docs: add documentation for prebuild scheduling feature (#18462)

Follow-up to https://github.com/coder/coder/pull/18126

Changes:
- address issue mentioned here:
https://github.com/coder/coder/pull/18126#discussion_r2144557600
- add docs for prebuilds scheduling

---------

Co-authored-by: Danny Kopping <danny@coder.com>
Co-authored-by: Atif Ali <atif@coder.com>

coderd/prebuilds/preset_snapshot.go

@@ -267,14 +267,14 @@ func (p PresetSnapshot) CalculateState() *ReconciliationState {
 // - ActionTypeBackoff: Only BackoffUntil is set, indicating when to retry
 // - ActionTypeCreate: Only Create is set, indicating how many prebuilds to create
 // - ActionTypeDelete: Only DeleteIDs is set, containing IDs of prebuilds to delete
-func (p PresetSnapshot) CalculateActions(clock quartz.Clock, backoffInterval time.Duration) ([]*ReconciliationActions, error) {
+func (p PresetSnapshot) CalculateActions(backoffInterval time.Duration) ([]*ReconciliationActions, error) {
 	// TODO: align workspace states with how we represent them on the FE and the CLI
 	//	     right now there's some slight differences which can lead to additional prebuilds being created
 
 	// TODO: add mechanism to prevent prebuilds being reconciled from being claimable by users; i.e. if a prebuild is
 	// 		 about to be deleted, it should not be deleted if it has been claimed - beware of TOCTOU races!
 
-	actions, needsBackoff := p.needsBackoffPeriod(clock, backoffInterval)
+	actions, needsBackoff := p.needsBackoffPeriod(p.clock, backoffInterval)
 	if needsBackoff {
 		return actions, nil
 	}

coderd/prebuilds/preset_snapshot_test.go

@@ -86,12 +86,12 @@ func TestNoPrebuilds(t *testing.T) {
 		preset(true, 0, current),
 	}
 
-	snapshot := prebuilds.NewGlobalSnapshot(presets, nil, nil, nil, nil, nil, quartz.NewMock(t), testutil.Logger(t))
+	snapshot := prebuilds.NewGlobalSnapshot(presets, nil, nil, nil, nil, nil, clock, testutil.Logger(t))
 	ps, err := snapshot.FilterByPreset(current.presetID)
 	require.NoError(t, err)
 
 	state := ps.CalculateState()
-	actions, err := ps.CalculateActions(clock, backoffInterval)
+	actions, err := ps.CalculateActions(backoffInterval)
 	require.NoError(t, err)
 
 	validateState(t, prebuilds.ReconciliationState{ /*all zero values*/ }, *state)
@@ -108,12 +108,12 @@ func TestNetNew(t *testing.T) {
 		preset(true, 1, current),
 	}
 
-	snapshot := prebuilds.NewGlobalSnapshot(presets, nil, nil, nil, nil, nil, quartz.NewMock(t), testutil.Logger(t))
+	snapshot := prebuilds.NewGlobalSnapshot(presets, nil, nil, nil, nil, nil, clock, testutil.Logger(t))
 	ps, err := snapshot.FilterByPreset(current.presetID)
 	require.NoError(t, err)
 
 	state := ps.CalculateState()
-	actions, err := ps.CalculateActions(clock, backoffInterval)
+	actions, err := ps.CalculateActions(backoffInterval)
 	require.NoError(t, err)
 
 	validateState(t, prebuilds.ReconciliationState{
@@ -156,7 +156,7 @@ func TestOutdatedPrebuilds(t *testing.T) {
 
 	// THEN: we should identify that this prebuild is outdated and needs to be deleted.
 	state := ps.CalculateState()
-	actions, err := ps.CalculateActions(clock, backoffInterval)
+	actions, err := ps.CalculateActions(backoffInterval)
 	require.NoError(t, err)
 	validateState(t, prebuilds.ReconciliationState{
 		Actual: 1,
@@ -174,7 +174,7 @@ func TestOutdatedPrebuilds(t *testing.T) {
 
