merge: Expose metrics directly to user (#80)

This pull request adds the ability to access the metrics of resources modeled
by the OpenDC Compute, Workflow, FaaS, and TensorFlow services directly from
their corresponding interfaces. Previously, users would have to interact with
OpenTelemetry to obtain these values, which is complex and provides
significant overhead.

With this pull request, users can access the metrics of all cloud resources
modeled by OpenDC via methods such as `getSchedulerStats()`, etc.

** Breaking Changes **

- `ComputeService.hostCount` removed in favour of `ComputeService.hosts.size`

12 files changed, 321 insertions, 22 deletions

diff --git a/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/ComputeService.kt b/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/ComputeService.kt
index 2a1fbaa0..3a6baaa1 100644
--- a/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/ComputeService.kt
+++ b/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/ComputeService.kt
@@ -25,9 +25,11 @@ package org.opendc.compute.service
 import io.opentelemetry.api.metrics.Meter
 import io.opentelemetry.api.metrics.MeterProvider
 import org.opendc.compute.api.ComputeClient
+import org.opendc.compute.api.Server
 import org.opendc.compute.service.driver.Host
 import org.opendc.compute.service.internal.ComputeServiceImpl
 import org.opendc.compute.service.scheduler.ComputeScheduler
+import org.opendc.compute.service.telemetry.SchedulerStats
 import java.time.Clock
 import java.time.Duration
 import kotlin.coroutines.CoroutineContext
@@ -37,16 +39,11 @@ import kotlin.coroutines.CoroutineContext
  */
 public interface ComputeService : AutoCloseable {
     /**
-     * The hosts that are used by the compute service.
+     * The hosts that are registered with the "compute" service.
      */
     public val hosts: Set<Host>
 
     /**
-     * The number of hosts available in the system.
-     */
-    public val hostCount: Int
-
-    /**
      * Create a new [ComputeClient] to control the compute service.
      */
     public fun newClient(): ComputeClient
@@ -66,6 +63,16 @@ public interface ComputeService : AutoCloseable {
      */
     public override fun close()
 
+    /**
+     * Lookup the [Host] that currently hosts the specified [server].
+     */
+    public fun lookupHost(server: Server): Host?
+
+    /**
+     * Collect the statistics about the scheduler component of this service.
+     */
+    public fun getSchedulerStats(): SchedulerStats
+
     public companion object {
         /**
          * Construct a new [ComputeService] implementation.
diff --git a/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/driver/Host.kt b/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/driver/Host.kt
index bed15dfd..67b144d9 100644
--- a/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/driver/Host.kt
+++ b/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/driver/Host.kt
@@ -23,6 +23,10 @@
 package org.opendc.compute.service.driver
 
 import org.opendc.compute.api.Server
+import org.opendc.compute.service.driver.telemetry.GuestCpuStats
+import org.opendc.compute.service.driver.telemetry.GuestSystemStats
+import org.opendc.compute.service.driver.telemetry.HostCpuStats
+import org.opendc.compute.service.driver.telemetry.HostSystemStats
 import java.util.*
 
 /**
@@ -55,6 +59,11 @@ public interface Host {
     public val meta: Map<String, Any>
 
