feat(compute): Add support for affinity scheduling (#101)

This change adds support for (anti-)affinity scheduling of servers onto hosts, 
which happens at the compute service level. 
In the future, we might add support for server groups, which also enables 
soft (anti-)affinity scheduling.

Implements #26

## Implementation Notes :hammer_and_pick:

* Add `DifferentHostFilter` to schedule instances on different hosts from a set of instances.
* Add `SameHostFilter` to schedule instances on the same hosts as a set of instances.

1 files changed, 78 insertions, 0 deletions

diff --git a/opendc-compute/opendc-compute-service/src/test/kotlin/org/opendc/compute/service/scheduler/FilterSchedulerTest.kt b/opendc-compute/opendc-compute-service/src/test/kotlin/org/opendc/compute/service/scheduler/FilterSchedulerTest.kt
index 3f2ce43b..350ac944 100644
--- a/opendc-compute/opendc-compute-service/src/test/kotlin/org/opendc/compute/service/scheduler/FilterSchedulerTest.kt
+++ b/opendc-compute/opendc-compute-service/src/test/kotlin/org/opendc/compute/service/scheduler/FilterSchedulerTest.kt
@@ -323,6 +323,84 @@ internal class FilterSchedulerTest {
     }
 
     @Test
+    fun testAffinityFilter() {
+        val scheduler = FilterScheduler(
+            filters = listOf(SameHostFilter()),
+            weighers = emptyList(),
+        )
+
+        val serverA = mockk<Server>()
+        every { serverA.uid } returns UUID.randomUUID()
+        every { serverA.flavor.cpuCount } returns 2
+        every { serverA.flavor.memorySize } returns 1024
+
+        val hostA = mockk<HostView>()
+        every { hostA.host.state } returns HostState.UP
+        every { hostA.host.model } returns HostModel(4 * 2600.0, 4, 2048)
+        every { hostA.host.instances } returns emptySet()
+        every { hostA.provisionedCores } returns 3
+
+        val hostB = mockk<HostView>()
+        every { hostB.host.state } returns HostState.UP
+        every { hostB.host.model } returns HostModel(4 * 2600.0, 4, 2048)
+        every { hostB.host.instances } returns setOf(serverA)
+        every { hostB.provisionedCores } returns 0
+
+        scheduler.addHost(hostA)
+        scheduler.addHost(hostB)
+
+        val serverB = mockk<Server>()
+        every { serverB.flavor.cpuCount } returns 2
+        every { serverB.flavor.memorySize } returns 1024
+        every { serverB.meta } returns emptyMap()
+
+        assertEquals(hostA, scheduler.select(serverB))
+
+        every { serverB.meta } returns mapOf("scheduler_hint:same_host" to setOf(serverA.uid))
+
+        assertEquals(hostB, scheduler.select(serverB))
+    }
+
+    @Test
+    fun testAntiAffinityFilter() {
+        val scheduler = FilterScheduler(
+            filters = listOf(DifferentHostFilter()),
+            weighers = emptyList(),
+        )
+
+        val serverA = mockk<Server>()
+        every { serverA.uid } returns UUID.randomUUID()
+        every { serverA.flavor.cpuCount } returns 2
+        every { serverA.flavor.memorySize } returns 1024
+
+        val hostA = mockk<HostView>()
+        every { hostA.host.state } returns HostState.UP
+        every { hostA.host.model } returns HostModel(4 * 2600.0, 4, 2048)
+        every { hostA.host.instances } returns setOf(serverA)
+        every { hostA.provisionedCores } returns 3
+
+        val hostB = mockk<HostView>()
+        every { hostB.host.state } returns HostState.UP
+        every { hostB.host.model } returns HostModel(4 * 2600.0, 4, 2048)
+        every { hostB.host.instances } returns emptySet()
+        every { hostB.provisionedCores } returns 0
+
+        scheduler.addHost(hostA)
+        scheduler.addHost(hostB)
+
+        val serverB = mockk<Server>()
+        every { serverB.flavor.cpuCount } returns 2
+        every { serverB.flavor.memorySize } returns 1024
+        every { serverB.meta } returns emptyMap()
+
+        assertEquals(hostA, scheduler.select(serverB))
+
+        every { serverB.meta } returns mapOf("scheduler_hint:different_host" to setOf(serverA.uid))
+
+        assertEquals(hostB, scheduler.select(serverB))
+    }
+
+    @Test
     fun testRamWeigher() {
         val scheduler = FilterScheduler(
             filters = emptyList(),