 	// THEN: we should not be blocked from creating a new prebuild while the outdate one deletes.
 	state = ps.CalculateState()
-	actions, err = ps.CalculateActions(clock, backoffInterval)
+	actions, err = ps.CalculateActions(backoffInterval)
 	require.NoError(t, err)
 	validateState(t, prebuilds.ReconciliationState{Desired: 1}, *state)
 	validateActions(t, []*prebuilds.ReconciliationActions{
@@ -223,7 +223,7 @@ func TestDeleteOutdatedPrebuilds(t *testing.T) {
 	// THEN: we should identify that this prebuild is outdated and needs to be deleted.
 	// Despite the fact that deletion of another outdated prebuild is already in progress.
 	state := ps.CalculateState()
-	actions, err := ps.CalculateActions(clock, backoffInterval)
+	actions, err := ps.CalculateActions(backoffInterval)
 	require.NoError(t, err)
 	validateState(t, prebuilds.ReconciliationState{
 		Actual:   1,
@@ -467,7 +467,7 @@ func TestInProgressActions(t *testing.T) {
 
 			// THEN: we should identify that this prebuild is in progress.
 			state := ps.CalculateState()
-			actions, err := ps.CalculateActions(clock, backoffInterval)
+			actions, err := ps.CalculateActions(backoffInterval)
 			require.NoError(t, err)
 			tc.checkFn(*state, actions)
 		})
@@ -510,7 +510,7 @@ func TestExtraneous(t *testing.T) {
 
 	// THEN: an extraneous prebuild is detected and marked for deletion.
 	state := ps.CalculateState()
-	actions, err := ps.CalculateActions(clock, backoffInterval)
+	actions, err := ps.CalculateActions(backoffInterval)
 	require.NoError(t, err)
 	validateState(t, prebuilds.ReconciliationState{
 		Actual: 2, Desired: 1, Extraneous: 1, Eligible: 2,
@@ -685,13 +685,13 @@ func TestExpiredPrebuilds(t *testing.T) {
 			}
 
 			// WHEN: calculating the current preset's state.
-			snapshot := prebuilds.NewGlobalSnapshot(presets, nil, running, nil, nil, nil, quartz.NewMock(t), testutil.Logger(t))
+			snapshot := prebuilds.NewGlobalSnapshot(presets, nil, running, nil, nil, nil, clock, testutil.Logger(t))
 			ps, err := snapshot.FilterByPreset(current.presetID)
 			require.NoError(t, err)
 
 			// THEN: we should identify that this prebuild is expired.
 			state := ps.CalculateState()
-			actions, err := ps.CalculateActions(clock, backoffInterval)
+			actions, err := ps.CalculateActions(backoffInterval)
 			require.NoError(t, err)
 			tc.checkFn(running, *state, actions)
 		})
@@ -727,7 +727,7 @@ func TestDeprecated(t *testing.T) {
 
 	// THEN: all running prebuilds should be deleted because the template is deprecated.
 	state := ps.CalculateState()
-	actions, err := ps.CalculateActions(clock, backoffInterval)
+	actions, err := ps.CalculateActions(backoffInterval)
 	require.NoError(t, err)
 	validateState(t, prebuilds.ReconciliationState{
 		Actual: 1,
@@ -774,13 +774,13 @@ func TestLatestBuildFailed(t *testing.T) {
 	}
 
 	// WHEN: calculating the current preset's state.
-	snapshot := prebuilds.NewGlobalSnapshot(presets, nil, running, inProgress, backoffs, nil, quartz.NewMock(t), testutil.Logger(t))
+	snapshot := prebuilds.NewGlobalSnapshot(presets, nil, running, inProgress, backoffs, nil, clock, testutil.Logger(t))
 	psCurrent, err := snapshot.FilterByPreset(current.presetID)
 	require.NoError(t, err)
 
 	// THEN: reconciliation should backoff.
 	state := psCurrent.CalculateState()
-	actions, err := psCurrent.CalculateActions(clock, backoffInterval)
+	actions, err := psCurrent.CalculateActions(backoffInterval)
 	require.NoError(t, err)
 	validateState(t, prebuilds.ReconciliationState{
 		Actual: 0, Desired: 1,
@@ -798,7 +798,7 @@ func TestLatestBuildFailed(t *testing.T) {
 