     /**
+     * The [Server] instances known to the host.
+     */
+    public val instances: Set<Server>
+
+    /**
      * Determine whether the specified [instance][server] can still fit on this host.
      */
     public fun canFit(server: Server): Boolean
@@ -100,4 +109,30 @@ public interface Host {
      * Remove a [HostListener] from this host.
      */
     public fun removeListener(listener: HostListener)
+
+    /**
+     * Query the system statistics of the host.
+     */
+    public fun getSystemStats(): HostSystemStats
+
+    /**
+     * Query the system statistics of a [Server] that is located on this host.
+     *
+     * @param server The [Server] to obtain the system statistics of.
+     * @throws IllegalArgumentException if the server is not present on the host.
+     */
+    public fun getSystemStats(server: Server): GuestSystemStats
+
+    /**
+     * Query the CPU statistics of the host.
+     */
+    public fun getCpuStats(): HostCpuStats
+
+    /**
+     * Query the CPU statistics of a [Server] that is located on this host.
+     *
+     * @param server The [Server] to obtain the CPU statistics of.
+     * @throws IllegalArgumentException if the server is not present on the host.
+     */
+    public fun getCpuStats(server: Server): GuestCpuStats
 }
diff --git a/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/driver/telemetry/GuestCpuStats.kt b/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/driver/telemetry/GuestCpuStats.kt
new file mode 100644
index 00000000..b5d63471
--- /dev/null
+++ b/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/driver/telemetry/GuestCpuStats.kt
@@ -0,0 +1,44 @@
+/*
+ * Copyright (c) 2022 AtLarge Research
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package org.opendc.compute.service.driver.telemetry
+
+/**
+ * Statistics about the CPUs of a guest.
+ *
+ * @property activeTime The cumulative time (in seconds) that the CPUs of the guest were actively running.
+ * @property idleTime The cumulative time (in seconds) the CPUs of the guest were idle.
+ * @property stealTime The cumulative CPU time (in seconds) that the guest was ready to run, but not granted time by the host.
+ * @property lostTime The cumulative CPU time (in seconds) that was lost due to interference with other machines.
+ * @property capacity The available CPU capacity of the guest (in MHz).
+ * @property usage Amount of CPU resources (in MHz) actually used by the guest.
+ * @property utilization Utilization of the CPU resources (in %) relative to the total CPU capacity.
+ */
+public data class GuestCpuStats(
+    val activeTime: Long,
+    val idleTime: Long,
+    val stealTime: Long,
+    val lostTime: Long,
+    val capacity: Double,
+    val usage: Double,
+    val utilization: Double
+)
diff --git a/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/driver/telemetry/GuestSystemStats.kt b/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/driver/telemetry/GuestSystemStats.kt
new file mode 100644
index 00000000..b3958473
--- /dev/null
+++ b/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/driver/telemetry/GuestSystemStats.kt
@@ -0,0 +1,39 @@
+/*
+ * Copyright (c) 2022 AtLarge Research
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package org.opendc.compute.service.driver.telemetry
+
+import java.time.Duration
+import java.time.Instant
+
+/**
+ * System-level statistics of a guest.
+ *
+ * @property uptime The cumulative uptime of the guest since last boot (in ms).
+ * @property downtime The cumulative downtime of the guest since last boot (in ms).
+ * @property bootTime The time at which the guest booted.
+ */
+public data class GuestSystemStats(
+    val uptime: Duration,
+    val downtime: Duration,
+    val bootTime: Instant
+)
diff --git a/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/driver/telemetry/HostCpuStats.kt b/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/driver/telemetry/HostCpuStats.kt
new file mode 100644
index 00000000..55e23c0e
--- /dev/null
+++ b/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/driver/telemetry/HostCpuStats.kt
@@ -0,0 +1,47 @@
+/*
+ * Copyright (c) 2022 AtLarge Research
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package org.opendc.compute.service.driver.telemetry
+
+/**
+ * Statistics about the CPUs of a host.
+ *
+ * @property activeTime The cumulative time (in seconds) that the CPUs of the host were actively running.
+ * @property idleTime The cumulative time (in seconds) the CPUs of the host were idle.
+ * @property stealTime The cumulative CPU time (in seconds) that virtual machines were ready to run, but were not able to.
+ * @property lostTime The cumulative CPU time (in seconds) that was lost due to interference between virtual machines.
+ * @property capacity The available CPU capacity of the host (in MHz).
+ * @property demand Amount of CPU resources (in MHz) the guests would use if there were no CPU contention or CPU
+ *                  limits.
+ * @property usage Amount of CPU resources (in MHz) actually used by the host.
+ * @property utilization Utilization of the CPU resources (in %) relative to the total CPU capacity.
+ */
+public data class HostCpuStats(
+    val activeTime: Long,
+    val idleTime: Long,
+    val stealTime: Long,
+    val lostTime: Long,
+    val capacity: Double,
+    val demand: Double,
+    val usage: Double,
+    val utilization: Double
+)
diff --git a/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/driver/telemetry/HostSystemStats.kt b/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/driver/telemetry/HostSystemStats.kt
new file mode 100644
index 00000000..1c07023f
--- /dev/null
+++ b/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/driver/telemetry/HostSystemStats.kt
@@ -0,0 +1,51 @@
+/*
+ * Copyright (c) 2022 AtLarge Research
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package org.opendc.compute.service.driver.telemetry
+
+import java.time.Duration
+import java.time.Instant
+
+/**
+ * System-level statistics of a host.
+
+ * @property uptime The cumulative uptime of the host since last boot (in ms).
+ * @property downtime The cumulative downtime of the host since last boot (in ms).
+ * @property bootTime The time at which the server started.
+ * @property powerUsage Instantaneous power usage of the system (in W).
+ * @property energyUsage The cumulative energy usage of the system (in J).
+ * @property guestsTerminated The number of guests that are in a terminated state.
+ * @property guestsRunning The number of guests that are in a running state.
+ * @property guestsError The number of guests that are in an error state.
+ * @property guestsInvalid The number of guests that are in an unknown state.
+ */
+public data class HostSystemStats(
+    val uptime: Duration,
+    val downtime: Duration,
+    val bootTime: Instant,
+    val powerUsage: Double,
+    val energyUsage: Double,
+    val guestsTerminated: Int,
+    val guestsRunning: Int,
+    val guestsError: Int,
+    val guestsInvalid: Int,
+)
diff --git a/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/internal/ClientServer.kt b/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/internal/ClientServer.kt
index f2929bf3..45775640 100644
--- a/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/internal/ClientServer.kt
+++ b/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/internal/ClientServer.kt
@@ -27,6 +27,7 @@ import org.opendc.compute.api.Image
 import org.opendc.compute.api.Server
 import org.opendc.compute.api.ServerState
 import org.opendc.compute.api.ServerWatcher
+import java.time.Instant
 import java.util.*
 
