feat(unit tests): add `expected_event_count` field to test outputs by pront · Pull Request #25186 · vectordotdev/vector

pront · 2026-04-14T14:21:19Z

Summary

Adds an optional expected_event_count field to the TestOutput config struct, allowing users to assert on the number of events emitted by a transform in unit tests.
Useful for transforms that emit multiple events (e.g., remap), where previously there was no way to verify the event count.
Fully backwards-compatible: omitting the field skips the check entirely.

This is a low hanging since it was requested multiple times across various channels and it's easy to implement.

Vector configuration

tests:
  - name: split produces three events
    inputs:
      - insert_at: my_transform
        value: "one,two,three"
    outputs:
      - extract_from: my_transform
        expected_event_count: 3
        conditions:
          - type: vrl
            source: |
              assert!(exists(.message), "message field must exist")

How did you test this PR?

Added two unit tests in src/config/unit_test/tests.rs:

expected_event_count_correct - verifies a matching count passes
expected_event_count_wrong - verifies a mismatched count produces a clear error message

Also ran make check-clippy, make check-fmt, and make check-generated-docs locally.

Change Type

Is this a breaking change?

Yes
No

Does this PR include user facing changes?

Yes. Please add a changelog fragment based on our guidelines.
No. A maintainer will apply the no-changelog label to this PR.

References

Closes: tests: allow to test the number of events emitted by a transform #20278

🤖 Generated with Claude Code

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Add an optional `expected_event_count` field to the `TestOutput` config struct, allowing users to assert on the number of events emitted by a transform in unit tests. The count check is independent of conditions and is verified first. - Reject `expected_event_count: 0` with conditions at build time - Error on conflicting counts when multiple outputs share extract_from - Allow `expected_event_count: 0` to suppress the "no events received" error when zero events are genuinely expected Closes #20278 Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

A config could bypass the zero-count/conditions validation by splitting expected_event_count: 0 and conditions across two output entries sharing the same extract_from. After merge the combined entry had count 0 with non-empty conditions, causing conditions to pass vacuously against zero events. Add a post-merge check to catch this. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

thomasqueirozb

Very nice!

thomasqueirozb · 2026-04-17T16:35:39Z

+                transforms:
+                  foo:
+                    inputs:
+                      - ignored
+                    type: remap
+                    source: |
+                      . = [{"message": "one"}, {"message": "two"}, {"message": "three"}]
+                tests:
+                  - name: wrong event count
+                    inputs:
+                      - insert_at: foo
+                        value: doesnt matter
+                    outputs:
+                      - extract_from: foo
+                        expected_event_count: 2
+                        conditions:
+                          - type: vrl
+                            source: |
+                              assert!(exists(.message), "message field must exist")


nit: this is too far indented. Doesn't matter because of how indoc works

thomasqueirozb · 2026-04-17T16:37:44Z

    test_outputs: &[TestOutput],
-) -> Result<IndexMap<Vec<OutputId>, Vec<Vec<Condition>>>, Vec<String>> {
-    let mut outputs: IndexMap<Vec<OutputId>, Vec<Vec<Condition>>> = IndexMap::new();
+) -> Result<IndexMap<Vec<OutputId>, (Option<usize>, Vec<Vec<Condition>>)>, Vec<String>> {


This type is too big, we should probably use a struct here

pront force-pushed the pavlos/expected-event-count-unit-tests branch from 4e0e025 to 25de938 Compare April 14, 2026 15:00

pront marked this pull request as ready for review April 14, 2026 15:00

pront requested a review from a team as a code owner April 14, 2026 15:00

chatgpt-codex-connector Bot reviewed Apr 14, 2026

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Comment thread src/config/unit_test/unit_test_components.rs

Comment thread src/config/unit_test/mod.rs

pront force-pushed the pavlos/expected-event-count-unit-tests branch from 25de938 to c0e636c Compare April 14, 2026 15:39

chatgpt-codex-connector Bot reviewed Apr 14, 2026

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Comment thread src/config/unit_test/mod.rs

pront and others added 2 commits April 14, 2026 11:54

fix(unit tests): add missing authors line to changelog fragment

8a31be2

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pront mentioned this pull request Apr 14, 2026

tests: allow to test the number of events emitted by a transform #20278

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feat(unit tests): add `expected_event_count` field to test outputs#25186

feat(unit tests): add `expected_event_count` field to test outputs#25186
pront wants to merge 4 commits intomasterfrom
pavlos/expected-event-count-unit-tests

pront commented Apr 14, 2026 •

edited

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pront commented Apr 14, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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Summary

Vector configuration

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pront commented Apr 14, 2026 •

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