Solve: How to Modularize Your SAM Templates Without Breaking sam sync

Solve: How to Modularize Your SAM Templates Without Breaking sam sync

The Tension Between Modularity and Speed

The AWS SAM CLI has grown into a powerful developer toolset, and sam sync is one of its most welcome additions. It lets you update your deployed application fast, skipping a full CloudFormation round trip and keeping your edit-deploy-test loop tight. But there's a catch: sam sync expects a flat, ready-to-go template.yaml. That can make modularization difficult. If you're breaking your SAM template into smaller files for clarity and reuse, you risk breaking compatibility with sam sync—unless you finish the thought.

What sam sync Expects From You

Under the hood, sam sync is watching a specific file: template.yaml. It assumes that file defines the whole application—no includes, no file stitching, just a single YAML document describing all resources and configuration. If your team splits its infrastructure across multiple partial files, you'll need to reassemble them before sam sync gets involved. Otherwise, it can't tell what's changed or what to redeploy.

This isn't a limitation so much as a design tradeoff. sam sync is meant to accelerate your dev loop, not synthesize or resolve complex architectures. So if you want modular templates and sam sync, you’ll need a merge layer that runs before syncing starts.

Merging Modular Templates the Right Way

The most reliable strategy is to create a small build step that merges your modular YAML files into a single template.yaml. You can use yq, a Python script, or even a make command—whatever suits your team's comfort level. That build step becomes part of your workflow: write modularly, then compile before syncing.

Here’s a simple structure to get started: 

bash

project-root/
├── templates/
│   ├── lambda.yaml
│   ├── api.yaml
│   └── dynamo.yaml
├── scripts/
│   └── merge_templates.py
├── Makefile
└── template.yaml  # generated 

Then your dev loop might look like this: 

bash

make merge  # outputs merged template.yaml
sam sync --stack-name mystack --watch 

You get fast iteration, a tidy repo, and a structure that scales with your team.

Read Our Related Post Here

We cover multiple layout options and give full examples of the merge script in our earlier post: 

Solve: Teaching SAM to Share—Splitting template.yaml into Multiple Files

Finish the Thought, Then Sync

Modularization and sam sync don’t have to be at odds. You just need to be deliberate: let your developers write clean, isolated template files, and give your tooling the merged result it expects. This isn’t a hack—it’s a bridge between clean repo design and fast deployment tooling. And it honors both worlds without compromise.

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Written by Aaron Rose, software engineer and technology writer at Tech-Reader.blog.