remove expired remote write metrics on export

client/app/src/main/java/com/birdthedeveloper/prometheus/android/prometheus/android/exporter/worker/RemoteWriteSenderStorage.kt

@@ -106,10 +106,36 @@ data class TimeSeriesSample(
 private typealias ConverterHashMap = HashMap<List<TimeSeriesLabel>, MutableList<TimeSeriesSample>>
 
 abstract class RemoteWriteSenderStorage {
-    private val remoteWriteLabel: TimeSeriesLabel = TimeSeriesLabel(
-        name = "backfill",
-        value = "true",
-    )
+    companion object{
+        private const val maxMetricsAge : Int = 58 * 60 // 58 minutes
+
+        private val remoteWriteLabel: TimeSeriesLabel = TimeSeriesLabel(
+            name = "backfill",
+            value = "true",
+        )
+
+        fun filterExpiredMetrics(metrics : MutableList<MetricsScrape>){
+            val oldestMetricTimeMs : Long = System.currentTimeMillis() - maxMetricsAge * 1000
+            var howManyMetricsRemove : Int = 0
+
+            // count how many metrics to remove
+            for (i in 0 until metrics.size){
+                val scrape : MetricsScrape = metrics[i]
+                if(scrape.timeSeriesList.isNotEmpty()){
+                    if(scrape.timeSeriesList.first().sample.timeStampMs < oldestMetricTimeMs){
+                        howManyMetricsRemove++
+                    }else{
+                        break; // I suppose scrapes were performed one after another
+                    }
+                }
+            }
+
+            // remove metrics
+            for (i in 1..howManyMetricsRemove){
+                metrics.removeFirst()
+            }
+        }
+    }
 
     protected fun encodeWithSnappy(data: ByteArray): ByteArray {
         return Snappy.compress(data)
@@ -205,6 +231,8 @@ class RemoteWriteSenderSimpleMemoryStorage : RemoteWriteSenderStorage() {
         }
         Log.d(TAG, "Getting scraped samples: ${scrapedMetrics.size} samples")
 
+        filterExpiredMetrics(scrapedMetrics)
+
         val writeRequest: WriteRequest = this.metricsScrapeListToProtobuf(scrapedMetrics.toList())
         val bytes: ByteArray = writeRequest.toByteArray()
         return this.encodeWithSnappy(bytes)

client/app/src/main/java/com/birdthedeveloper/prometheus/android/prometheus/android/exporter/worker/Util.kt

@@ -18,7 +18,6 @@ class Util {
                 }
             }
             return false
-
         }
     }
 }

client/app/src/test/java/com/birdthedeveloper/prometheus/android/prometheus/android/exporter/worker/RemoteWriteSenderStorageTest.kt

@@ -0,0 +1,41 @@
+package com.birdthedeveloper.prometheus.android.prometheus.android.exporter.worker
+
+import org.junit.Assert.*
+import org.junit.Test
+
+class RemoteWriteSenderStorageTest{
+    @Test
+    fun `basic test filterExpiredMetrics`(){
+        val metrics : MutableList<MetricsScrape> = mutableListOf(
+            // MetricSamples must be ordered
+            createDummyMetricsScrape(70),
+            createDummyMetricsScrape(50),
+            createDummyMetricsScrape(30),
+        )
+
+        // execute SUT
+        RemoteWriteSenderStorage.filterExpiredMetrics(metrics)
+
+        assertEquals(2, metrics.size)
+
+        // assert the right order
+        val firstTimeStamp = metrics[0].timeSeriesList[0].sample.timeStampMs
+        val secondTimeStamp = metrics[1].timeSeriesList[0].sample.timeStampMs
+        assertTrue(firstTimeStamp < secondTimeStamp)
+    }
+
+    private fun createDummyMetricsScrape(ageInMinutes : Int) : MetricsScrape{
+        return MetricsScrape(
+            timeSeriesList = listOf(
+                StorageTimeSeries(
+                    labels = listOf<TimeSeriesLabel>(),
+                    sample = TimeSeriesSample(
+                        // too old
+                        timeStampMs = System.currentTimeMillis() - ageInMinutes * 60 * 1000L,
+                        value = 0.0,
+                    ),
+                )
+            )
+        )
+    }
+}