 	// THEN: it should NOT be in backoff because all is OK.
 	state = psOther.CalculateState()
-	actions, err = psOther.CalculateActions(clock, backoffInterval)
+	actions, err = psOther.CalculateActions(backoffInterval)
 	require.NoError(t, err)
 	validateState(t, prebuilds.ReconciliationState{
 		Actual: 1, Desired: 1, Eligible: 1,
@@ -812,7 +812,7 @@ func TestLatestBuildFailed(t *testing.T) {
 	psCurrent, err = snapshot.FilterByPreset(current.presetID)
 	require.NoError(t, err)
 	state = psCurrent.CalculateState()
-	actions, err = psCurrent.CalculateActions(clock, backoffInterval)
+	actions, err = psCurrent.CalculateActions(backoffInterval)
 	require.NoError(t, err)
 	validateState(t, prebuilds.ReconciliationState{
 		Actual: 0, Desired: 1,
@@ -867,7 +867,7 @@ func TestMultiplePresetsPerTemplateVersion(t *testing.T) {
 		},
 	}
 
-	snapshot := prebuilds.NewGlobalSnapshot(presets, nil, nil, inProgress, nil, nil, quartz.NewMock(t), testutil.Logger(t))
+	snapshot := prebuilds.NewGlobalSnapshot(presets, nil, nil, inProgress, nil, nil, clock, testutil.Logger(t))
 
 	// Nothing has to be created for preset 1.
 	{
@@ -875,7 +875,7 @@ func TestMultiplePresetsPerTemplateVersion(t *testing.T) {
 		require.NoError(t, err)
 
 		state := ps.CalculateState()
-		actions, err := ps.CalculateActions(clock, backoffInterval)
+		actions, err := ps.CalculateActions(backoffInterval)
 		require.NoError(t, err)
 
 		validateState(t, prebuilds.ReconciliationState{
@@ -891,7 +891,7 @@ func TestMultiplePresetsPerTemplateVersion(t *testing.T) {
 		require.NoError(t, err)
 
 		state := ps.CalculateState()
-		actions, err := ps.CalculateActions(clock, backoffInterval)
+		actions, err := ps.CalculateActions(backoffInterval)
 		require.NoError(t, err)
 
 		validateState(t, prebuilds.ReconciliationState{
@@ -995,7 +995,7 @@ func TestPrebuildScheduling(t *testing.T) {
 				require.NoError(t, err)
 
 				state := ps.CalculateState()
-				actions, err := ps.CalculateActions(clock, backoffInterval)
+				actions, err := ps.CalculateActions(backoffInterval)
 				require.NoError(t, err)
 
 				validateState(t, prebuilds.ReconciliationState{
@@ -1016,7 +1016,7 @@ func TestPrebuildScheduling(t *testing.T) {
 				require.NoError(t, err)
 
 				state := ps.CalculateState()
-				actions, err := ps.CalculateActions(clock, backoffInterval)
+				actions, err := ps.CalculateActions(backoffInterval)
 				require.NoError(t, err)
 
 				validateState(t, prebuilds.ReconciliationState{

docs/admin/templates/extending-templates/prebuilt-workspaces.md

@@ -12,6 +12,7 @@ Prebuilt workspaces are:
 - Created and maintained automatically by Coder to match your specified preset configurations.
 - Claimed transparently when developers create workspaces.
 - Monitored and replaced automatically to maintain your desired pool size.
+- Automatically scaled based on time-based schedules to optimize resource usage.
 
 ## Relationship to workspace presets
 
@@ -111,6 +112,105 @@ prebuilt workspace can remain before it is considered expired and eligible for c
 Expired prebuilt workspaces are removed during the reconciliation loop to avoid stale environments and resource waste.
 New prebuilt workspaces are only created to maintain the desired count if needed.
 
