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feat(topic): add Topic.latest() for a single-read value + timestamp - #31
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Checking whether a published value is stale needs two calls:
long stamp = topic.latestPublishNanos();
T v = topic.latestValueOr(null);
long age = System.nanoTime() - stamp;
Both accessors are individually correct, but a publish can land between
them, leaving the caller holding a value from one publish and a timestamp
from another. Reading the timestamp first makes that safe in one
direction -- the age can only be over-reported, never under-reported --
but over-reporting still discards a fresh value as stale, by up to a full
publish interval.
How often that happens depends on how long the two reads take. Normally
they are nanoseconds apart and a publish landing between them is
vanishingly rare. It stops being rare when the reader is preempted
between the two calls by a GC pause or the scheduler, since the window
becomes milliseconds. This library configures a small young gen, so young
GCs are part of the deployment rather than a hypothetical.
`latest()` returns the value and its timestamp from one snapshot read, so
the pair provably comes from a single publish and the window does not
exist:
Optional<Topic.Latest<T>> snap = topic.latest();
T v = snap.map(Topic.Latest::value).orElse(defaultValue);
long age = snap.map(Topic.Latest::ageNanos).orElse(Long.MAX_VALUE);
Additive only. No existing signature or behaviour changes. Callers that
never check an age keep using `latestValueOr` -- the same single volatile
read, and allocation-free.
`Topic.Latest` becomes public with `value()`, `publishNanos()` and
`ageNanos()`. `ageNanos()` is the age of *this* value computed from its
own stamp, which also removes the pre-publish `0` sentinel that the
two-call form has to guard by hand.
`latestPublishNanos()` still returns 0 before the first publish, so that
guard stays necessary for anyone composing by hand, and it is documented
as a heuristic rather than a proof -- System.nanoTime() is permitted to
return 0 too. It is stated as such in the javadoc, the CHANGELOG and the
topics guide.
The concurrency test's reader now uses `latest()`, which is the stronger
property: no straddle is representable, so the "timestamp first"
ordering rule has nothing left to guard. Its cross-check between
`latest()` being empty and `latestPublishNanos()` being nonzero is gone,
because a publish can legitimately land between those two reads.
Suite: 86 tests, 0 failures. javadoc: no errors, no new warnings.
Reviewer's GuideAdds Sequence diagram for the consistent Topic.latest snapshotsequenceDiagram
participant Caller
participant Topic
participant Latest
Caller->>Topic: latest()
Topic-->>Topic: read latest snapshot once
alt No publish yet
Topic-->>Caller: Optional.empty()
else Published value exists
Topic->>Latest: value and publishNanos
Topic-->>Caller: Optional<Latest<T>>
Caller->>Latest: value()
Caller->>Latest: ageNanos()
Latest-->>Caller: value and age from same publish
end
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📝 WalkthroughWalkthrough
ChangesTopic snapshot API
Priority: ⬇️ Low Estimated code review effort: 3 (Moderate) | ~20 minutes Change: Feature Merge Risk: 🔵 Low · up to The snapshot API has no established functional defect, but two assertions can fail for valid results. Remove those assertions before merging to avoid misleading test failures. Security Architecture ReviewSecurity architecture risk: 🔵 Low · up to The inspected API improves consistency without granting new publication authority or exposing data unavailable through existing accessors. No concrete security issue was identified. External caller exposure and the precise before-and-after publication behavior remain partially established. Retained concerns Security review detailsSecurity Blast Radius
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Hey - I've found 2 issues
Prompt for AI Agents
Please address the comments from this code review:
## Individual Comments
### Comment 1
<location path="src/test/java/com/aaravlabs/synapse/TopicLockFreeTest.java" line_range="277" />
<code_context>
+ // age >= 0, not > 0: a snapshot read in the same clock tick as its own publish
+ // stamp legitimately reports age 0. Only a negative age is impossible here, and
+ // that is all this check needs to catch.
+ long age = first.ageNanos();
+ assertTrue(age >= 0 && age < 1_000_000_000L, "ageNanos out of range: " + age);
+ assertTrue(age <= System.nanoTime() - first.publishNanos(),
+ "ageNanos " + age + " exceeds a delta measured later from the same stamp");
</code_context>
<issue_to_address>
**nitpick (testing):** The test rejects a valid snapshot whenever the test thread is descheduled for at least one second between publishing and calling `ageNanos()`, even though the API permits the age to be arbitrarily large. A long GC pause or scheduler delay therefore causes a flaky test failure unrelated to snapshot consistency.