 /**
@@ -55,6 +56,9 @@ internal class ClientServer(private val delegate: Server) : Server, ServerWatche
     override var state: ServerState = delegate.state
         private set
 
+    override var launchedAt: Instant? = null
+        private set
+
     override suspend fun start() {
         delegate.start()
         refresh()
@@ -95,6 +99,7 @@ internal class ClientServer(private val delegate: Server) : Server, ServerWatche
         labels = delegate.labels
         meta = delegate.meta
         state = delegate.state
+        launchedAt = delegate.launchedAt
     }
 
     override fun onStateChanged(server: Server, newState: ServerState) {
diff --git a/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/internal/ComputeServiceImpl.kt b/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/internal/ComputeServiceImpl.kt
index 144b6573..e8664e5c 100644
--- a/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/internal/ComputeServiceImpl.kt
+++ b/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/internal/ComputeServiceImpl.kt
@@ -36,8 +36,10 @@ import org.opendc.compute.service.driver.Host
 import org.opendc.compute.service.driver.HostListener
 import org.opendc.compute.service.driver.HostState
 import org.opendc.compute.service.scheduler.ComputeScheduler
+import org.opendc.compute.service.telemetry.SchedulerStats
 import java.time.Clock
 import java.time.Duration
+import java.time.Instant
 import java.util.*
 import kotlin.coroutines.CoroutineContext
 import kotlin.math.max
@@ -126,6 +128,9 @@ internal class ComputeServiceImpl(
     private val _schedulingAttemptsSuccessAttr = Attributes.of(AttributeKey.stringKey("result"), "success")
     private val _schedulingAttemptsFailureAttr = Attributes.of(AttributeKey.stringKey("result"), "failure")
     private val _schedulingAttemptsErrorAttr = Attributes.of(AttributeKey.stringKey("result"), "error")
+    private var _attemptsSuccess = 0L
+    private var _attemptsFailure = 0L
+    private var _attemptsError = 0L
 
     /**
      * The response time of the service.
@@ -145,6 +150,8 @@ internal class ComputeServiceImpl(
         .build()
     private val _serversPendingAttr = Attributes.of(AttributeKey.stringKey("state"), "pending")
     private val _serversActiveAttr = Attributes.of(AttributeKey.stringKey("state"), "active")
+    private var _serversPending = 0
+    private var _serversActive = 0
 
     /**
      * The [Pacer] to use for scheduling the scheduler cycles.
@@ -154,9 +161,6 @@ internal class ComputeServiceImpl(
     override val hosts: Set<Host>
         get() = hostToView.keys
 
-    override val hostCount: Int
-        get() = hostToView.size
-
     init {
         val upState = Attributes.of(AttributeKey.stringKey("state"), "up")
         val downState = Attributes.of(AttributeKey.stringKey("state"), "down")
@@ -165,7 +169,7 @@ internal class ComputeServiceImpl(
             .setDescription("Number of hosts registered with the scheduler")
             .setUnit("1")
             .buildWithCallback { result ->
-                val total = hostCount
+                val total = hosts.size
                 val available = availableHosts.size.toLong()
 
                 result.record(available, upState)
@@ -322,17 +326,35 @@ internal class ComputeServiceImpl(
         }
     }
 
+    override fun lookupHost(server: Server): Host? {
+        val internal = requireNotNull(servers[server.uid]) { "Invalid server passed to lookupHost" }
+        return internal.host
+    }
+
     override fun close() {
         scope.cancel()
     }
 
