Serverless Jetpack

A faster JavaScript packager for Serverless applications

Serverless Jetpack 🚀

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A faster JavaScript packager for Serverless applications.

  • ⚡ Drop-in replacement for serverless package|deploy
  • 💻 Lambda Functions packaging
  • 🍰 Lambda Layers packaging
  • 📦 Per-function packaging
  • 🐉 Monorepo (lerna, yarn workspace) support
  • 🔀 Tunable, multi-cpu parallelization
  • 🔎 Dependency tracing options (faster packaging, slimmer bundles)


The Serverless framework is a fantastic one-stop-shop for taking your code and packing up all the infrastructure around it to deploy it to the cloud. Unfortunately, for many JavaScript applications, some aspects of packaging are slow, hindering deployment speed and developer happiness.

With the serverless-jetpack plugin, many common, slow Serverless packaging scenarios can be dramatically sped up. All with a very easy, seamless integration into your existing Serverless projects.

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The short, short version

First, install the plugin:

$ yarn add --dev serverless-jetpack
$ npm install --save-dev serverless-jetpack

Add to serverless.yml

  - serverless-jetpack

... and you're off to faster packaging awesomeness! 🚀

A little more detail...

The plugin supports all normal built-in Serverless framework packaging configurations in serverless.yml like:

  # Any `include`, `exclude` logic is applied to the whole service, the same
  # as built-in serverless packaging.
  # include: ...
    - "*"
    - "**/node_modules/aws-sdk/**" # included on Lambda.
    - "!package.json"

  # Add the plugin here.
  - serverless-jetpack

    # ...
    # ...
      # These work just like built-in serverless packaging - added to the
      # service-level exclude/include fields.
        - "src/**"
        - "!**/node_modules/aws-sdk/**" # Faster way to exclude
        - "package.json"


Most Serverless framework projects should be able to use Jetpack without any extra configuration besides the plugins entry. However, there are some additional options that may be useful in some projects (e.g., lerna monorepos, yarn workspaces)...

Service-level configurations available via custom.jetpack:

  • base (string): The base directory (relative to servicePath / CWD) at which dependencies may be discovered by Jetpack. This is useful in some bespoke monorepo scenarios where dependencies may be hoisted/flattened to a root node_modules directory that is the parent of the directory serverless is run from. (default: Serverless' servicePath / CWD).

    • WARNING: See our discussion below about the dangers of including files below the current working directory / Serverless servicePath.
    • Layers: Layers are a bit of an oddity with built-in Serverless Framework packaging in that the current working directory is layer.NAME.path (and not servicePath like usual), yet things like include|exclude apply relatively to the layer path, not the servicePath. Jetpack has a similar choice and applies base applies to the root servicePath for everything (layers, functions, and service packaging), which seems to be the best approach given that monorepo consumers may well lay out projects like functions/* and layers/* and need dependency inference to get all the way to the root irrespective of a child layer path.
  • roots (Array<string>): A list of paths (relative to servicePath / CWD) at which there may additionally declared and/or installed node_modules. (default: [Serverless' servicePath / CWD]).

    • Setting a value here replaces the default [servicePath] with the new array, so if you want to additionally keep the servicePath in the roots array, set as: [".", ADDITION_01, ADDITION_02, ...].
    • This typically occurs in a monorepo project, wherein dependencies may be located in e.g. packages/{NAME}/node_modules and/or hoisted to the node_modules at the project base. It is important to specify these additional dependency roots so that Jetpack can (1) find and include the right dependencies and (2) hone down these directories to just production dependencies when packaging. Otherwise, you risk having a slow serverless package execution and/or end up with additional/missing dependencies in your final application zip bundle.
    • You only need to declare roots of things that aren't naturally inferred in a dependency traversal. E.g., if starting at packages/{NAME}/package.json causes a traversal down to node_modules/something then symlinked up to lib/something-else/node_modules/even-more these additional paths don't need to be separately declared because they're just part of the dependency traversal.
    • Layers: Similar to base, both the project/service- and layer-level roots declarations will be relative to the project servicePath directory and not the layers.NAME.path directory.
  • preInclude (Array<string>): A list of glob patterns to be added before Jetpack's dependency pattern inclusion and Serverless' built-in service-level and then function-level package.includes. This option most typically comes up in a monorepo scenario where you want a broad base exclusion like !functions/** or !packages/** at the service level and then inclusions in later functions.
  • concurrency (Number): The number of independent package tasks (per function and service) to run off the main execution thread. If 1, then run tasks serially in main thread. If 2+ run off main thread with concurrency number of workers. (default: 1).

