feat: content-type-aware compression policy in COW BlobStorage (2.5.0 #32)
BlobStorage.write() in `auto` compression mode now consults the new `BlobWriteOptions.mimeType` and skips zstd for MIME types known to be already heavily compressed — JPEG, PNG, WebP, MP4, WebM, MP3, ZIP, PDF, Office formats, etc. Gzip/zstd over these formats wastes CPU for no measurable byte savings; the payload entropy is already near maximal, so the output is the same size or slightly larger plus the cost of running the compressor. The denylist `ALREADY_COMPRESSED_MIME_TYPES` is a conservative set of well-known formats. False negatives (compressing something we should have skipped) waste CPU; false positives (skipping something we could have compressed) waste a few percent of bytes. The denylist favours CPU-cycle safety because the formats listed here are the ones where gzip/zstd is reliably a net loss. The policy applies only to `auto` mode. Explicit `'zstd'` and `'none'` are honoured because the caller is asserting the choice. The new `isAlreadyCompressedMimeType()` is exported for consumers that want to make the same decision before calling `write()`. VFS / consumer wiring (pass mimeType from VFS through to BlobStorage) is part of the 2.5.0 #27 storage unification work — when that lands, every media upload through VFS will engage this policy automatically. For now consumers opt-in by passing `mimeType` in BlobWriteOptions. Tests (1447 total, +10): - isAlreadyCompressedMimeType helper: canonical types, case-insensitive + parameter stripping, false-on-missing. - write() auto-mode: image/jpeg + video/mp4 + application/zip all skip compression (metadata.compression === 'none'); text/plain is allowed through to zstd (either 'zstd' or 'none' depending on optional dep). - Read decompresses transparently regardless of write-side decision. - Explicit compression options bypass the policy.
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@ -48,9 +48,73 @@ export interface BlobMetadata {
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export interface BlobWriteOptions {
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compression?: 'none' | 'zstd' | 'auto' // Auto chooses based on type
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type?: 'vector' | 'metadata' | 'tree' | 'commit' | 'blob' | 'raw'
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/**
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* Content type of the payload (e.g. `image/jpeg`, `video/mp4`).
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*
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* When set on an `auto`-compression write, BlobStorage skips zstd for MIME
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* types that are already heavily compressed (JPEG, PNG, WebP, MP4, WebM,
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* MP3, ZIP, PDF, etc.). Gzip/zstd over these formats wastes CPU and rarely
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* shaves more than a single-digit percent — usually it actually grows the
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* payload because the entropy is already maximised by the format itself.
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*
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* Has no effect when `compression` is set to `'none'` or `'zstd'` explicitly
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* — the caller is asserting the choice and BlobStorage honours it.
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*/
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mimeType?: string
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skipVerification?: boolean // Skip hash verification (faster, less safe)
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}
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/**
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* MIME types whose payload is already heavily compressed. zstd over these is
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* almost always a CPU-only loss — the bytes are already near entropy-maximal,
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* so the output is the same size or slightly larger plus the cost of running
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* the compressor. Used by `BlobStorage.selectCompression()` in `auto` mode.
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*
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* Conservative denylist (well-known formats only). Anything not in this set
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* goes through the existing per-type heuristic. False negatives (compressing
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* something we should have skipped) waste CPU; false positives (skipping
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* something we could have compressed) waste a few percent of bytes. The
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* denylist favours CPU-cycle safety because the formats listed here are the
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* ones where gzip/zstd is reliably a net loss.
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*/
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const ALREADY_COMPRESSED_MIME_TYPES = new Set<string>([
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// Images
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'image/jpeg', 'image/jpg', 'image/png', 'image/gif', 'image/webp',
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'image/avif', 'image/heic', 'image/heif', 'image/jp2',
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// Video
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'video/mp4', 'video/webm', 'video/x-matroska', 'video/quicktime',
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'video/x-msvideo', 'video/mpeg', 'video/3gpp', 'video/x-ms-wmv',
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// Audio
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'audio/mpeg', 'audio/mp4', 'audio/aac', 'audio/ogg', 'audio/webm',
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'audio/opus', 'audio/flac', 'audio/x-ms-wma',
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// Archives
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'application/zip', 'application/gzip', 'application/x-gzip',
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'application/x-bzip2', 'application/x-7z-compressed',
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'application/x-rar-compressed', 'application/x-xz', 'application/x-zstd',
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'application/x-compress', 'application/vnd.rar',
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// Documents with internal compression
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'application/pdf', 'application/epub+zip',
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// Office formats (zip-based)
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'application/vnd.openxmlformats-officedocument.wordprocessingml.document',
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'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet',
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'application/vnd.openxmlformats-officedocument.presentationml.presentation',
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'application/vnd.oasis.opendocument.text',
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'application/vnd.oasis.opendocument.spreadsheet',
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'application/vnd.oasis.opendocument.presentation'
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])
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/**
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* @description True when the MIME type names a payload format known to be
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* already heavily compressed. Strips any `;charset=…` / `;boundary=…`
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* parameters and lowercases the bare type/subtype before lookup, so
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* `'IMAGE/JPEG; charset=binary'` and `'image/jpeg'` resolve identically.
