Skipping cache generation, cache already exists for key - caching

Question

Using CircleCI - version: 2.1 - for continuous deployment where caching installed dependencies. Based on save_cache documentation:
Generates and stores a cache of a file or directory of files such as dependencies or source code in our object storage. Later jobs can restore this cache.
Current scenario:
See the simplified caching step below in .circleci/config.yml file:
steps:
- node/with-cache:
steps:
- checkout
- run: npm install
- save_cache:
key: dependencies
paths: node_modules
The problem is coming once adding new package to the project thus package.json file is changing. In the same time CircleCI shows the message for Saving Cache step:
Skipping cache generation, cache already exists for key: dependenciesFound one created at 2020-05-23 19:29:29 +0000 UTC
Then once restoring the cache obviously does not find the newly added package in the build step:
./src/index.tsxCannot find module: 'package-name'. Make sure this package is installed.
Questions:
Is there any way to check package.json changes in the pipeline? Ideally I would install the dependencies only in those cases, so the cache can be purged and updated.
Maybe I did not see something in the documentation. Any help is appreciated, thank you!

Answer 1

The problem is the cache key you used is "dependencies", a plain string. This key never changes, so you will always use the same exact cache.
You need to use a cache key that changes, preferably based on package.lock. Please read the section of cache keys in the CircleCI Docs for more information: https://circleci.com/docs/2.0/caching/#using-keys-and-templates