    • This option is most useful for Serverless projects that (1) have many individually packaged functions, and (2) large numbers of files and dependencies. E.g., start considering this option if your per-function packaging time takes more than 10 seconds and you have more than one service and/or function package.
  • collapsed.bail (Boolean): Terminate serverless program with an error if collapsed file conflicts are detected. See discussion below regarding collapsed files.

The following function and layer-level configurations available via functions.{FN_NAME}.jetpack and layers.{LAYER_NAME}.jetpack:

  • roots (Array<string>): This option adds more dependency roots to the service-level roots option.
  • preInclude (Array<string>): This option adds more glob patterns to the service-level preInclude option.
  • collapsed.bail (Boolean): Terminate serverless program with an error if collapsed file conflicts are detected if the function is being packaged individually.

Here are some example configurations:

Additional roots

# serverless.yml
  - serverless-jetpack

    # ...
    # This example monorepo project has:
    # - `packages/another/src`: JS source code to include
    # - `packages/another/package.json`: Declares production dependencies
    # - `packages/another/node_modules`: One location prod deps may be.
    # - `node_modules`: Another location prod deps may be if hoisted.
    # ...
      individually: true
        # If you want to keep prod deps from servicePath/CWD package.json
        # - "."
        # Different root to infer prod deps from package.json
        - "packages/another"
      # Ex: Typically you'll also add in sources from a monorepo package.
      - "packages/another/src/**"

Different base root

# serverless.yml
  - serverless-jetpack

    # Search for hoisted dependencies to one parent above normal.
    base: ".."

  # ...
    # **NOTE**: The include patterns now change to allow the underlying
    # globbing libraries to reach below the working directory to our base,
    # so patterns should be of the format:
    # - "!{BASE/,}{**/,}NORMAL_PATTERN"
    # - "!{BASE/,}{**/,}node_modules/aws-sdk/**"
    # - "!{BASE/,}{**/,}node_modules/{@*/*,*}/"
    # ... here with a BASE of `..` that means:
    # General
    - "!{../,}{**/,}.DS_Store"
    - "!{../,}{**/,}.vscode/**"
    # Dependencies
    - "!{../,}{**/,}node_modules/aws-sdk/**"
    - "!{../,}{**/,}node_modules/{@*/*,*}/"
    - "!{../,}{**/,}node_modules/{@*/*,*}/"

    # ...

With custom pre-includes

# 1. `preInclude` comes first after internal `**` pattern.
      - "!**" # Start with absolutely nothing (typical in monorepo scenario)

# 2. Jetpack then dynamically adds in production dependency glob patterns.

# 3. Then, we apply the normal serverless `include`s.
  individually: true
    - "!**/node_modules/aws-sdk/**"

  - serverless-jetpack

    # ...
        - "packages/another"
        # Tip: Could then have a service-level `include` negate subfiles.
        - "packages/another/dist/**"
      - "packages/another/src/**"


# serverless.yml
  - serverless-jetpack

    # A typical pattern is `NAME/nodejs/node_modules` that expands to
    # `/opt/nodejs/node_modules` which is included in `NODE_PATH` and available
    # to running lambdas. Here, we use `jetpack.roots` to properly exclude
    # `devDependencies` that built-in Serverless wouldn't.
    path: layers/vendor
        # Instruct Jetpack to review and exclude devDependencies originating
        # from this `package.json` directory.
        - "layers/vendor/nodejs"

How Jetpack's faster dependency filtering works

Serverless built-in packaging slows to a crawl in applications that have lots of files from devDependencies. Although the excludeDevDependencies option will ultimately remove these from the target zip bundle, it does so only after the files are read from disk, wasting a lot of disk I/O and time.

The serverless-jetpack plugin removes this bottleneck by performing a fast production dependency on-disk discovery via the inspectdep library before any globbing is done. The discovered production dependencies are then converted into patterns and injected into the otherwise normal Serverless framework packaging heuristics to efficiently avoid all unnecessary disk I/O due to devDependencies in node_modules.

Process-wise, the serverless-jetpack plugin detects when built-in packaging applies and then takes over the packaging process. The plugin then sets appropriate internal Serverless artifact fields to cause Serverless to skip the (slower) built-in packaging.

