perf: draw the grids from thumbnails made at ingest - #377
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Measured on production: opening "Approve variations" spends ~4.5s in matchingSiblings before a single thumbnail is requested. For a screen with 34 locales that call fetches 70 full-size screenshots back out of S3 and decodes them, eight at a time. The concurrency bound added with the worker pool was chosen to protect memory and CPU; on S3 the work is network-bound, so eight is simply the number of round trips it waits on. None of that has to happen at review time. The diff already decodes both screenshots at ingest, so the signature is now produced by that same pass and stored on the run. Grouping a screen becomes a column read. - computePixelmatchDiff optionally returns the signature, computed from the images it has already decoded and ignore-masked. Both paths call one signatureOfDecoded, and a test pins the fused answer to be identical to the standalone job's — if they drifted, stored signatures would silently stop matching computed ones and variations would stop grouping. - It is asked for on every comparison rather than only when the project has bulk approve switched on, so turning the flag on later does not leave a build's runs unsigned. - saveDiffResult writes it, and always overwrites: recomputing a diff after the reviewer edits the ignore areas must not leave the old signature describing a change that no longer exists. - Runs with nothing stored — ingested before this, or compared by something other than pixelmatch — fall back to the existing worker-pool path, so no backfill is needed and old builds keep working. The column is nullable with no default, so the migration is catalogue-only: no table rewrite, no long lock on a TestRun table holding a hundred builds.
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📝 WalkthroughWalkthroughChangesThe change adds nullable change signatures and thumbnail filenames to Diff metadata pipeline
Estimated code review effort: 4 (Complex) | ~45 minutes Merge Risk: 🟡 Moderate · up to Thumbnail persistence can leave broken diff-image references after a failed recalculation and can leak newly written storage objects when an upload fails. Failure handling should preserve existing assets until replacement persistence succeeds and clean up partial writes before merge. Sequence Diagram(s)sequenceDiagram
participant PixelmatchService
participant diffWorkerPool
participant staticService
participant TestRunsService
participant TestRunDatabase
PixelmatchService->>diffWorkerPool: Run comparison with signature and thumbnail flags
diffWorkerPool-->>PixelmatchService: Return diff metadata and thumbnail buffers
PixelmatchService->>staticService: Save screenshot and diff thumbnails
staticService-->>PixelmatchService: Return thumbnail filenames
TestRunsService->>TestRunDatabase: Persist signature and thumbnail filenames
TestRunsService->>TestRunDatabase: Read and validate stored signature
🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
✨ Finishing Touches🧪 Generate unit tests (beta)
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Actionable comments posted: 2
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In `@src/compare/libs/pixelmatch/pixelmatch.service.spec.ts`:
- Line 296: Update the thumbnail persistence tests around saveDiffAsFile to
parameterize that option, keeping the existing persistence assertions for the
enabled case and adding a disabled case that verifies thumbnail buffers are not
saved when diff persistence is off.
In `@src/compare/libs/pixelmatch/pixelmatch.service.ts`:
- Line 81: Require output.diffBuffer alongside output.imageThumbnail and
output.diffThumbnail before saving thumbnails in pixelmatch.service.ts at lines
81-81. In test-runs.service.ts at lines 396-396, delete the previous diff and
its thumbnail names before recalculating and saving the new DiffResult, using
the existing saveDiffResult flow.
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prisma/migrations/20260904120000_add_test_run_change_signature/migration.sqlprisma/migrations/20260904140000_add_test_run_thumbnails/migration.sqlprisma/schema.prismasrc/_data_/index.tssrc/compare/libs/pixelmatch/pixelmatch.core.spec.tssrc/compare/libs/pixelmatch/pixelmatch.core.tssrc/compare/libs/pixelmatch/pixelmatch.service.spec.tssrc/compare/libs/pixelmatch/pixelmatch.service.tssrc/compare/libs/pixelmatch/signature.core.tssrc/compare/libs/pixelmatch/thumbnail.core.spec.tssrc/compare/libs/pixelmatch/thumbnail.core.tssrc/compare/utils/index.tssrc/test-runs/diffResult.tssrc/test-runs/dto/testRun.dto.tssrc/test-runs/test-runs.service.spec.tssrc/test-runs/test-runs.service.ts
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A signature only means anything under the threshold it was computed with. The in-memory memo has always keyed on threshold and includeAA; when the same value started being persisted, that guard was not carried over. So a project whose imageComparisonConfig is edited after a build was ingested could compare a stored signature, taken under the old settings, against a sibling's computed fresh under the new ones. Nothing breaks loudly: variations simply stop grouping as well as they did, and the reviewer has no way to tell why. The settings now travel with the signature, and a stored one whose settings no longer match the project's is discarded — which falls back to computing it, exactly as for a run that never had one. The optimisation now knows what it was computed under. Found by CodeRabbit on Visual-Regression-Tracker#376.
