diff --git a/go.mod b/go.mod
index 41b6df0..e95b389 100644
--- a/go.mod
+++ b/go.mod
@@ -1,6 +1,6 @@
 module github.com/ClusterCockpit/cc-metric-store
 
-go 1.16
+go 1.18
 
 require (
 	github.com/golang-jwt/jwt/v4 v4.0.0
diff --git a/internal/checkpoints/checkpoints.go b/internal/checkpoints/checkpoints.go
new file mode 100644
index 0000000..4830baf
--- /dev/null
+++ b/internal/checkpoints/checkpoints.go
@@ -0,0 +1,134 @@
+package checkpoints
+
+import (
+	"encoding/binary"
+	"io"
+	"reflect"
+	"unsafe"
+
+	"github.com/ClusterCockpit/cc-metric-store/internal/types"
+)
+
+const writeBufferSize int = 8192
+
+// A single checkpoint file is defined by this structure.
+type Checkpoint struct {
+	Reserved []byte           // Reserved for future use/metadata.
+	From, To uint64           // Time covered by this checkpoint.
+	Root     *CheckpointLevel // Root of the internal tree structure.
+}
+
+// A single level in the tree of the in-memory store is defined by this structure.
+type CheckpointLevel struct {
+	Reserved  []byte                      // Reserved for future use/metadata.
+	Metrics   map[string]*Metrics         // Payload...
+	Sublevels map[string]*CheckpointLevel // Child in the internal tree structure.
+}
+
+// Payload/Data
+type Metrics struct {
+	Reserved  []byte        // Reserved for future use/metadata.
+	Frequency uint64        // Frequency of measurements for this metric.
+	From      uint64        // Timestamp of the first measurement of this metric.
+	Data      []types.Float // Payload...
+}
+
+func (c *Checkpoint) Serialize(w io.Writer) error {
+	// Write magic value (8 byte):
+	if _, err := w.Write([]byte("CCMSv1.0")); err != nil {
+		return err
+	}
+
+	// The header:
+	buf := make([]byte, 0, writeBufferSize*2)
+	buf = encodeBytes(buf, c.Reserved)
+	buf = encodeUint64(buf, c.From)
+	buf = encodeUint64(buf, c.To)
+
+	// The rest:
+	buf, err := c.Root.serialize(w, buf)
+	if err != nil {
+		return err
+	}
+
+	if _, err := w.Write(buf); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+func (c *CheckpointLevel) serialize(w io.Writer, buf []byte) ([]byte, error) {
+	// The reserved data:
+	buf = encodeBytes(buf, c.Reserved)
+
+	var err error
+	// metrics:
+	buf = encodeUint32(buf, uint32(len(c.Metrics)))
+	for key, metric := range c.Metrics {
+		buf = encodeString(buf, key)
+		buf, err = metric.serialize(w, buf)
+		if err != nil {
+			return nil, err
+		}
+
+		if len(buf) >= writeBufferSize {
+			if _, err = w.Write(buf); err != nil {
+				return nil, err
+			}
+
+			buf = buf[0:]
+		}
+	}
+
+	// Sublevels:
+	buf = encodeUint32(buf, uint32(len(c.Sublevels)))
+	for key, sublevel := range c.Sublevels {
+		buf = encodeString(buf, key)
+		buf, err = sublevel.serialize(w, buf)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	return buf, nil
+}
+
+func (m *Metrics) serialize(w io.Writer, buf []byte) ([]byte, error) {
+	buf = encodeBytes(m.Reserved, buf)           // Reserved
+	buf = encodeUint64(buf, m.Frequency)         // Frequency
+	buf = encodeUint64(buf, m.From)              // First measurmenet timestamp
+	buf = encodeUint32(buf, uint32(len(m.Data))) // Number of elements
+
+	var x types.Float
+	elmsize := unsafe.Sizeof(x)
+	sh := (*reflect.SliceHeader)(unsafe.Pointer(&m.Data))
+	bytes := unsafe.Slice((*byte)(unsafe.Pointer(sh.Data)), sh.Len*int(elmsize))
+
+	buf = append(buf, bytes...)
+	return buf, nil
+}
+
+func encodeBytes(buf []byte, bytes []byte) []byte {
+	buf = encodeUint32(buf, uint32(len(bytes)))
+	buf = append(buf, bytes...)
+	return buf
+}
+
+func encodeString(buf []byte, str string) []byte {
+	buf = encodeUint32(buf, uint32(len(str)))
+	buf = append(buf, str...)
+	return buf
+}
+
+func encodeUint64(buf []byte, i uint64) []byte {
+	buf = append(buf, 0, 0, 0, 0, 0, 0, 0, 0) // 8 bytes
+	binary.LittleEndian.PutUint64(buf, i)
+	return buf[8:]
+}
+
+func encodeUint32(buf []byte, i uint32) []byte {
+	buf = append(buf, 0, 0, 0, 0) // 4 bytes
+	binary.LittleEndian.PutUint32(buf, i)
+	return buf[4:]
+}
diff --git a/internal/types/float.go b/internal/types/float.go
new file mode 100644
index 0000000..a8c52ad
--- /dev/null
+++ b/internal/types/float.go
@@ -0,0 +1,63 @@
+package types
+
+import (
+	"math"
+	"strconv"
+)
+
+// Go's JSON encoder for floats does not support NaN (https://github.com/golang/go/issues/3480).
+// This program uses NaN as a signal for missing data.
+// For the HTTP JSON API to be able to handle NaN values,
+// we have to use our own type which implements encoding/json.Marshaler itself.
+type Float float64
+
+var NaN Float = Float(math.NaN())
+var nullAsBytes []byte = []byte("null")
+
+func (f Float) IsNaN() bool {
+	return math.IsNaN(float64(f))
+}
+
+func (f Float) MarshalJSON() ([]byte, error) {
+	if math.IsNaN(float64(f)) {
+		return nullAsBytes, nil
+	}
+
+	return strconv.AppendFloat(make([]byte, 0, 10), float64(f), 'f', 3, 64), nil
+}
+
+func (f *Float) UnmarshalJSON(input []byte) error {
+	if string(input) == "null" {
+		*f = NaN
+		return nil
+	}
+
+	val, err := strconv.ParseFloat(string(input), 64)
+	if err != nil {
+		return err
+	}
+	*f = Float(val)
+	return nil
+}
+
+// Same as `[]Float`, but can be marshaled to JSON with less allocations.
+type FloatArray []Float
+
+func (fa FloatArray) MarshalJSON() ([]byte, error) {
+	buf := make([]byte, 0, 2+len(fa)*8)
+	buf = append(buf, '[')
+	for i := 0; i < len(fa); i++ {
+		if i != 0 {
+			buf = append(buf, ',')
+		}
+
+		if fa[i].IsNaN() {
+			buf = append(buf, `null`...)
+		} else {
+			buf = strconv.AppendFloat(buf, float64(fa[i]), 'f', 3, 64)
+
+		}
+	}
+	buf = append(buf, ']')
+	return buf, nil
+}