diff --git a/sdks/go/pkg/beam/runners/prism/internal/engine/strategy.go b/sdks/go/pkg/beam/runners/prism/internal/engine/strategy.go
new file mode 100644
index 0000000000000..44e6064958c09
--- /dev/null
+++ b/sdks/go/pkg/beam/runners/prism/internal/engine/strategy.go
@@ -0,0 +1,50 @@
+// Licensed to the Apache Software Foundation (ASF) under one or more
+// contributor license agreements.  See the NOTICE file distributed with
+// this work for additional information regarding copyright ownership.
+// The ASF licenses this file to You under the Apache License, Version 2.0
+// (the "License"); you may not use this file except in compliance with
+// the License.  You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package engine
+
+import (
+	"fmt"
+	"time"
+
+	"github.com/apache/beam/sdks/v2/go/pkg/beam/core/graph/mtime"
+	"github.com/apache/beam/sdks/v2/go/pkg/beam/core/typex"
+)
+
+type winStrat interface {
+	EarliestCompletion(typex.Window) mtime.Time
+}
+
+type defaultStrat struct{}
+
+func (ws defaultStrat) EarliestCompletion(w typex.Window) mtime.Time {
+	return w.MaxTimestamp()
+}
+
+func (defaultStrat) String() string {
+	return "default"
+}
+
+type sessionStrat struct {
+	GapSize time.Duration
+}
+
+func (ws sessionStrat) EarliestCompletion(w typex.Window) mtime.Time {
+	return w.MaxTimestamp().Add(ws.GapSize)
+}
+
+func (ws sessionStrat) String() string {
+	return fmt.Sprintf("session[GapSize:%v]", ws.GapSize)
+}
diff --git a/sdks/go/pkg/beam/runners/prism/internal/engine/strategy_test.go b/sdks/go/pkg/beam/runners/prism/internal/engine/strategy_test.go
new file mode 100644
index 0000000000000..9d558396f8067
--- /dev/null
+++ b/sdks/go/pkg/beam/runners/prism/internal/engine/strategy_test.go
@@ -0,0 +1,45 @@
+// Licensed to the Apache Software Foundation (ASF) under one or more
+// contributor license agreements.  See the NOTICE file distributed with
+// this work for additional information regarding copyright ownership.
+// The ASF licenses this file to You under the Apache License, Version 2.0
+// (the "License"); you may not use this file except in compliance with
+// the License.  You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package engine
+
+import (
+	"testing"
+	"time"
+
+	"github.com/apache/beam/sdks/v2/go/pkg/beam/core/graph/mtime"
+	"github.com/apache/beam/sdks/v2/go/pkg/beam/core/graph/window"
+	"github.com/apache/beam/sdks/v2/go/pkg/beam/core/typex"
+)
+
+func TestEarliestCompletion(t *testing.T) {
+	tests := []struct {
+		strat winStrat
+		input typex.Window
+		want  mtime.Time
+	}{
+		{defaultStrat{}, window.GlobalWindow{}, mtime.EndOfGlobalWindowTime},
+		{defaultStrat{}, window.IntervalWindow{Start: 0, End: 4}, 3},
+		{defaultStrat{}, window.IntervalWindow{Start: mtime.MinTimestamp, End: mtime.MaxTimestamp}, mtime.MaxTimestamp - 1},
+		{sessionStrat{}, window.IntervalWindow{Start: 0, End: 4}, 3},
+		{sessionStrat{GapSize: 3 * time.Millisecond}, window.IntervalWindow{Start: 0, End: 4}, 6},
+	}
+
+	for _, test := range tests {
+		if got, want := test.strat.EarliestCompletion(test.input), test.want; got != want {
+			t.Errorf("%v.EarliestCompletion(%v)) = %v, want %v", test.strat, test.input, got, want)
+		}
+	}
+}