fix(store): omit empty statuses from findOldestCreatedAt (false lag-gauge reading)

okapi-core/src/main/kotlin/com/softwaremill/okapi/core/OutboxStore.kt

@@ -19,7 +19,13 @@ interface OutboxStore {
      */
     fun removeDeliveredBefore(time: Instant, limit: Int): Int
 
-    /** Returns the oldest createdAt per status (useful for lag metrics). */
+    /**
+     * For each of the given [statuses] that has at least one entry, maps that status to the
+     * [OutboxEntry.createdAt] of its oldest entry (the minimum `createdAt` among entries in
+     * that status). Statuses with no entries are omitted from the result -- callers rely on
+     * absence to mean "no backlog" (e.g. the lag gauge reports 0 for an omitted status).
+     * Useful for lag metrics.
+     */
     fun findOldestCreatedAt(statuses: Set<OutboxStatus>): Map<OutboxStatus, Instant>
 
     /** Returns entry count per status. */

okapi-integration-tests/src/test/kotlin/com/softwaremill/okapi/test/store/OutboxStoreContractTests.kt

@@ -225,6 +225,27 @@ fun FunSpec.outboxStoreContractTests(
         counts shouldContain (OutboxStatus.FAILED to 1L)
     }
 
+    test("[$dbName] findOldestCreatedAt returns the oldest entry only for statuses that have rows") {
+        // Regression guard: a status with no rows must be ABSENT from the result map, not defaulted
+        // to "now". The lag gauge (okapi-micrometer) relies on absence to report 0 for empty statuses;
+        // if the store returns a near-now timestamp for an empty status the gauge shows a tiny false lag.
+        val older = Instant.parse("2024-01-01T00:00:00Z")
+        val newer = Instant.parse("2024-01-02T00:00:00Z")
+        jdbc.withTransaction {
+            store.persist(createTestEntry(now = older, messageType = "type.older"))
+            store.persist(createTestEntry(now = newer, messageType = "type.newer"))
+        }
+
+        val oldest = jdbc.withTransaction {
+            store.findOldestCreatedAt(setOf(OutboxStatus.PENDING, OutboxStatus.DELIVERED, OutboxStatus.FAILED))
+        }
+
+        // PENDING has two entries -> oldest createdAt is the earlier one.
+        oldest shouldContain (OutboxStatus.PENDING to older)
+        // DELIVERED / FAILED have no rows -> omitted entirely, not seeded with the current time.
+        oldest.keys shouldBe setOf(OutboxStatus.PENDING)
+    }
+
     test("[$dbName] claimPending returns empty when no PENDING entries") {
         val claimed = jdbc.withTransaction { store.claimPending(10) }
 

okapi-mysql/src/main/kotlin/com/softwaremill/okapi/mysql/MysqlOutboxStore.kt

@@ -107,7 +107,7 @@ class MysqlOutboxStore(
     }
 
     override fun findOldestCreatedAt(statuses: Set<OutboxStatus>): Map<OutboxStatus, Instant> {
-        val result = statuses.associateWith { clock.instant() }.toMutableMap()
+        val result = mutableMapOf<OutboxStatus, Instant>()
         val placeholders = statuses.joinToString(",") { "?" }
         val sql = "SELECT status, MIN(created_at) AS min_created_at FROM okapi_outbox WHERE status IN ($placeholders) GROUP BY status"
 

okapi-postgres/src/main/kotlin/com/softwaremill/okapi/postgres/PostgresOutboxStore.kt

@@ -101,7 +101,7 @@ class PostgresOutboxStore(
     }
 
     override fun findOldestCreatedAt(statuses: Set<OutboxStatus>): Map<OutboxStatus, Instant> {
-        val result = statuses.associateWith { clock.instant() }.toMutableMap()
+        val result = mutableMapOf<OutboxStatus, Instant>()
         val placeholders = statuses.joinToString(",") { "?" }
         val sql = "SELECT status, MIN(created_at) AS min_created_at FROM okapi_outbox WHERE status IN ($placeholders) GROUP BY status"