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Two ways the thumbnails could be written and then orphaned, both found by CodeRabbit on Visual-Regression-Tracker#377. shouldAutoApprove compares a run against each past baseline with saveDiffAsFile off and throws the result away. The thumbnails were tied to the diff being over tolerance rather than to the diff actually being kept, so every one of those attempts stored two objects that nothing referenced and nothing deletes. They are no longer even asked for when the caller is not keeping the diff, and are stored only once the diff itself has landed. Recalculating a diff — which happens whenever a reviewer edits the ignore areas — deleted the old diff file but not the old thumbnails, while saveDiffResult replaced all three names on the row. That stranded two more objects per edit. All three now go together. The first of these was hiding behind a test that compared with saveDiffAsFile off and still expected thumbnail names, which is exactly the combination that should produce none. It now runs with the flag on, and a negative case covers the flag off.
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Inline comments:
In `@src/compare/libs/pixelmatch/pixelmatch.service.ts`:
- Around line 95-98: Update getDiff in
src/compare/libs/pixelmatch/pixelmatch.service.ts:95-98 to track all thumbnail
write outcomes, delete every successfully created thumbnail plus result.diffName
when either saveImage call fails, then propagate the original failure. Add
corresponding failure cases in
src/compare/libs/pixelmatch/pixelmatch.service.spec.ts:357-364 for each
thumbnail write, asserting successful sibling writes and the full diff are
deleted.
In `@src/test-runs/test-runs.service.ts`:
- Around line 399-401: Update calculateDiff in
src/test-runs/test-runs.service.ts (lines 399-401) to retain old asset names
until getDiff and saveDiffResult complete, then delete the old assets; if
persistence fails, compensate by deleting newly created assets. Update
src/test-runs/test-runs.service.spec.ts (lines 777-795) to verify old assets
remain when comparison fails and are deleted only after a successful commit.
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🟡 Changes recommended
There are a couple of confirmed correctness/robustness issues (nullable thumbnail deletions) and a confirmed unnecessary thumbnail-encoding cost path that should be addressed before merging.
Once you've addressed the issues Copilot identified, you can request another Copilot review.
Pull request overview
This PR reduces review-time bandwidth and latency by generating and persisting small “grid” thumbnails (and reusing stored change signatures) during ingest/recompute, so the UI can render run/variation grids without fetching full-size screenshots/diffs from storage.
Changes:
- Extend pixelmatch diff computation to optionally return a stored change signature and thumbnail PNGs.
- Persist signature + thumbnail image names on
TestRun, and delete thumbnail objects when runs/diffs are recomputed or removed. - Add Prisma schema + migrations and expand backend specs to cover new plumbing.
File summaries
| File | Description |
|---|---|
| src/test-runs/test-runs.service.ts | Persists stored signature/thumbnail names and deletes thumbnail objects on recompute/delete. |
| src/test-runs/test-runs.service.spec.ts | Adds specs covering signature persistence, thumbnail persistence, and cleanup behavior. |
| src/test-runs/dto/testRun.dto.ts | Exposes thumbnail names on the API DTO for UI consumption. |
| src/test-runs/diffResult.ts | Extends diff result contract to carry stamped signature + thumbnail names. |
| src/compare/utils/index.ts | Centralizes RawImage and downscale for shared signature/thumbnail logic. |
| src/compare/libs/pixelmatch/thumbnail.core.ts | Implements thumbnail encoding at ingest time from already-decoded pixels. |
| src/compare/libs/pixelmatch/thumbnail.core.spec.ts | Verifies thumbnail sizing/proportions/no-enlarge behavior. |
| src/compare/libs/pixelmatch/signature.core.ts | Refactors to share signatureOfDecoded with fused diff job. |
| src/compare/libs/pixelmatch/pixelmatch.service.ts | Requests signatures on all comparisons; saves thumbnails only when keeping a diff. |
| src/compare/libs/pixelmatch/pixelmatch.service.spec.ts | Updates expectations for additional saves; adds signature/thumbnail plumbing tests. |
| src/compare/libs/pixelmatch/pixelmatch.core.ts | Produces signature and thumbnails as part of the worker-thread diff computation. |
| src/compare/libs/pixelmatch/pixelmatch.core.spec.ts | Pins fused signature equality vs standalone job; validates thumbnail outputs. |
| src/data/index.ts | Updates test data factory with new nullable fields. |
| prisma/schema.prisma | Adds nullable columns for stored signature + thumbnail names. |
| prisma/migrations/20260904140000_add_test_run_thumbnails/migration.sql | Adds imageThumbnailName and diffThumbnailName columns. |
| prisma/migrations/20260904120000_add_test_run_change_signature/migration.sql | Adds changeSignature column. |
Review details
Suppressed comments (1)
src/test-runs/test-runs.service.ts:477
imageThumbnailName/diffThumbnailNameare nullable, but they’re passed directly intodeleteImageinsidePromise.all(). Filtering out null/undefined avoids calling the storage backend with invalid keys and makes the intent clearer.