**Triggers:** When the test JVM experiences a long GC pause or scheduler descheduling after the first publish.
**Suggested fix:** Remove the arbitrary one-second upper bound, or derive the bound from an explicit test timeout while asserting only that the age is non-negative and no greater than a later clock delta.
```suggestion
assertTrue(age >= 0, "ageNanos out of range: " + age);
```
</issue_to_address>
### Comment 2
<location path="src/main/java/com/aaravlabs/synapse/Topic.java" line_range="261-263" />
<code_context>
+ *
+ * <p>Returns empty before the first publish.
+ *
+ * <p><b>Allocates one small {@link Optional} wrapper per call.</b> If you only want
+ * the value and never check its age, use {@link #latestValueOr(Object)}, which is
+ * allocation-free.
+ *
</code_context>
<issue_to_address>
**nitpick:** The documentation says `latest()` allocates one `Optional` wrapper per call, but calls before the first publish return the cached `Optional.empty()` instance and allocate no wrapper. This gives callers an incorrect allocation estimate for the empty-read path.
**Triggers:** When `latest()` is called before the topic has received its first publish.
**Suggested fix:** Document that non-empty calls allocate an `Optional` wrapper, while empty calls return the shared empty instance.
```suggestion
* <p><b>Non-empty calls allocate one small {@link Optional} wrapper.</b> Empty calls
* before the first publish return the shared {@link Optional#empty()} instance. If you
* only want the value and never check its age, use {@link #latestValueOr(Object)}, which is
* allocation-free.
```
</issue_to_address>Sourcery assessment
Approved.
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Actionable comments posted: 2
- 🪄 Fix CodeRabbit comments on this PR
🤖 Prompt to fix review comments
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only still-valid issues, skip the rest with a brief reason, keep changes
minimal, and validate.
Inline comments:
Review comments at @src/test/java/com/aaravlabs/synapse/TopicLockFreeTest.java:
- Line 277: Update the ageNanos assertion in TopicLockFreeTest to remove the
one-second upper bound while retaining the nonnegative check and the separate
later-clock-delta checks.
- Around line 266-267: Remove the nonzero publishNanos assertion from this test;
the present Optional already verifies that publication occurred, and a zero
System.nanoTime() value is valid.
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CHANGELOG.mdsrc/main/java/com/aaravlabs/synapse/Topic.javasrc/test/java/com/aaravlabs/synapse/TopicLockFreeTest.javawebsite/src/content/docs/api/index.mdxwebsite/src/content/docs/concepts/topics.mdx
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- Remove the one-second age ceiling. A GC pause between the publish and the ageNanos() read legitimately makes the age arbitrarily large, so the bound imposed a scheduling deadline on the test. Both bounds are now relative to the clock. - Remove the nonzero publishNanos() assertion. System.nanoTime() has an arbitrary origin, so a published snapshot may carry 0; the present Optional already proves a publish happened. - latest() allocates an Optional wrapper only when it returns a value. Empty reads return the shared Optional.empty() and allocate nothing, which is now what the javadoc and the topics guide say. Suite 86/86, javadoc clean.
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All reported issues were addressed
Reply with feedback, questions, or to request a fix.
Fix all with cubic | Re-trigger cubic
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All reported issues were addressed across 5 files
Requires human review: Auto-approval blocked because this review re-detected 1 unresolved issue already reported by Cubic.
Fix all with cubic | Re-trigger cubic
Three review findings on the snapshot tests: - ageNanos() was only bounded from above, so an implementation returning a constant 0 satisfied the check while reporting nothing about the real age. Bracket the call between two clock reads in both the unit test and the concurrency reader, and require the age to fall between both deltas from the snapshot's own stamp. - second.ageNanos() < first.ageNanos() sampled at two different moments, so a pause between the publish and the second read could make the newer value look older. Derive both ages from a single clock reading: from one now, a strictly later stamp is a strictly smaller age. The one-second wall-clock ceiling and the nonzero-stamp assertion reported earlier were already removed in the previous commit; these reviews landed on the older SHA. Suite 86/86, tear test green on a 2-core-pinned runner, javadoc clean.
The value-and-age example called Optional.map() once per field, so each non-empty read allocated two more Optional wrappers and boxed the long age -- in a guide aimed at periodic loops, teaching the allocating form three times over (the latest() wrapper plus two map() calls). Every example now unwraps with isPresent()/get() and calls value() and ageNanos() directly: the topics guide, the javadoc and the CHANGELOG.
