backfill/README.md

# backfill

A JavaScript caching library for reducing build time.

**🔌 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

_backfill is under active development and should probably not be used in
production, yet. We will initially focus on stability improvements. We will look
into various optimization strategies, adding more customization, and introducing
an API for only running scripts in packages that have changed and skipping
others altogether._

_Current prerequisites:_

- git (for running `--audit`)
- yarn.lock and yarn workspaces (for optimized hashing)

These prerequisites can easily be loosened to make backfill work with npm, Rush,
and Lerna.

## 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.

## Install

Install backfill using yarn:

```
$ yarn add --dev backfill
```

## Usage - CLI

```
backfill -- [command]
```

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

```json
{
  "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` in the package it was called from
and among parent folders recursively and then combine those configs together.

To configure backfill, simply export a config object with the properties you
wish to override:

```js
module.exports = {
  cacheStorageConfig: {
    provider: "azure-blob",
    options: { ... }
  }
};
```

The default configuration object is:

```js
{
  cacheStorageConfig: { provider: "local" },
  clearOutputFolder: false,
  internalCacheFolder: "node_modules/.cache/backfill",
  logFolder: "node_modules/.cache/backfill",
  logLevel: "info",
  mode: "READ_WRITE",
  name: "[name-of-package]",
  outputGlob: ["lib/**"],
  packageRoot: "path/to/package",
  producePerformanceLogs: false,
  validateOutput: false
}
```

The `outputGlob` is a list of globs describing the files you want to cache.
`outputGlob` should be expressed as a relative path from the root of each
package. If you want to cache `package-a/lib`, for instance, you'd write
`outputGlob: ["lib/**"]`. If you also want to cache the `pacakge-a/dist/bundles`
folder, you'd write `outputGlob: ["lib/**", "dist/bundles/**"]`.

The configuration type is:

```ts
export type Config = {
  cacheStorageConfig: CacheStorageConfig;
  clearOutputFolder: boolean;
  internalCacheFolder: string;
  logFolder: string;
  logLevel: LogLevels;
  mode: "READ_ONLY" | "WRITE_ONLY" | "READ_WRITE" | "PASS";
  name: string;
  outputGlob: string[];
  packageRoot: string;
  performanceReportName?: string;
  producePerformanceLogs: boolean;
  validateOutput: boolean;
};
```

#### Environment variable

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.

See `getEnvConfig()` in
[`./packages/config/src/envConfig.ts`](https://github.com/microsoft/backfill/blob/master/packages/config/src/envConfig.ts#L15).

## Set up remote cache

### Microsoft Azure Blog Storage

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

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

#### Options

<dl>
  <dt>connectionString</dt>
  <dd>retrieve this from the Azure Portal interface</dd>

  <dt>container</dt>
  <dd>the name of the blob storage container</dd>
  
  <dt>maxSize (<em>optional</em>)</dt>
  <dd>
    max size of a single package cache, in the number of bytes
  </dd>
</dl>

You can also configure Microsoft Azure Blog Storage using environment variables.

```
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:

```js
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:

```js
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"
```

## API

Backfill provides an API, this allows for 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

You can optionally output performance logs to disk. If turned on, backfill will
output a log file after each run with performance metrics. Each log file is
formatted as a JSON file. You can turn performance logging by setting
`producePerformanceLogs: true` in `backfill.config.js`.

## Contributing

### Ways to contribute

This project welcomes contributions and suggestions.

- [Submit bugs](https://github.com/microsoft/backfill/issues) and help us verify
  fixes as they are checked in.
- Review the [source code changes](https://github.com/microsoft/backfill/pulls).

### Describing your changes

When submitting source code changes, be sure to accompany the changes with a
change file. Change files can be generated with the `yarn change` command.

### Contributor License Agreement (CLA)

Most contributions require you to agree to a Contributor License Agreement (CLA)
declaring that you have the right to, and actually do, grant us the rights to
use your contribution. For details, visit https://cla.opensource.microsoft.com.

When you submit a pull request, a CLA bot will automatically determine whether
you need to provide a CLA and decorate the PR appropriately (e.g., status check,
comment). Simply follow the instructions provided by the bot. You will only need
to do this once across all repos using our CLA.

This project has adopted the
[Microsoft Open Source Code of Conduct](https://opensource.microsoft.com/codeofconduct/).
For more information see the
[Code of Conduct FAQ](https://opensource.microsoft.com/codeofconduct/faq/) or
contact [opencode@microsoft.com](mailto:opencode@microsoft.com) with any
additional questions or comments.