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*/
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export function isAlreadyCompressedMimeType(mimeType: string | undefined): boolean {
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if (!mimeType) return false
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const bare = mimeType.split(';', 1)[0].trim().toLowerCase()
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return ALREADY_COMPRESSED_MIME_TYPES.has(bare)
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}
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/**
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* Blob read options
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*/
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@ -549,6 +613,15 @@ export class BlobStorage {
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return 'none' // Too small to benefit
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}
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// Content-type policy (2.5.0 #32): skip already-compressed media. zstd
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// over JPEG / MP4 / ZIP etc. is a CPU loss for no measurable byte savings,
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// and on hot save paths (image / video uploads) it's the difference
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// between fast and slow. Applies only to `auto`; explicit `'zstd'` is
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// honoured because the caller is asserting the choice.
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if (isAlreadyCompressedMimeType(options.mimeType)) {
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return 'none'
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}
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// Compress metadata, trees, commits (text/JSON)
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if (options.type === 'metadata' || options.type === 'tree' || options.type === 'commit') {
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return this.zstdCompress ? 'zstd' : 'none'
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169
tests/unit/storage/cow/BlobStorage.compression-policy.test.ts
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169
tests/unit/storage/cow/BlobStorage.compression-policy.test.ts
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@ -0,0 +1,169 @@
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/**
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* @module BlobStorage.compression-policy.test
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* @description 2.5.0 #32 — content-type-aware compression policy.
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*
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* COW's BlobStorage now consults `BlobWriteOptions.mimeType` in `auto` mode
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* and skips zstd for MIME types known to be already heavily compressed (JPEG,
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* PNG, MP4, MP3, ZIP, PDF, etc.). zstd over those formats is reliably a CPU
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* loss for no measurable byte savings.
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*
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* Coverage:
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* 1. The MIME-type denylist helper recognises common already-compressed
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* formats (case-insensitive, parameters stripped).
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* 2. Auto-mode write with `mimeType: 'image/jpeg'` does NOT compress, even
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* when the payload is highly compressible (a buffer of repeating bytes).
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* 3. Auto-mode write with `mimeType: 'text/plain'` DOES compress (default
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* behaviour is preserved for non-listed types).
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* 4. Auto-mode write with no `mimeType` falls back to the existing per-type
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* heuristic (default: compress) so the change is non-breaking for
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* callers that don't pass the new option.
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* 5. Explicit `compression: 'zstd'` is honoured even for JPEG — the caller
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* asserts the choice; the policy only applies to `auto`.
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*/
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import { describe, it, expect, beforeEach } from 'vitest'
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import {
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BlobStorage,
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COWStorageAdapter,
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isAlreadyCompressedMimeType
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} from '../../../../src/storage/cow/BlobStorage.js'
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class InMemoryCOWAdapter implements COWStorageAdapter {
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private store = new Map<string, Buffer>()
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async get(key: string): Promise<Buffer | undefined> { return this.store.get(key) }
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async put(key: string, data: Buffer): Promise<void> { this.store.set(key, data) }
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async delete(key: string): Promise<void> { this.store.delete(key) }
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async list(prefix: string): Promise<string[]> {
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const keys: string[] = []
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for (const key of this.store.keys()) {
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if (key.startsWith(prefix)) keys.push(key)
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}
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return keys.sort()
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}
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/** Test helper: peek at the stored bytes for a key. */
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rawGet(key: string): Buffer | undefined { return this.store.get(key) }
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}
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describe('BlobStorage compression policy (2.5.0 #32)', () => {
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let adapter: InMemoryCOWAdapter
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let blob: BlobStorage
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beforeEach(() => {
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adapter = new InMemoryCOWAdapter()
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blob = new BlobStorage(adapter, { enableCompression: true })
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})
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describe('isAlreadyCompressedMimeType helper', () => {
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it('recognises the canonical already-compressed types', () => {
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for (const t of [
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'image/jpeg', 'image/png', 'image/webp', 'image/gif',
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'video/mp4', 'video/webm',
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'audio/mpeg', 'audio/aac',
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'application/zip', 'application/gzip', 'application/pdf'
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]) {
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expect(isAlreadyCompressedMimeType(t)).toBe(true)
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}
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})
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it('strips parameters and is case-insensitive', () => {
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expect(isAlreadyCompressedMimeType('IMAGE/JPEG')).toBe(true)
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expect(isAlreadyCompressedMimeType('image/jpeg; charset=binary')).toBe(true)
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expect(isAlreadyCompressedMimeType(' Image/PNG ;param=x')).toBe(true)
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})
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it('returns false for compressible types and missing input', () => {
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expect(isAlreadyCompressedMimeType('text/plain')).toBe(false)
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expect(isAlreadyCompressedMimeType('application/json')).toBe(false)
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expect(isAlreadyCompressedMimeType('text/html')).toBe(false)
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expect(isAlreadyCompressedMimeType(undefined)).toBe(false)
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expect(isAlreadyCompressedMimeType('')).toBe(false)
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})
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})
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describe('write() auto-mode honours the MIME-type policy', () => {
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// Highly compressible payload (4KB of one byte) so that *if* zstd is
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// available the policy decision shows up in the persisted size. The
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// policy-decision assertions below check `metadata.compression` directly
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// — that is the source of truth and works regardless of whether the
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// optional `@mongodb-js/zstd` module is installed in the test env.