+### Scheduling
+
+Prebuilt workspaces support time-based scheduling to scale the number of instances up or down.
+This allows you to reduce resource costs during off-hours while maintaining availability during peak usage times.
+
+Configure scheduling by adding a `scheduling` block within your `prebuilds` configuration:
+
+```tf
+data "coder_workspace_preset" "goland" {
+   name = "GoLand: Large"
+   parameters {
+     jetbrains_ide = "GO"
+     cpus          = 8
+     memory        = 16
+   }
+
+   prebuilds {
+     instances = 0                  # default to 0 instances
+
+     scheduling {
+       timezone = "UTC"             # only a single timezone may be used for simplicity
+
+       # scale to 3 instances during the work week
+       schedule {
+         cron = "* 8-18 * * 1-5"    # from 8AM-6:59PM, Mon-Fri, UTC
+         instances = 3              # scale to 3 instances
+       }
+
+       # scale to 1 instance on Saturdays for urgent support queries
+       schedule {
+         cron = "* 8-14 * * 6"      # from 8AM-2:59PM, Sat, UTC
+         instances = 1              # scale to 1 instance
+       }
+     }
+   }
+}
+```
+
+**Scheduling configuration:**
+
+- **`timezone`**: The timezone for all cron expressions (required). Only a single timezone is supported per scheduling configuration.
+- **`schedule`**: One or more schedule blocks defining when to scale to specific instance counts.
+  - **`cron`**: Cron expression interpreted as continuous time ranges (required).
+  - **`instances`**: Number of prebuilt workspaces to maintain during this schedule (required).
+
+**How scheduling works:**
+
+1. The reconciliation loop evaluates all active schedules every reconciliation interval (`CODER_WORKSPACE_PREBUILDS_RECONCILIATION_INTERVAL`).
+2. The schedule that matches the current time becomes active. Overlapping schedules are disallowed by validation rules.
+3. If no schedules match the current time, the base `instances` count is used.
+4. The reconciliation loop automatically creates or destroys prebuilt workspaces to match the target count.
+
+**Cron expression format:**
+
+Cron expressions follow the format: `* HOUR DOM MONTH DAY-OF-WEEK`
+
+- `*` (minute): Must always be `*` to ensure the schedule covers entire hours rather than specific minute intervals
+- `HOUR`: 0-23, range (e.g., 8-18 for 8AM-6:59PM), or `*`
+- `DOM` (day-of-month): 1-31, range, or `*`
+- `MONTH`: 1-12, range, or `*`
+- `DAY-OF-WEEK`: 0-6 (Sunday=0, Saturday=6), range (e.g., 1-5 for Monday to Friday), or `*`
+
+**Important notes about cron expressions:**
+
+- **Minutes must always be `*`**: To ensure the schedule covers entire hours
+- **Time ranges are continuous**: A range like `8-18` means from 8AM to 6:59PM (inclusive of both start and end hours)
+- **Weekday ranges**: `1-5` means Monday through Friday (Monday=1, Friday=5)
+- **No overlapping schedules**: The validation system prevents overlapping schedules.
+
+**Example schedules:**
+
+```tf
+# Business hours only (8AM-6:59PM, Mon-Fri)
+schedule {
+  cron = "* 8-18 * * 1-5"
+  instances = 5
+}
+
+# 24/7 coverage with reduced capacity overnight and on weekends
+schedule {
+  cron = "* 8-18 * * 1-5"  # Business hours (8AM-6:59PM, Mon-Fri)
+  instances = 10
+}
+schedule {
+  cron = "* 19-23,0-7 * * 1,5"  # Evenings and nights (7PM-11:59PM, 12AM-7:59AM, Mon-Fri)
+  instances = 2
+}
+schedule {
+  cron = "* * * * 6,0"  # Weekends
+  instances = 2
+}
+
+# Weekend support (10AM-4:59PM, Sat-Sun)
+schedule {
+  cron = "* 10-16 * * 6,0"
+  instances = 1
+}
+```
+
 ### Template updates and the prebuilt workspace lifecycle
 
 Prebuilt workspaces are not updated after they are provisioned.
@@ -195,12 +295,6 @@ The prebuilt workspaces feature has these current limitations:
 
   [View issue](https://github.com/coder/internal/issues/364)
 
-- **Autoscaling**
-
-  Prebuilt workspaces remain running until claimed. There's no automated mechanism to reduce instances during off-hours.
-
-  [View issue](https://github.com/coder/internal/issues/312)
-
 ### Monitoring and observability
 
 #### Available metrics

enterprise/coderd/prebuilds/reconcile.go

@@ -518,7 +518,7 @@ func (c *StoreReconciler) CalculateActions(ctx context.Context, snapshot prebuil
 		return nil, ctx.Err()
 	}
 
-	return snapshot.CalculateActions(c.clock, c.cfg.ReconciliationBackoffInterval.Value())
+	return snapshot.CalculateActions(c.cfg.ReconciliationBackoffInterval.Value())
 }
 
 func (c *StoreReconciler) WithReconciliationLock(