The nitty gritty of why it's faster

Let's start by looking at how Serverless packages (more or less):

  1. If the excludeDevDependencies option is set, use synchronous globby() for on disk I/O calls to find all the package.json files in node_modules, then infer which are devDependencies. Use this information to enhance the include|exclude configured options.
  2. Glob files from disk using globby with a root ** (all files) and the include pattern, following symlinks, and create a list of files (no directories). This is again disk I/O.
  3. Filter the in-memory list of files using nanomatch via service + function exclude, then include patterns in order to decide what is included in the package zip file.

This is potentially slow if node_modules contains a lot of ultimately removed files, yielding a lot of completely wasted disk I/O time.

Jetpack, by contrast does the following:

  1. Efficiently infer production dependencies from disk without globbing, and without reading any devDependencies.
  2. Glob files from disk with a root ** (all files), !node_modules/** (exclude all by default), node_modules/PROD_DEP_01/**, node_modules/PROD_DEP_02/**, ... (add in specific directories of production dependencies), and then the normal include patterns. This small nuance of limiting the node_modules globbing to just production dependencies gives us an impressive speedup.
  3. Apply service + function exclude, then include patterns in order to decide what is included in the package zip file.

This ends up being way faster in most cases, and particularly when you have very large devDependencies. It is worth pointing out the minor implication that:

  • If your include|exclude logic intends to glob in devDependencies, this won't work anymore. But, you're not really planning on deploying non-production dependencies are you? 😉


Other Serverless plugins that set package.artifact

The serverless-jetpack plugin hooks into the Serverless packaging lifecycle by being the last function run in the before:package:createDeploymentArtifacts lifecycle event. This means that if a user configures package.artifact directly in their Serverless configuration or another plugin sets package.artifact before Jetpack runs then Jetpack will skip the unit of packaging (service, function, layer, etc.).

Some notable plugins that do set package.artifact and thus don't need and won't use Jetpack (or vanilla Serverless packaging for that matter):

Minor differences vs. Serverless globbing

Our benchmark correctness tests highlight a number of various files not included by Jetpack that are included by serverless in packaging our benchmark scenarios. Some of these are things like node_modules/.yarn-integrity which Jetpack knowingly ignores because you shouldn't need it. All of the others we've discovered to date are instances in which serverless incorrectly includes devDependencies...


Jetpack supports layer packaging as close to serverless as it can. However, there are a couple of very wonky things with serverless' approach that you probably want to keep in mind:

  • Service level package.include|exclude patterns are applied at the layers.NAME.path level for a given layer. So, e.g., if you have a service-level include pattern of "!*" to remove ROOT/foo.txt, this will apply at a different root path from layers.NAME.path of like ROOT/layers/NAME/foo.txt.
  • As mentioned in our options configuration section above, Jetpack applies the base and roots options to the root project servicePath for dependency searching and not relatively to layer paths.

Be careful with include configurations and node_modules

Let's start with how include|exclude work for both Serverless built-in packaging and Jetpack:

  1. Disk read phase with globby(). Assemble patterns in order from below and then return a list of files matching the total patterns.

    1. Start at ** (everything).
    2. (Jetpack only) Add in service and function-level jetpack.preInclude patterns.
    3. (Jetpack only) Add in dynamic patterns to include production node_modules.
    4. Add in service and function-level package.include patterns.
  2. File filtering phase with nanomatch(). Once we have a list of files read from disk, we apply patterns in order as follows to decide whether to include them (last positive match wins).

    1. (Jetpack only) Add in service and function-level jetpack.preInclude patterns.
    2. (Jetpack only) Add in dynamic patterns to include production node_modules.
    3. Add in service and function-level package.exclude patterns.
    4. (Serverless only) Add in dynamic patterns to exclude development node_modules
    5. Add in service and function-level package.include patterns.

The practical takeaway here is the it is typically faster to prefer include exclusions like !foo/** than to use exclude patterns like foo/** because the former avoids a lot of unneeded disk I/O.

Let's consider a pattern like this:

  - "node_modules/**"
  - # ... a whole bunch of stuff ...

This would likely be just as slow as built-in Serverless packaging because all of node_modules gets read from disk.

Thus, the best practice here when crafting service or function include configurations is: don't include anything extra in node_modules. It's fine to do extra exclusions like:

# Good. Remove dependency provided by lambda from zip
  - "**/node_modules/aws-sdk/**"

# Better! Never even read the files from disk during globbing in the first place!
  - "!**/node_modules/aws-sdk/**"

Packaging files Outside CWD

How files are zipped

A potentially serious situation that comes up with adding files to a Serverless package zip file is if any included files are outside of Serverless' servicePath / current working directory. For example, if you have files like:

- src/foo/bar.js
- ../node_modules/lodash/index.js

Any file below CWD is collapsed into starting at CWD and not outside. So, for the above example, we package / later expand:

- src/foo/bar.js                # The same.
- node_modules/lodash/index.js  # Removed `../`!!!