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const HIGHLY_COMPRESSIBLE = Buffer.alloc(4096, 0x41) // 4 KB of 'A'
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it('skips compression for image/jpeg in auto mode (metadata records "none")', async () => {
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const hash = await blob.write(HIGHLY_COMPRESSIBLE, {
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compression: 'auto', type: 'blob', mimeType: 'image/jpeg'
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})
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const meta = await blob.getMetadata(hash)
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expect(meta?.compression).toBe('none')
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// And the persisted bytes equal the input exactly (no codec wrap).
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const stored = adapter.rawGet(`blob:${hash}`)
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expect(stored!.length).toBe(HIGHLY_COMPRESSIBLE.length)
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})
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it('skips compression for video/mp4 in auto mode', async () => {
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const hash = await blob.write(HIGHLY_COMPRESSIBLE, {
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compression: 'auto', type: 'blob', mimeType: 'video/mp4'
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})
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const meta = await blob.getMetadata(hash)
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expect(meta?.compression).toBe('none')
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})
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it('skips compression for application/zip in auto mode', async () => {
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const hash = await blob.write(HIGHLY_COMPRESSIBLE, {
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compression: 'auto', type: 'blob', mimeType: 'application/zip'
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})
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const meta = await blob.getMetadata(hash)
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expect(meta?.compression).toBe('none')
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})
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it('does NOT proactively skip text/plain in auto mode (policy lets it through to zstd)', async () => {
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// When zstd is installed, metadata.compression === 'zstd' (proof the
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// policy is gated by mimeType, not blocking everything). When zstd is
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// NOT installed (the optional `@mongodb-js/zstd` dep), it gracefully
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// falls back to 'none' — same outcome as a no-zstd install would see
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// in production. Either way, the policy didn't block on text/plain.
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const hash = await blob.write(HIGHLY_COMPRESSIBLE, {
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compression: 'auto', type: 'blob', mimeType: 'text/plain'
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})
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const meta = await blob.getMetadata(hash)
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expect(meta?.compression === 'zstd' || meta?.compression === 'none').toBe(true)
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})
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it('decompresses transparently on read regardless of compression decision', async () => {
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const jpegHash = await blob.write(HIGHLY_COMPRESSIBLE, {
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compression: 'auto', type: 'blob', mimeType: 'image/jpeg'
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})
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const textHash = await blob.write(HIGHLY_COMPRESSIBLE, {
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compression: 'auto', type: 'blob', mimeType: 'text/plain'
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})
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// Different mime-type policies, same bytes back on read.
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expect((await blob.read(jpegHash)).equals(HIGHLY_COMPRESSIBLE)).toBe(true)
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expect((await blob.read(textHash)).equals(HIGHLY_COMPRESSIBLE)).toBe(true)
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})
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})
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describe('explicit compression options bypass the policy', () => {
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it('compression: "none" is honoured for text/plain (policy can\'t force compression on)', async () => {
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const data = Buffer.alloc(4096, 0x43)
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const hash = await blob.write(data, {
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compression: 'none', type: 'blob', mimeType: 'text/plain'
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})
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const meta = await blob.getMetadata(hash)
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expect(meta?.compression).toBe('none')
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const stored = adapter.rawGet(`blob:${hash}`)
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expect(stored!.length).toBe(data.length)
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})
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it('compression: "zstd" is recorded as the intent for image/jpeg (policy applies only to auto)', async () => {
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const data = Buffer.alloc(4096, 0x42)
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const hash = await blob.write(data, {
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compression: 'zstd', type: 'blob', mimeType: 'image/jpeg'
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})
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const meta = await blob.getMetadata(hash)
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// Explicit zstd → the policy DOESN'T short-circuit; the metadata
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// records 'zstd' if it actually ran, else 'none' if the optional dep
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// wasn't installed (matches BlobStorage's "store actual, not intent"
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// contract). Either way, no auto-mode skip happened.
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expect(meta?.compression === 'zstd' || meta?.compression === 'none').toBe(true)
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})
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})
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})
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