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…call The previous bounds sampled a clock reading outside the ageNanos() call, so they could not bound the result in either direction: a pause between the sampling and the call breaks the lower bound, and the age correctly exceeds a reading taken before it. Both the unit test and the concurrency reader now take one clock reading immediately before the call and one immediately after, and require the age to fall between the two deltas from the snapshot's own stamp. That is the only bracketing that constrains the value, and it holds regardless of how long the call itself takes. Suite 86/86 across 8 consecutive runs, tear test green pinned to two cores, javadoc clean.
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| // Comparing ageNanos() against `now - publishNanos()` cannot establish that: | ||
| // both sides recompute the same subtraction, so the relation holds for any pair | ||
| // and a Latest carrying a foreign stamp still passes. What actually pins the | ||
| // stamp to this value is the cross-check above -- latest().publishNanos() equals | ||
| // latestPublishNanos() equals the stamp of the "a" publish -- combined with a | ||
| // non-negative age and a stamp that is not in the future. |
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Tautological stamp validation in src/test/java/com/aaravlabs/synapse/TopicLockFreeTest.java leaves Latest.publishNanos() unverified for the "a" publish: latest() (Topic.java:287, Optional.ofNullable(latest)) and latestPublishNanos() (Topic.java:163-166, latest.publishNanos()) read the same volatile field, so the assertEquals at lines 269-270 passes for any stamp and the replacement age assertion cannot substitute. The bracket at lines 297-300 remains offset-invariant because a Latest built with stamp + K shifts age and both bounds by exactly K, letting a wrong-but-consistent timestamp—the staleness failure latest() exists to prevent—pass the whole method; bracket orchestrator.publish("snap", "a") with same-thread System.nanoTime() reads and assert that first.publishNanos() falls between them, the only check that rejects a constant offset.
// The cross-check above only proves the two accessors read the same field; it is
// tautological for the stamp's identity, so the stamp is pinned here instead by
// bracketing the publish itself between two clock reads on this thread.
long publishBefore = System.nanoTime();
orchestrator.publish("snap", "a");
Topic.Latest<String> first = t.latest().orElseThrow();
long publishAfter = System.nanoTime();
assertTrue(first.publishNanos() >= publishBefore
&& first.publishNanos() <= publishAfter,
"stamp " + first.publishNanos() + " is not the clock reading taken during the publish"
+ " between " + publishBefore + " and " + publishAfter);
// ageNanos must be the age of THIS value, from ITS OWN stamp: a constant 0 or a
// constant offset would fall outside this bracket, while a GC pause between the
// reads only moves T inside it.Prompt for LLM
File src/test/java/com/aaravlabs/synapse/TopicLockFreeTest.java:
Line 279 to 284:
Tautological stamp validation in `src/test/java/com/aaravlabs/synapse/TopicLockFreeTest.java` leaves `Latest.publishNanos()` unverified for the `"a"` publish: `latest()` (`Topic.java:287`, `Optional.ofNullable(latest)`) and `latestPublishNanos()` (`Topic.java:163-166`, `latest.publishNanos()`) read the same volatile field, so the `assertEquals` at lines 269-270 passes for any stamp and the replacement age assertion cannot substitute. The bracket at lines 297-300 remains offset-invariant because a `Latest` built with `stamp + K` shifts `age` and both bounds by exactly K, letting a wrong-but-consistent timestamp—the staleness failure `latest()` exists to prevent—pass the whole method; bracket `orchestrator.publish("snap", "a")` with same-thread `System.nanoTime()` reads and assert that `first.publishNanos()` falls between them, the only check that rejects a constant offset.
Suggested Code:
// The cross-check above only proves the two accessors read the same field; it is
// tautological for the stamp's identity, so the stamp is pinned here instead by
// bracketing the publish itself between two clock reads on this thread.
long publishBefore = System.nanoTime();
orchestrator.publish("snap", "a");
Topic.Latest<String> first = t.latest().orElseThrow();
long publishAfter = System.nanoTime();
assertTrue(first.publishNanos() >= publishBefore
&& first.publishNanos() <= publishAfter,
"stamp " + first.publishNanos() + " is not the clock reading taken during the publish"
+ " between " + publishBefore + " and " + publishAfter);
// ageNanos must be the age of THIS value, from ITS OWN stamp: a constant 0 or a
// constant offset would fall outside this bracket, while a GC pause between the
// reads only moves T inside it.