This most often happens with node_modules in monorepos where node_modules roots are scattered across different directories and nested. In particular, if you are using the custom.jetpack.base option this is likely going to come into play. Fortunately, in most cases, it's not that big of a deal. For example:

- node_modules/chalk/index.js
- ../node_modules/lodash/index.js

will collapse when zipped to:

- node_modules/chalk/index.js
- node_modules/lodash/index.js

... but Node.js resolution rules should resolve and load the collapsed package the same as if it were in the original location.

Zipping problems

The real problems occur if there is a path conflict where files collapse to the same location. For example, if we have:

- node_modules/lodash/index.js
- ../node_modules/lodash/index.js

this will append files with the same path in the zip file:

- node_modules/lodash/index.js
- node_modules/lodash/index.js

that when expanded leave only one file actually on disk!

How to detect zipping problems

The first level is detecting potentially collapsed files that conflict. Jetpack does this automatically with log warnings like:

Serverless: [serverless-jetpack] WARNING: Found 1 collapsed dependencies in .serverless/! Please fix, with hints at:
Serverless: [serverless-jetpack] .serverless/ collapsed dependencies:
- lodash (Packages: 2, Files: 108 unique, 216 total): [node_modules/lodash@4.17.11, ../node_modules/lodash@4.17.15]`

In the above example, 2 different versions of lodash were installed and their files were collapsed into the same path space. A total of 216 files will end up collapsed into 108 when expanded on disk in your cloud function. Yikes!

A good practice if you are using tracing mode is to set: jetpack.collapsed.bail = true so that Jetpack will throw an error and kill the serverless program if any collapsed conflicts are detected.

How to solve zipping problems

So how do we fix the problem?

A first starting point is to generate a full report of the packaging step. Instead of running serverless deploy|package <OPTIONS>, try out serverless jetpack package --report <OPTIONS>. This will produce a report at the end of packaging that gives a full list of files. You can then use the logged message above as a starting point to examine the actual files collapsed in the zip file. Then, spend a little time figuring out the dependencies of how things ended up where.

With a better understanding of what the files are and why we can turn to avoiding collapses. Some options:

  • Don't allow node_modules in intermediate directories: Typically, a monorepo has ROOT/package.json and packages/NAME/package.json or something, which doesn't typically lead to collapsed files. A situation that runs into trouble is something like:


    with serverless being run from backend as CWD then ROOT/node_modules and ROOT/backend/node_modules will present potential collapsing conflicts. So, if possible, just remove the backend/package.json dependencies and stick them all either in the root or further nested into the functions/packages of the monorepo.

  • Mirror exact same dependencies in package.jsons: In our above example, even if lodash isn't declared in either ../package.json or package.json we can manually add it to both at the same pinned version (e.g., "lodash": "4.17.15") to force it to be the same no matter where npm or Yarn place the dependency on disk.

  • Use Yarn Resolutions: If you are using Yarn and resolutions are an option that works for your project, they are a straightforward way to ensure that only one of a dependency exists on disk, solving collapsing problems.

  • Use package.include|exclude: You can manually adjust packaging by excluding files that would be collapsed and then allowing the other ones to come into play. In our example above, a negative package.include for !node_modules/lodash/** would solve our problem in a semver-acceptable way by leaving only root-level lodash.

Tracing mode

ℹ️ Experimental: Although we have a wide array of tests, tracing mode is still considered experimental as we roll out the feature. You should be sure to test all the execution code paths in your deployed serverless functions and verify your bundled package contents before using in production.

Jetpack speeds up the underlying dependencies filtering approach of serverless packaging while providing completely equivalent bundles. However, this approach has some fundamental limitations:

  • Over-inclusive: All production dependencies include many individual files that are not needed at runtime.
  • Speed: For large sets of dependencies, copying lots of files is slow at packaging time.

Thus, we pose the question: What if we packaged only the files we needed at runtime?

Welcome to tracing mode!

Tracing mode is an alternative way to include dependencies in a serverless application. It works by using Acorn to parse out all dependencies in entry point files (require, require.resolve, static import) and then resolves them with resolve according to the Node.js resolution algorithm. This produces a list of the files that will actually be used at runtime and Jetpack includes these instead of traversing production dependencies. The engine for all of this work is a small, dedicated library, trace-deps.

Tracing configuration

The most basic configuration is just to enable custom.jetpack.trace (service-wide) or functions.{FN_NAME}.jetpack.trace (per-function) set to true. By default, tracing mode will trace just the entry point file specified in functions.{FN_NAME}.handler.

  - serverless-jetpack

    trace: true

The trace field can be a Boolean or object containing further configuration information.

Tracing options

The basic trace Boolean field should hopefully work for most cases. Jetpack provides several additional options for more flexibility:

Service-level configurations available via custom.jetpack.trace:

  • trace (Boolean | Object): If trace: true or trace: { /* other options */ } then tracing mode is activated at the service level.

  • trace.ignores (Array<string>): A set of package path prefixes up to a directory level (e.g., react or mod/lib) to skip tracing on. This is particularly useful when you are excluding a package like aws-sdk that is already provided for your lambda.

  • trace.allowMissing (Object.<string, Array<string>>): A way to allow certain packages to have potentially failing dependencies. Specify each object key as either (1) an source file path relative to servicePath / CWD that begins with a ./ or (2) a package name and provide a value as an array of dependencies that might be missing on disk. If the sub-dependency is found, then it is included in the bundle (this part distinguishes this option from ignores). If not, it is skipped without error.

  • trace.include (Array<string>): Additional file path globbing patterns (relative to servicePath) to be included in the package and be further traced for dependencies to include. Applies to functions that are part of a service or function (individually) packaging.

    • Note: These patterns are in addition to the handler inferred file path. If you want to exclude the handler path you could technically do a !file/path.js exclusion, but that would be a strange case in that your handler files would no longer be present.
  • trace.dynamic.bail (Boolean): Terminate serverless program with an error if dynamic import misses are detected. See discussion below regarding handling.

  • trace.dynamic.resolutions (Object.<string, Array<string>>): Handle dynamic import misses by providing a key to match misses on and an array of additional glob patterns to trace and include in the application bundle.

    • Application source files: If a miss is an application source file (e.g., not within node_modules), specify the relative path (from servicePath / CWD) to it like "./src/server/router.js": [/* array of patterns */].

      • Note: To be an application source path, it must be prefixed with a dot (e.g., ./src/server.js, ../lower/src/server.js). Basically, like the Node.js require() rules go for a local path file vs. a package dependency.
    • Dependency packages: If a miss is part of a dependency (e.g., an npm package placed within node_modules), specify the package name first (without including node_modules) and then trailing path to file at issue like "bunyan/lib/bunyan.js": [/* array of patterns */].
    • Ignoring dynamic import misses: If you just want to ignore the missed dynamic imports for a given application source file or package, just specify and empty array [] or falsey value.

    A way to allow certain packages to have potentially failing dependencies. Specify each object key as a package name and value as an array of dependencies that might be missing on disk. If the sub-dependency is found, then it is included in the bundle (this part distinguishes this option from ignores). If not, it is skipped without error.

The following function-level configurations available via functions.{FN_NAME}.jetpack.trace and layers.{LAYER_NAME}.jetpack.trace:

  • trace (Boolean | Object): If trace: true or trace: { /* other options */ } then tracing mode is activated at the function level if the function is being packaged individually.
  • trace.ignores (Array<string>): A set of package path prefixes up to a directory level (e.g., react or mod/lib) to skip tracing if the function is being packaged individually. If there are service-level trace.ignores then the function-level ones will be added to the list.
  • trace.allowMissing (Object.<string, Array<string>>): An object of package path prefixes mapping to lists of packages that are allowed to be missing if the function is being packaged individually. If there is a service-level trace.allowMissing object then the function-level ones will be smart merged into the list.
  • trace.include (Array<string>): Additional file path globbing patterns (relative to servicePath) to be included in the package and be further traced for dependencies to include. Applies to functions that are part of a service or function (individually) packaging. If there are service-level trace.includes then the function-level ones will be added to the list.
  • trace.dynamic.bail (Boolean): Terminate serverless program with an error if dynamic import misses are detected if the function is being packaged individually.
  • trace.dynamic.resolutions (Object.<string, Array<string>>): An object of application source file or package name keys mapping to lists of pattern globs that are traced and included in the application bundle if the function is being packaged individually. If there is a service-level trace.dynamic.resolutions object then the function-level ones will be smart merged into the list.

Let's see the advanced options in action:

  - serverless-jetpack

      - "!**"
        # Unconditionally skip `aws-sdk` and all dependencies
        # (Because it already is installed in target Lambda)
        - "aws-sdk"
        # For just the `ws` package allow certain lazy dependencies to be
        # skipped without error if not found on disk.
          - "bufferutil"
          - "utf-8-validate"
        # Force errors if have unresolved dynamic imports
        bail: true
        # Resolve encountered dynamic import misses, either by tracing
        # additional files, or ignoring after confirmation of safety.
          # **Application Source**
          # Specify keys as relative path to application source files starting
          # with a dot.
            # Manually trace all configuration files for bespoke configuration
            # application code. (Note these are relative to the file key!)
            - "../../config/default.js"
            - "../../config/production.js"

          # Ignore dynamic import misses with empty array.
          "./src/something-else.js": []

          # **Dependencies**
          # Specify keys as `PKG_NAME/path/to/file.js`.
            # - node_modules/bunyan/lib/bunyan.js [79:17]: require('dtrace-provider' + '')
            # - node_modules/bunyan/lib/bunyan.js [100:13]: require('mv' + '')
            # - node_modules/bunyan/lib/bunyan.js [106:27]: require('source-map-support' + '')
            # These are all just try/catch-ed permissive require's meant to be
            # excluded in browser. We manually add them in here.
            - "dtrace-provider"
            - "mv"
            - "source-map-support"

          # Ignore: we aren't using themes.
          # - node_modules/colors/lib/colors.js [127:29]: require(theme)
          "colors/lib/colors.js": []

    - "a/manual/file-i-want.js"

  # Functions in service package.
  # - `jetpack.trace.ignores` does not apply.
  # - `jetpack.trace.include` **will** include and trace additional files.
    handler: app1.handler

    handler: app2.handler
      # - `jetpack.trace.allowMissing` additions are merged into service level
        # Trace and include: `app2.js` + `extra/**.js` patterns
          - "extra/**.js"

  # Individually with no trace configuration will be traced from service-level config
    handler: ind1.handler
      individually: true
      # Normal package include|exclude work the same, but are not traced.
        - "some/stuff/**"
        # When individually, `ignores` from fn are added: `["aws-sdk", "react-ssr-prepass"]`
          - "react-ssr-prepass"
        # When individually, `allowMissing` smart merges like:
        # `{ "ws": ["bufferutil", "utf-8-validate", "another"] }`
            - "another"

  # Individually with explicit `false` will not be traced
    handler: ind1.handler
      individually: true
      trace: false

Tracing caveats

  • Works best for large, unused production dependencies: Tracing mode is best suited for an application wherein many / most of the files specified in package.json:dependencies are not actually used. When there is a large discrepancy between "specific dependencies" and "actually used files" you'll see the biggest speedups. Conversely, when production dependencies are very tight and almost every file is used you won't see a large speedup versus Jetpack's normal dependency mode.

  • Only works with JavaScript handlers + code: Tracing mode only works with functions.{FN_NAME}.handler and trace.include files that are real JavaScript ending in the suffixes of .js or .mjs. If you have TypeScript, JSX, etc., please transpile it first and point your handler at that file. By default tracing mode will search on PATH/TO/HANDLER_FILE.{js,mjs} to then trace, and will throw an error if no matching files are found for a function that has runtime: node* when tracing mode is enabled.

  • Only works with imports/requires: trace-deps only works with a supported set of require, require.resolve and import dependency specifiers. That means if your application code or a dependency does something like: const styles = fs.readFileSync(path.join(__dirname, "styles.css")) then the dependency of node_modules/<pkg>/<path>/styles.css will not be included in your serverless bundle. To remedy this you presently must manually detect and find any such missing files and use a standard service or function level package.include as appropriate to explicitly include the specific files in your bundle.

  • Service/function-level Applications: Tracing mode at the service level and individually configurations work as follows:

    • If service level custom.jetpack.trace is set (true or config object), then the service will be traced. All functions are packaged in tracing mode except for those with both individually enabled (service or function level) and functions.{FN_NAME}.jetpack.trace=false explicitly.
    • If service level custom.jetpack.trace is false or unset, then the service will not be traced. All functions are packaged in normal dependency-filtering mode except for those with both individually enabled (service or function level) and functions.{FN_NAME}.jetpack.trace is set which will be in tracing mode.
  • Replaces Package Introspection: Enabling tracing mode will replace all package.json production dependency inspection and add a blanket exclusion pattern for node_modules meaning things that are traced are the only thing that will be included by your bundle.

  • Works with other include|excludess: The normal package include|excludes work like normal and are a means of bring in other files as appropriate to your application. And for many cases, you will want to include other files via the normal serverless configurations, just without tracing and manually specified.

  • Layers are not traced: Because Layers don't have a distinct entry point, they will not be traced. Instead Jetpack does normal pattern-based production dependency inference.

  • Static analysis by default: Out of the box, tracing will only detect files included via require("A_STRING"), require.resolve("A_STRING"), import "A_STRING", and import NAME from "A_STRING". It will not work with dynamic import()s or requires that dynamically inject a variable etc. like require(myVariable).

    • Note: Jetpack will log warnings for files found that have dynamic imports that tracing missed. See WARNING log output for the list of files and read our section below on handling dynamic imports.

Handling dynamic import misses

Dynamic imports that use variables or runtime execution like require(A_VARIABLE) or import(`template_${VARIABLE}`) cannot be used by Jetpack to infer what the underlying dependency files are for inclusion in the bundle. That means some level of developer work to handle.


The first step is to be aware and watch for dynamic import misses. Conveniently, Jetpack logs warnings like the following:

Serverless: [serverless-jetpack] WARNING: Found 6 dependency packages with tracing misses in .serverless/! Please see logs and read:
Serverless: [serverless-jetpack] .serverless/ dependency package tracing misses: [* ... */,"colors","bunyan",/* ... */]

and produces combined --report output like:

### Tracing Dynamic Misses (`6` packages): Dependencies

- ../node_modules/aws-xray-sdk-core/node_modules/colors/lib/colors.js [127:29]: require(theme)
- ../node_modules/bunyan/lib/bunyan.js [79:17]: require('dtrace-provider' + '')
- ../node_modules/bunyan/lib/bunyan.js [100:13]: require('mv' + '')
- ../node_modules/bunyan/lib/bunyan.js [106:27]: require('source-map-support' + '')

which gives you the line + column number of the dynamic dependency in a given source file and snippet of the code in question.

In addition to just logging this information, you can ensure you have no unaccounted for dynamic import misses by setting jetpack.trace.dynamic.bail = true in your applicable service or function-level configuration.


With the --report output in hand, the recommended course is to identify what the impact is of these missed dynamic imports. For example, in node_modules/bunyan/lib/bunyan.js the interesting require('mv' + '') import is within a permissive try/catch block to allow conditional import of the library if found (and prevent browserify from bundling the library). For our Serverless application we could choose to ignore these dynamic imports or manually add in the imported libraries.

For other dependencies, there may well be "hidden" dependencies that you will need to add to your Serverless bundle for runtime correctness. Things like node-config which dynamically imports various configuration files from environment variable information, etc.


Once we have logging information and the --report output, we can start remedying dynamic import misses via the Jetpack feature jetpack.trace.dynamic.resolutions. Resolutions are keys to files with dynamic import misses that allow a developer to specify what imports should be included manually or to simply ignore the dynamic import misses.

Keys: Resolutions take a key value to match each file with missing dynamic imports. There are two types of keys that are used:

  • Application Source File: Something that is within your application and not node_modules. Specify these files with a dot prefix as appropriate relative to the Serverless service path (usually CWD) like ./src/server.js or ../outside/file.js.
  • Package Dependencies: A file from a dependency within node_modules. Specify these files without a dot and just PKG_NAME/path/to/file.js or @SCOPE/PKG_NAME/path/to/file.js.

Values: Values are an array of extra imports to add in from each file as if they were declared in that very file with require("EXTRA_IMPORT") or import "EXTRA_IMPORT". This means the values should either be relative paths within that package (./lib/auth/noop.js) or other package dependencies (lodash or lodash/map.js). * Note: We choose to support "additional imports" and not just file additions like package.include or jetpack.trace.include. The reason is that for package dependency import misses, the packages can be flattened to unpredictable locations in the node_modules trees and doubly so in monorepos. An import will always be resolved to the correct location, and that's why we choose it. At the same time, tools like package.include or jetpack.trace.includeare still available to use!