+    override fun getSchedulerStats(): SchedulerStats {
+        return SchedulerStats(
+            availableHosts.size,
+            hostToView.size - availableHosts.size,
+            _attemptsSuccess,
+            _attemptsFailure,
+            _attemptsError,
+            _serversPending,
+            _serversActive
+        )
+    }
+
     internal fun schedule(server: InternalServer): SchedulingRequest {
         logger.debug { "Enqueueing server ${server.uid} to be assigned to host." }
         val now = clock.millis()
         val request = SchedulingRequest(server, now)
 
-        server.lastProvisioningTimestamp = now
+        server.launchedAt = Instant.ofEpochMilli(now)
         queue.add(request)
+        _serversPending++
         _servers.add(1, _serversPendingAttr)
         requestSchedulingCycle()
         return request
@@ -371,6 +393,7 @@ internal class ComputeServiceImpl(
 
             if (request.isCancelled) {
                 queue.poll()
+                _serversPending--
                 _servers.add(-1, _serversPendingAttr)
                 continue
             }
@@ -383,7 +406,9 @@ internal class ComputeServiceImpl(
                 if (server.flavor.memorySize > maxMemory || server.flavor.cpuCount > maxCores) {
                     // Remove the incoming image
                     queue.poll()
+                    _serversPending--
                     _servers.add(-1, _serversPendingAttr)
+                    _attemptsFailure++
                     _schedulingAttempts.add(1, _schedulingAttemptsFailureAttr)
 
                     logger.warn { "Failed to spawn $server: does not fit [${clock.instant()}]" }
@@ -399,6 +424,7 @@ internal class ComputeServiceImpl(
 
             // Remove request from queue
             queue.poll()
+            _serversPending--
             _servers.add(-1, _serversPendingAttr)
             _schedulingLatency.record(now - request.submitTime, server.attributes)
 
@@ -417,6 +443,8 @@ internal class ComputeServiceImpl(
                     activeServers[server] = host
 
                     _servers.add(1, _serversActiveAttr)
+                    _serversActive++
+                    _attemptsSuccess++
                     _schedulingAttempts.add(1, _schedulingAttemptsSuccessAttr)
                 } catch (e: Throwable) {
                     logger.error(e) { "Failed to deploy VM" }
@@ -425,6 +453,7 @@ internal class ComputeServiceImpl(
                     hv.provisionedCores -= server.flavor.cpuCount
                     hv.availableMemory += server.flavor.memorySize
 
+                    _attemptsError++
                     _schedulingAttempts.add(1, _schedulingAttemptsErrorAttr)
                 }
             }
@@ -481,6 +510,7 @@ internal class ComputeServiceImpl(
             logger.info { "[${clock.instant()}] Server ${server.uid} ${server.name} ${server.flavor} finished." }
 
             if (activeServers.remove(server) != null) {
+                _serversActive--
                 _servers.add(-1, _serversActiveAttr)
             }
 
@@ -503,7 +533,8 @@ internal class ComputeServiceImpl(
      */
     private fun collectProvisionTime(result: ObservableLongMeasurement) {
         for ((_, server) in servers) {
-            result.record(server.lastProvisioningTimestamp, server.attributes)
+            val launchedAt = server.launchedAt ?: continue
+            result.record(launchedAt.toEpochMilli(), server.attributes)
         }
     }
 }
diff --git a/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/internal/InternalServer.kt b/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/internal/InternalServer.kt
index f1b92c66..d2a2d896 100644
--- a/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/internal/InternalServer.kt
+++ b/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/internal/InternalServer.kt
@@ -28,6 +28,7 @@ import io.opentelemetry.semconv.resource.attributes.ResourceAttributes
 import mu.KotlinLogging
 import org.opendc.compute.api.*
 import org.opendc.compute.service.driver.Host
+import java.time.Instant
 import java.util.UUID
 
 /**
@@ -75,7 +76,7 @@ internal class InternalServer(
     /**
      * The most recent timestamp when the server entered a provisioning state.
      */
-    @JvmField internal var lastProvisioningTimestamp: Long = Long.MIN_VALUE
+    override var launchedAt: Instant? = null
 