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Summary
Adds
Topic.latest(), which returns the latest value and its publish timestamp from a single snapshot read, so a staleness check cannot combine halves from two different publishes.Follow-up to #29, which made the topic's internals safe and documented an ordering rule for composing the existing accessors. This removes the need for that rule.
The problem it solves
Checking staleness today takes two calls:
Both accessors are individually correct. But a publish can land between them:
Reading the timestamp first makes that safe in one direction — the age can only be over-reported, never under-reported — but over-reporting by a full publish interval still makes you reject a fresh value as stale. On a 100 Hz sensor with a 50 ms budget that is 20% of your margin, spent for nothing.
How often this happens depends on how long the two reads take. Normally they are nanoseconds apart and the odds are negligible. It stops being negligible when the reader is preempted between the two calls — a GC pause or scheduler preemption — because the window becomes milliseconds. This project deliberately configures a small young gen (
-Xmn16m,-Xmx256m), so young GCs are part of the deployment, not a hypothetical. On a Control Hub also running the driver station and telemetry, a reader landing in a GC pause between two adjacent reads is not exotic.The API
Unwrap once rather than calling
map()per field — eachmap()allocates its ownOptionaland boxes thelong.Topic.Latestbecomes public with three accessors:value()publishNanos()System.nanoTime()at which it was recordedageNanos()publishNanos()— the age of this valueageNanos()is computed from the object's own stamp, so it also removes the pre-publish0sentinel that the two-call form has to guard by hand.latestPublishNanos()still returns0before the first publish, so that guard remains necessary for anyone composing by hand — it is now documented as a heuristic rather than a proof, sinceSystem.nanoTime()is permitted to return0too.Additive only. No existing signature or behaviour changes. Callers who never check an age keep using
latestValueOr— identical single volatile read, and allocation-free.It allocates.
Optional.ofNullablewraps every non-empty result, solatest()costs one small object per call; empty reads return the sharedOptional.empty()and allocate nothing. The examples unwrap once rather than callingmap()per field, since eachmap()allocates anotherOptionaland boxes thelong. That matters in a periodic loop on the small young gen this project configures, so the javadoc says so and points atlatestValueOrfor value-only reads. An earlier draft of this description claimedlatest()"allocates nothing"; that was wrong and is corrected here.Why the ordering rule disappears
#29's rule was "sample the timestamp first, because two calls can straddle." With one read there is no straddle to guard, so:
latestValue()javadoc points atlatest()rather than teaching a rulelatest(), so the rule has no remaining subjectThat is the part that matters most: it is not that the rule became easier to remember, it is that there is no longer anything to remember.
Tests
latestReturnsValueAndTimestampFromOneConsistentPublish: empty before the first publish; agrees with both single-field accessors; a published stamp is never the0sentinel;ageNanos()is in range and never exceeds a delta measured later from the same stamp; a later publish replaces both halves together and reports a younger age.Two assertions were wrong when first written, both caught in review: a strictly positive age, and a fixed
Thread.sleep(2)to guarantee clock movement.System.nanoTime()may return the same tick for adjacent reads, soageNanos() == 0is legitimate and the sleep guarantees nothing. The wait is now a bounded loop on the clock.The concurrency test's reader switched to
latest(). Its old cross-check between "latest()empty" and "latestPublishNanos()nonzero" had to be deleted — a publish can legitimately land between those two reads, so a nonzero stamp there says nothing about consistency.Verification
gradle test→ 86 tests, 0 failures, from cleantaskset -c 0,1)gradle javadoc→ 0 errors, no new warningsOpen questions for the maintainer
latest()earn public API space, or should it stay internal? It is the honest fix for the straddle, but it is also a new type in the public surface with a name (Latest) that could collide with user vocabulary.latestValue()andlatestPublishNanos()be deprecated? They are still correct and still right when you only want one. I have deliberately not deprecated anything — that is a maintainer decision, and it would break the "purely additive" property of this PR.Optional<Topic.Latest<T>>allocates a wrapper per call, which is a real cost in a periodic loop on a small young gen; a nullable-returninglatestOrNull()would be allocation-free. If hot-path staleness checks matter more than the ergonomics, the allocation-free shape is the better one and I would rather you chose it than have me guess.Summary by Sourcery
Provide an atomic latest-value snapshot so callers can check freshness without straddling a publish.
New Features:
Topic.latest()API that returns the latest value and publish timestamp from a single snapshot read.Topic.Latest<T>accessors for the value, publish timestamp, and value-specific age.Bug Fixes:
Enhancements:
Documentation:
Tests: