backfill

A JavaScript caching library for reducing build time.

⚠️ As of 2025, backfill is primarily used as the caching layer for lage. Most information about options and remote cache setup also applies to lage's cacheOptions configuration.

• 🔌 Easy to install: Simply wrap your build commands inside backfill -- [command]
• ☁️ Remote cache: Store your cache on Azure Blob or as an npm package
• ⚙️ Fully configurable: Smart defaults with cross-package and per-package configuration and environment variable overrides

Prerequisites:

• git
• package manager with lock file (for optimized hashing)

Why

When you're working in a multi-package repo you don't want to re-build packages that haven't changed. By wrapping your build scripts inside backfill you enable storing and fetching of build output to and from a local or remote cache.

Backfill is based on two concepts:

1. Hashing: It will hash the files of a package, its dependencies and the build command
2. Caching: Using the hash key, it will look for build output from a local or remote cache. If there's a match, it will backfill the package using the cache. Otherwise, it will run the build command and persist the output to the cache.

Usage - CLI

backfill -- [command]

Typically you would wrap your npm scripts inside backfill, like this:

{
  "name": "package",
  "scripts": {
    "build": "backfill -- tsc -b"
  }
}

--audit

Backfill can only bring back build output from the folders it was asked to cache. A package that modifies or adds files outside of the cached folder will not be brought back to the same state as when it was initially built. To help you debug this you can add --audit to your backfill command. It will listen to all file changes in your repo (it assumes you're in a git repo) while running the build command and then report on any files that got changed outside of the cache folder.

Configuration

Backfill will look for backfill.config.js (CJS only) in the package it was called from and parent folders, then combine those configs together.

To configure backfill, simply export a config object with the properties you wish to override. All properties in are optional in the config file.

/** @type {Partial<import("backfill").Config>} */
const config = {
  cacheStorageConfig: {
    provider: "azure-blob",
    options: { ... }
  },
  outputGlob: ["lib/**/*", "dist/bundles/**/*"]
};
module.exports = config;

Options

Notable options:

• outputGlob (string[]): A list of glob patterns for the built/generated files that should be hashed and cached, relative to the root of each package. For example, if you want to cache package-a/lib and package-a/dist/bundles, you'd write "outputGlob: ["lib/**/*", "dist/bundles/**/*"].
• cacheStorageConfig: See set up remote cache below.

All options:

export type Config = {
  /**
   * Glob patterns for the built/generated files that should be hashed and
   * cached, relative to the root of each package. (see example above)
   *
   * Defaults to `["lib/**"]`.
   */
  outputGlob: string[];

  /**
   * Cache storage provider name and potentially configuration.
   * See below for details.
   * @default { provider: "local" }
   */
  cacheStorageConfig: CacheStorageConfig;

  /**
   * Whether to delete the `outputGlob` files on completion.
   * @default false
   */
  clearOutput: boolean;

  /**
   * Absolute path to local cache folder.
   * @default "[packageRoot]/node_modules/.cache/backfill"
   */
  internalCacheFolder: string;

  /**
   * Absolute path to local log folder.
   * @default "[packageRoot]/node_modules/.cache/backfill"
   */
  logFolder: string;

  /**
   * Log level.
   * @default "info"
   */
  logLevel: "silly" | "verbose" | "info" | "warn" | "error" | "mute";

  /**
   * Name of the package, used for logging and performance reports.
   * Defaults to name from `package.json`.
   */
  name: string;

  /**
   * Cache operation mode.
   * @default "READ_WRITE"
   */
  mode: "READ_ONLY" | "WRITE_ONLY" | "READ_WRITE" | "PASS";

  /**
   * Package root path.
   * Defaults to searching for `package.json` in the current working directory.
   */
  packageRoot: string;

  /**
   * If true, write performance logs to `logFolder`.
   * @default false
   */
  producePerformanceLogs: boolean;

  /**
   * If true, write the hash of the output files to the performance report.
   * @default false
   */
  validateOutput: boolean;

  /**
   * Compute hashes to only cache changed files.
   * @default false
   */
  incrementalCaching: boolean;
};

Environment variables

You can override configuration with environment variables. Backfill will also look for a .env-file in the root of your repository, and load those into the environment. This can be useful when you don't want to commit keys and secrets to your remote cache, or if you want to commit a read-only cache access key in the repo and override with a write and read access key in the PR build, for instance.

• BACKFILL_CACHE_PROVIDER: Cache provider name (Config.cacheStorageConfig.provider)
• BACKFILL_CACHE_PROVIDER_OPTIONS: Cache provider options (the rest of Config.cacheStorageConfig)
• For other Config properties, BACKFILL_* snake case version of option, e.g. BACKFILL_LOG_LEVEL for Config.logLevel

Set up remote cache

Backfill supports multiple cache storage providers:

• Local folder (local), the default option
• Azure blob storage (azure-blob)
• NPM package (npm)
• Skip cache locally (local-skip)
• Custom (custom)

Azure Blob Storage

To cache to Microsoft Azure Blob Storage, you need to provide a connection string and the container name. If you are configuring via backfill.config.js, use the following syntax:

module.exports = {
  cacheStorageConfig: {
    provider: "azure-blob",
    options: {
      connectionString: "...",
      container: "..."
      maxSize: 12345
    }
  }
};

Optionally, you can pass a credential along with a connectionString which does not have a SAS token. This is useful if you want to use a managed identity or interactive browser login. For example:

import { InteractiveBrowserCredential } from '@azure/identity'

module.exports = {
  cacheStorageConfig: {
    provider: "azure-blob",
    options: {
      connectionString: "https://<your-storage-account-name>.blob.core.windows.net",
      credential: new InteractiveBrowserCredential()
      container: "..."
      maxSize: 12345
    }
  }
};

azure-blob options

• connectionString: retrieve this from the Azure Portal interface
• container: the name of the blob storage container
• maxSize (optional): max size of a single package cache, in the number of bytes
• credential (optional): one of the credential types from @azure/identity.

You can also use environment variables for configuration.

BACKFILL_CACHE_PROVIDER="azure-blob"
BACKFILL_CACHE_PROVIDER_OPTIONS='{"connectionString":"...","container":"..."}'

NPM package

To cache to an NPM package you need to provide a package name and the registry URL of your package feed. This feed should probably be private. If you are configuring via backfill.config.js, you can use the following syntax:

module.exports = {
  cacheStorageConfig: {
    provider: "npm",
    options: {
      npmPackageName: "...",
      registryUrl: "...",
    },
  },
};

You can also provide a path to the .npmrc user config file, to provide auth details related to your package feed using the npmrcUserconfig field in options.

You can also configure NPM package cache using environment variables.

BACKFILL_CACHE_PROVIDER="npm"
BACKFILL_CACHE_PROVIDER_OPTIONS='{"npmPackageName":"...","registryUrl":"..."}'

Skipping cache locally

Sometimes in a local build environment, it is useful to compare hashes to determine whether to execute the task without having to explicitly use a separate directory for the cache.

One caveat, this is using output that the task produced and one could possibly modify the output on a local development environment. For this reason, this is an opt-in behavior rather than the default.

The main benefit of using this strategy is a significant speed boost. Backfill can skip file copying of the cached outputs if it can rely on the built artifacts. Hashing is CPU-bound while caching is I/O-bound. Using this strategy results in speed gains but at the cost of needing to trust the outputs have not be altered by the user. While this usually is true, it is prudent to also provide a command in your repository to clean the output along with the saved hashes.

You can configure this from the backfill.config.js file this way:

module.exports = {
  cacheStorageConfig: {
    provider: "local-skip",
  },
};

Like other cases, you can also use the environment variable to choose this storage strategy:

BACKFILL_CACHE_PROVIDER="local-skip"

Custom storage providers

It is also possible to use a custom storage provider. This allows ultimate flexibility in how to handle cache fetching and putting.

Configure the custom cache provider this way:

// @ts-check
// See the types for details of the signatures
/** @implements {import("backfill").ICacheStorage} */
class CustomStorageProvider {
  constructor(providerOptions, logger, cwd) {
    // do what is needed in regards to the options
  }

  async fetch(hash) {
    // some fetch logic
  }

  async put(hash, filesToCache) {
    // some putting logic
  }
}

// backfill.config.js
/** @type {Partial<import("backfill").Config>} */
const config = {
  cacheStorageConfig: {
    provider: (logger, cwd) =>
      new CustomStorageProvider(
        {
          key1: "value1",
          key2: "value2",
        },
        logger,
        cwd
      ),
  },
};
module.exports = config;

API

Backfill provides an API to support more complex scenarios and performance optimizations.

const backfill = require("backfill/lib/api");

const packagePath = getPath(packageName);

const logger = backfill.makeLogger("verbose", process.stdout, process.stderr);
const packagehash = await backfill.computeHash(packagePath, logger);

const fetchSuccess = await backfill.fetch(packagePath, packageHash, logger);

if (!fetchSuccess) {
  await runBuildCommand();
  await backfill.put(packagePath, packageHash, logger);
}

Performance logs

Backfill provides the option to output a JSON log file after each run with performance metrics. Enable this by setting producePerformanceLogs: true in backfill.config.js.