     /**
      * The current scheduling request.
diff --git a/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/telemetry/SchedulerStats.kt b/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/telemetry/SchedulerStats.kt
new file mode 100644
index 00000000..4dc70286
--- /dev/null
+++ b/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/telemetry/SchedulerStats.kt
@@ -0,0 +1,46 @@
+/*
+ * Copyright (c) 2022 AtLarge Research
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package org.opendc.compute.service.telemetry
+
+import org.opendc.compute.service.ComputeService
+
+/**
+ * Statistics about the scheduling component of the [ComputeService].
+ *
+ * @property hostsAvailable The number of hosts currently available for scheduling.
+ * @property hostsUnavailable The number of hosts unavailable for scheduling.
+ * @property attemptsSuccess Scheduling attempts that resulted into an allocation onto a host.
+ * @property attemptsFailure The number of failed scheduling attempt due to insufficient capacity at the moment.
+ * @property attemptsError The number of scheduling attempts that failed due to system error.
+ * @property serversPending The number of servers that are pending to be scheduled.
+ * @property serversActive The number of servers that are currently managed by the service and running.
+ */
+public data class SchedulerStats(
+    val hostsAvailable: Int,
+    val hostsUnavailable: Int,
+    val attemptsSuccess: Long,
+    val attemptsFailure: Long,
+    val attemptsError: Long,
+    val serversPending: Int,
+    val serversActive: Int
+)
diff --git a/opendc-compute/opendc-compute-service/src/test/kotlin/org/opendc/compute/service/ComputeServiceTest.kt b/opendc-compute/opendc-compute-service/src/test/kotlin/org/opendc/compute/service/ComputeServiceTest.kt
index 7b8d0fe2..eb106817 100644
--- a/opendc-compute/opendc-compute-service/src/test/kotlin/org/opendc/compute/service/ComputeServiceTest.kt
+++ b/opendc-compute/opendc-compute-service/src/test/kotlin/org/opendc/compute/service/ComputeServiceTest.kt
@@ -24,7 +24,6 @@ package org.opendc.compute.service
 
 import io.mockk.*
 import io.opentelemetry.api.metrics.MeterProvider
-import kotlinx.coroutines.ExperimentalCoroutinesApi
 import kotlinx.coroutines.delay
 import org.junit.jupiter.api.Assertions.assertEquals
 import org.junit.jupiter.api.Assertions.assertNull
@@ -48,10 +47,9 @@ import java.util.*
 /**
  * Test suite for the [ComputeService] interface.
  */
-@OptIn(ExperimentalCoroutinesApi::class)
 internal class ComputeServiceTest {
-    lateinit var scope: SimulationCoroutineScope
-    lateinit var service: ComputeService
+    private lateinit var scope: SimulationCoroutineScope
+    private lateinit var service: ComputeService
 
     @BeforeEach
     fun setUp() {
@@ -128,14 +126,12 @@ internal class ComputeServiceTest {
         every { host.model } returns HostModel(4 * 2600.0, 4, 2048)
         every { host.state } returns HostState.UP
 
-        assertEquals(0, service.hostCount)
         assertEquals(emptySet<Host>(), service.hosts)
 
         service.addHost(host)
 
         verify(exactly = 1) { host.addListener(any()) }
 
-        assertEquals(1, service.hostCount)
         assertEquals(1, service.hosts.size)
 
         service.removeHost(host)
@@ -150,7 +146,6 @@ internal class ComputeServiceTest {
         every { host.model } returns HostModel(4 * 2600.0, 4, 2048)
         every { host.state } returns HostState.DOWN
 
-        assertEquals(0, service.hostCount)
         assertEquals(emptySet<Host>(), service.hosts)
 
         service.addHost(host)
diff --git a/opendc-compute/opendc-compute-service/src/test/kotlin/org/opendc/compute/service/InternalServerTest.kt b/opendc-compute/opendc-compute-service/src/test/kotlin/org/opendc/compute/service/InternalServerTest.kt
index dfd3bc67..4e5a37ae 100644
--- a/opendc-compute/opendc-compute-service/src/test/kotlin/org/opendc/compute/service/InternalServerTest.kt
+++ b/opendc-compute/opendc-compute-service/src/test/kotlin/org/opendc/compute/service/InternalServerTest.kt
@@ -23,7 +23,6 @@
 package org.opendc.compute.service
 
 import io.mockk.*
-import kotlinx.coroutines.ExperimentalCoroutinesApi
 import kotlinx.coroutines.yield
 import org.junit.jupiter.api.Assertions.*
 import org.junit.jupiter.api.Test
@@ -41,7 +40,6 @@ import java.util.*
 /**
  * Test suite for the [InternalServer] implementation.
  */
-@OptIn(ExperimentalCoroutinesApi::class)
 class InternalServerTest {
     @Test
     fun testEquality() {