// left behind, these would outlive every build that referenced them and
// nothing would ever collect them
this.staticService.deleteImage(testRun.imageThumbnailName),
this.staticService.deleteImage(testRun.diffThumbnailName),
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| const thumbnails = input.withThumbnails | ||
| ? { imageThumbnail: encodeThumbnail(imageIgnored), diffThumbnail: encodeThumbnail(diff) } | ||
| : {}; |
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Good catch — fixed in 49e7495.
Thumbnails now share the exact condition the diff buffer already had: over tolerance and the caller keeping it. The guard I added earlier stopped them being stored; this stops them being made, which is where the cost actually was. On a build that is mostly passing runs that is two resizes saved per run across the majority of an ingest.
Two tests cover it: none produced when the change is under tolerance, and none when saveDiff is off.
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| this.staticService.deleteImage(testRun.diffName); | ||
| this.staticService.deleteImage(testRun.imageThumbnailName); | ||
| this.staticService.deleteImage(testRun.diffThumbnailName); |
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Both storage backends already guard this, so there is nothing to add.
hdd.service.ts opens deleteImage with if (!imageName) return; and s3.service.ts with if (!imageName) return false;. A null or undefined name never reaches the filesystem or the S3 client. The pre-existing deleteImage(testRun.diffName) call relies on exactly that — diffName is nullable too, and always has been.
Adding a caller-side check would duplicate a guard the interface already makes, in two places, and imply the storage layer is stricter than it is.
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| // small copies for the card grids; null on runs ingested before these existed | ||
| imageThumbnailName?: string; | ||
| diffThumbnailName?: string; |
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Correct — fixed in 49e7495.
Every other field in TestRunDto is annotated and the file already imported ApiPropertyOptional; these two were the only ones without. Without the annotation the fields are absent from the generated OpenAPI schema, so the client SDKs built from it would not know they exist — which matters here because the whole point of the change is the UI reading them.
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calculateDiff deleted the old diff before running the comparison — from before this branch; I had only widened it to the two thumbnails as well. A comparison that then failed left the row pointing at files already gone, so the reviewer opened the run and found broken pictures with nothing to fall back on. The old names are now dropped only once the replacement has been persisted. Deleting late can leak bytes if the process dies in between; deleting early breaks what the reviewer sees. The first is much the cheaper failure. Found by CodeRabbit on Visual-Regression-Tracker#377.
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A build is mostly runs that pass. Their comparison still produced two resized PNGs, seconds before the diff they belonged to was thrown away — work nobody ever saw the result of, on the majority of every ingest. Thumbnails now share the exact condition the diff buffer already had: over tolerance, and the caller keeping it. The service-level guard added earlier stopped them being *stored*; this stops them being *made*. Also annotates the two new DTO fields with @ApiPropertyOptional. Every other field in TestRunDto is annotated and the file already imported it, so without this the generated OpenAPI schema — and the client SDKs built from it — would simply not know the fields exist. Both found by Copilot on Visual-Regression-Tracker#377.
A signature is a fixed-length histogram, and parseStoredSignature accepted any non-empty array as one. Comparing a stored vector of one length against a current one of another walks off the end of the shorter and scores the similarity over undefined entries — it returns a number, and the number means nothing. Nothing throws; grouping just quietly gets worse. That is the same failure the config stamp was added for, one level down: if COLOR_BUCKETS_PER_CHANNEL is ever changed, every signature already in the database becomes the wrong shape. The length and element types are now checked against what this build produces, so a stored signature that no longer fits is recomputed instead of half-used. Also types saveDiffResult's parameter as nullable. It genuinely is — a comparison that produced nothing clears the row back to "new", the body has always handled that, and a test passes null — but the signature claimed otherwise, which is the sort of small lie that invites a real one later. Both found by Copilot on Visual-Regression-Tracker#376.
