just making changes to the template file, making the feedback loop unnecessarily long.

[tamvm]

To me this only happens using Ruby once I've added a gem into the equation, with my functions having Gemfiles in their respective directories. Then I'm forced to build using --use-container parameter, which takes a lot of time; simply running sam build doesn't work because the app doesn't find the gems.

If I don't use gems at all, then I have live reloading; but once I need gems, everything only works by rebuilding inside a container. And since that takes about a minute, it makes SAM pretty much unusable for me. I can't build every time I make a small change, it just takes too long.

Hi @alxx,

So what is your workaround for that? Or can you share your experience on that?

Thank you,

[mims92]

I don't like to compare but Chalice as this feature if I'm not mistaken ;)

[justengland]

Assuming this is hard to build the watch option since this has been open for the better part of a year. It seems an easier option would be to use a docker volume to share your code rather than requiring a build, this way you do not have to have an actual watch.

[dgomesbr]

For a workaround, you're able to simply delete the .aws-sam folder at the root of your project and get back invoking your function.

ref: rm -rf .aws-sam/*at root.