Some examples:

bunyan: The popular logger library has some optional dependencies that are not meant only for Node.js. To prevent browser bundling tools from including, they use a curious require strategy of require('PKG_NAME' + '') to defeat parsing. For Jetpack, this means we get dynamic misses reports of:

- node_modules/bunyan/lib/bunyan.js [79:17]: require('dtrace-provider' + '')
- node_modules/bunyan/lib/bunyan.js [100:13]: require('mv' + '')
- node_modules/bunyan/lib/bunyan.js [106:27]: require('source-map-support' + '')

Using resolutions we can remedy these by simple adding imports for all three libraries like:

            - "dtrace-provider"
            - "mv"
            - "source-map-support"

express: The popular server framework dynamically imports engines which produces a dynamic misses report of:

- node_modules/express/lib/view.js [81:13]: require(mod)

In a common case, this is a non-issue if you aren't using engines, so we can simply "ignore" the import miss by setting an empty array resolutions value:

          "express/lib/view.js": []

Once we have analyzed all of our misses and added resolutions to either ignore the miss or add other imports, we can then set trace.dynamic.bail = true to make sure that if future dependency upgrades adds new, unhandled dynamic misses we will get a failed build notification so we know that we're always deploying known, good code.

Tracing results

The following is a table of generated packages using vanilla Serverless vs Jetpack with tracing (using yarn benchmark:sizes).

The relevant portions of our measurement chart.

  • Scenario: Same benchmark scenarios
  • Type: jetpack is this plugin in trace mode and baseline is Serverless built-in packaging.
  • Zips: The number of zip files generated per scenario (e.g., service bundle + individually packaged function bundles).
  • Files: The aggregated number of individual files in all zip files for a given scenario. This shows how Jetpack in tracing mode results in many less files.
  • Size: The aggregated total byte size of all zip files for a given scenario. This shows how Jetpack in tracing mode results in smaller bundle packages.
  • vs Base: Percentage difference of the aggregated zip bundle byte sizes for a given scenario of Jetpack vs. Serverless built-in packaging.


Scenario Type Zips Files Size vs Base
simple jetpack 1 200 529417 -42.78 %
simple baseline 1 433 925260
complex jetpack 2 1588 3835544 -18.20 %
complex baseline 2 2120 4688648

Command Line Interface

Jetpack also provides some CLI options.

serverless jetpack package

Package a function like serverless package does, just with better options.

$ serverless jetpack package -h
Plugin: Jetpack
jetpack package ............... Packages a Serverless service or function
    --function / -f .................... Function name. Packages a single function (see 'deploy function')
    --report / -r ...................... Generate full bundle report

So, to package all service / functions like serverless package does, use:

$ serverless jetpack package # OR
$ serverless package

... as this is basically the same built-in or custom.

The neat addition that Jetpack provides is:

$ serverless jetpack package -f|--function {NAME}

which allows you to package just one named function exactly the same as serverless deploy -f {NAME} does. (Curiously serverless deploy implements the -f {NAME} option but serverless package does not.)


The following is a simple, "on my machine" benchmark generated with yarn benchmark. It should not be taken to imply any real world timings, but more to express relative differences in speed using the serverless-jetpack versus the built-in baseline Serverless framework packaging logic.

As a quick guide to the results table:

  • Scenario: Contrived scenarios for the purpose of generating results. E.g.,

    • simple: Very small production and development dependencies.
    • complex: Many different serverless configurations all in one.
  • Pkg: Project installed via yarn or npm? This really only matters in that npm and yarn may flatten dependencies differently, so we want to make sure Jetpack is correct in both cases.
  • Type: jetpack is this plugin and baseline is Serverless built-in packaging.
  • Mode: For jetpack benchmarks, either:

    • deps: Dependency filtering with equivalent output to serverless (just faster).
    • trace: Tracing dependencies from specified source files. Not equivalent to serverless packaging, but functionally correct, way faster, and with smaller packages.
  • Time: Elapsed build time in milliseconds.
  • vs Base: Percentage difference of serverless-jetpack vs. Serverless built-in. Negative values are faster, positive values are slower.

Machine information:

  • os: darwin 18.7.0 x64
  • node: v12.14.1


Scenario Pkg Type Mode Time vs Base
simple yarn jetpack trace 4878 -74.25 %
simple yarn jetpack deps 3861 -79.62 %
simple yarn baseline 18941
simple npm jetpack trace 7290 -68.34 %
simple npm jetpack deps 4017 -82.55 %
simple npm baseline 23023
complex yarn jetpack trace 10475 -70.93 %
complex yarn jetpack deps 8821 -75.52 %
complex yarn baseline 36032
complex npm jetpack trace 15644 -59.13 %
complex npm jetpack deps 9896 -74.15 %
complex npm baseline 38282

Maintenance status

Active: Formidable is actively working on this project, and we expect to continue for work for the foreseeable future. Bug reports, feature requests and pull requests are welcome.

view on Github

Latest commit b2f54ec on Sep 24, 2017

New to serverless?

To get started, pop open your terminal & run:

npm install serverless -g