The variations dialog spends ~1.5s and ~7 MB on 68 requests after the matching call returns, pulling a full-size diff for each of 34 cards and letting CSS shrink it to about a hundred pixels wide. The card grid in the list does the same. The comparison already has both pictures decoded, and already draws the diff, so the small copies are made there and stored beside the run: - computePixelmatchDiff optionally returns a small PNG of the checkpoint and of the diff, resized with the same downscale the signature uses — moved to compare/utils so there is one implementation rather than two. - They are saved on the same condition as the diff itself. A thumbnail of a diff that was never saved would point at a file that does not exist. - Stored as ordinary images, so they follow the same storage, the same URLs and the same deletion as everything else, with no naming convention for the UI to guess at and nothing to probe for when one is missing. - delete() removes them with the run. Two extra objects per run left behind would outlive every build that referenced them, and nothing collects them. Runs from before this have no thumbnail and fall back to the full-size file, so old builds look exactly as they do now and no backfill is needed. The migration adds two nullable columns with no default: catalogue-only on Postgres 11+, no table rewrite, no long lock.
Two ways the thumbnails could be written and then orphaned, both found by CodeRabbit on Visual-Regression-Tracker#377. shouldAutoApprove compares a run against each past baseline with saveDiffAsFile off and throws the result away. The thumbnails were tied to the diff being over tolerance rather than to the diff actually being kept, so every one of those attempts stored two objects that nothing referenced and nothing deletes. They are no longer even asked for when the caller is not keeping the diff, and are stored only once the diff itself has landed. Recalculating a diff — which happens whenever a reviewer edits the ignore areas — deleted the old diff file but not the old thumbnails, while saveDiffResult replaced all three names on the row. That stranded two more objects per edit. All three now go together. The first of these was hiding behind a test that compared with saveDiffAsFile off and still expected thumbnail names, which is exactly the combination that should produce none. It now runs with the flag on, and a negative case covers the flag off.
calculateDiff deleted the old diff before running the comparison — from before this branch; I had only widened it to the two thumbnails as well. A comparison that then failed left the row pointing at files already gone, so the reviewer opened the run and found broken pictures with nothing to fall back on. The old names are now dropped only once the replacement has been persisted. Deleting late can leak bytes if the process dies in between; deleting early breaks what the reviewer sees. The first is much the cheaper failure. Found by CodeRabbit on Visual-Regression-Tracker#377.
A build is mostly runs that pass. Their comparison still produced two resized PNGs, seconds before the diff they belonged to was thrown away — work nobody ever saw the result of, on the majority of every ingest. Thumbnails now share the exact condition the diff buffer already had: over tolerance, and the caller keeping it. The service-level guard added earlier stopped them being *stored*; this stops them being *made*. Also annotates the two new DTO fields with @ApiPropertyOptional. Every other field in TestRunDto is annotated and the file already imported it, so without this the generated OpenAPI schema — and the client SDKs built from it — would simply not know the fields exist. Both found by Copilot on Visual-Regression-Tracker#377.
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Stacked on #376 — review that first; this branch contains its commit.
The measurement
After #376 removes the ~4.5 s the matching call spent recomputing signatures, what is left in the variations dialog is ~1.5 s and ~7 MB across 68 requests: a full-size diff pulled for each of 34 cards, only for CSS to shrink each one to about a hundred pixels wide. Each card costs two requests — the API redirect and the S3 GET — at ~200 kB apiece. The card grid in the run list does the same, and with
Show diffoff it pulls 1.4 MB screenshots instead.The fix
The comparison already has both pictures decoded, and already draws the diff, so the small copies are made there.
computePixelmatchDiffoptionally returns a small PNG of the checkpoint and of the diff, resized with the samedownscalethe signature uses — moved tocompare/utilsso there is one implementation rather than a second that could drift.delete()removes them with the run. Two extra objects per run left behind would outlive every build that referenced them, and nothing in the product collects them.Old builds
Runs from before this have no thumbnail and fall back to the full-size file, so existing builds look and behave exactly as they do now. No backfill required.
Storage
Two extra objects per run that has a saved diff, ~15–40 kB each against screenshots of 200 kB–1.5 MB. Runs under tolerance store none.
Migration
imageThumbnailName/diffThumbnailName, both nullable with no default: catalogue-only on Postgres 11+, no table rewrite and no long lock.Tests
Watched failing first: the encoder (bounds the longest side, keeps proportions, never enlarges a small image, is a fraction of the bytes); the fused job (returns both pictures, none when not asked for, none when the screenshots match); the service (saves both and reports the names, only when the diff is saved); and the run service (persists the names, and removes both files with the run).
Two existing assertions counted
saveImagecalls; they now name what the first call is rather than only how many there were.All 38 backend spec files pass.
tsc, prettier clean; eslint's 5 warnings are the onesmasteralready has.Paired with frontend nGervasyuk/frontend
perf/use-thumbnails.Summary by CodeRabbit
New Features
Bug Fixes