7 files changed, 214 insertions, 0 deletions
diff --git a/objectstore/chain/bytes.go b/objectstore/chain/bytes.go
new file mode 100644
index 00000000..007c7522
--- /dev/null
+++ b/objectstore/chain/bytes.go
@@ -0,0 +1,54 @@
+package chain
+
+import (
+	"errors"
+	"fmt"
+
+	"codeberg.org/lindenii/furgit/objectid"
+	"codeberg.org/lindenii/furgit/objectstore"
+	"codeberg.org/lindenii/furgit/objecttype"
+)
+
+// ReadBytesFull reads a full serialized object from the first backend that has it.
+func (chain *Chain) ReadBytesFull(id objectid.ObjectID) ([]byte, error) {
+	for i, backend := range chain.backends {
+		if backend == nil {
+			continue
+		}
+
+		full, err := backend.ReadBytesFull(id)
+		if err == nil {
+			return full, nil
+		}
+
+		if errors.Is(err, objectstore.ErrObjectNotFound) {
+			continue
+		}
+
+		return nil, fmt.Errorf("objectstore: backend %d read bytes full: %w", i, err)
+	}
+
+	return nil, objectstore.ErrObjectNotFound
+}
+
+// ReadBytesContent reads an object's type and content bytes from the first backend that has it.
+func (chain *Chain) ReadBytesContent(id objectid.ObjectID) (objecttype.Type, []byte, error) {
+	for i, backend := range chain.backends {
+		if backend == nil {
+			continue
+		}
+
+		ty, content, err := backend.ReadBytesContent(id)
+		if err == nil {
+			return ty, content, nil
+		}
+
+		if errors.Is(err, objectstore.ErrObjectNotFound) {
+			continue
+		}
+
+		return objecttype.TypeInvalid, nil, fmt.Errorf("objectstore: backend %d read bytes content: %w", i, err)
+	}
+
+	return objecttype.TypeInvalid, nil, objectstore.ErrObjectNotFound
+}
diff --git a/objectstore/chain/chain.go b/objectstore/chain/chain.go
new file mode 100644
index 00000000..2b17369d
--- /dev/null
+++ b/objectstore/chain/chain.go
@@ -0,0 +1,11 @@
+// Package chain provides a wrapper object storage backend to query a chain of backends.
+package chain
+
+import (
+	"codeberg.org/lindenii/furgit/objectstore"
+)
+
+// Chain queries multiple object databases in order.
+type Chain struct {
+	backends []objectstore.Store
+}
diff --git a/objectstore/chain/close.go b/objectstore/chain/close.go
new file mode 100644
index 00000000..440afb10
--- /dev/null
+++ b/objectstore/chain/close.go
@@ -0,0 +1,21 @@
+package chain
+
+import "errors"
+
+// Close closes all backends and joins close errors.
+func (chain *Chain) Close() error {
+	var errs []error
+
+	for _, backend := range chain.backends {
+		if backend == nil {
+			continue
+		}
+
+		err := backend.Close()
+		if err != nil {
+			errs = append(errs, err)
+		}
+	}
+
+	return errors.Join(errs...)
+}
diff --git a/objectstore/chain/header.go b/objectstore/chain/header.go
new file mode 100644
index 00000000..c1d4692c
--- /dev/null
+++ b/objectstore/chain/header.go
@@ -0,0 +1,32 @@
+package chain
+
+import (
+	"errors"
+	"fmt"
+
+	"codeberg.org/lindenii/furgit/objectid"
+	"codeberg.org/lindenii/furgit/objectstore"
+	"codeberg.org/lindenii/furgit/objecttype"
+)
+
+// ReadHeader reads object header data from the first backend that has it.
+func (chain *Chain) ReadHeader(id objectid.ObjectID) (objecttype.Type, int64, error) {
+	for i, backend := range chain.backends {
+		if backend == nil {
+			continue
+		}
+
+		ty, size, err := backend.ReadHeader(id)
+		if err == nil {
+			return ty, size, nil
+		}
+
+		if errors.Is(err, objectstore.ErrObjectNotFound) {
+			continue
+		}
+
+		return objecttype.TypeInvalid, 0, fmt.Errorf("objectstore: backend %d read header: %w", i, err)
+	}
+
+	return objecttype.TypeInvalid, 0, objectstore.ErrObjectNotFound
+}
diff --git a/objectstore/chain/new.go b/objectstore/chain/new.go
new file mode 100644
index 00000000..ca00e9be
--- /dev/null
+++ b/objectstore/chain/new.go
@@ -0,0 +1,10 @@
+package chain
+
+import "codeberg.org/lindenii/furgit/objectstore"
+
+// New creates an ordered object database chain.
+func New(backends ...objectstore.Store) *Chain {
+	return &Chain{
+		backends: append([]objectstore.Store(nil), backends...),
+	}
+}
diff --git a/objectstore/chain/reader.go b/objectstore/chain/reader.go
new file mode 100644
index 00000000..e005d929
--- /dev/null
+++ b/objectstore/chain/reader.go
@@ -0,0 +1,55 @@
+package chain
+
+import (
+	"errors"
+	"fmt"
+	"io"
+
+	"codeberg.org/lindenii/furgit/objectid"
+	"codeberg.org/lindenii/furgit/objectstore"
+	"codeberg.org/lindenii/furgit/objecttype"
+)
+
+// ReadReaderFull reads a full serialized object stream from the first backend that has it.
+func (chain *Chain) ReadReaderFull(id objectid.ObjectID) (io.ReadCloser, error) {
+	for i, backend := range chain.backends {
+		if backend == nil {
+			continue
+		}
+
+		reader, err := backend.ReadReaderFull(id)
+		if err == nil {
+			return reader, nil
+		}
+
+		if errors.Is(err, objectstore.ErrObjectNotFound) {
+			continue
+		}
+
+		return nil, fmt.Errorf("objectstore: backend %d read reader full: %w", i, err)
+	}
+
+	return nil, objectstore.ErrObjectNotFound
+}
+
+// ReadReaderContent reads an object's type, declared content length, and content stream from the first backend that has it.
+func (chain *Chain) ReadReaderContent(id objectid.ObjectID) (objecttype.Type, int64, io.ReadCloser, error) {
+	for i, backend := range chain.backends {
+		if backend == nil {
+			continue
+		}
+
+		ty, size, reader, err := backend.ReadReaderContent(id)
+		if err == nil {
+			return ty, size, reader, nil
+		}
+
+		if errors.Is(err, objectstore.ErrObjectNotFound) {
+			continue
+		}
+
+		return objecttype.TypeInvalid, 0, nil, fmt.Errorf("objectstore: backend %d read reader content: %w", i, err)
+	}
+
+	return objecttype.TypeInvalid, 0, nil, objectstore.ErrObjectNotFound
+}
diff --git a/objectstore/chain/size.go b/objectstore/chain/size.go
new file mode 100644
index 00000000..38aa7abc
--- /dev/null
+++ b/objectstore/chain/size.go
@@ -0,0 +1,31 @@
+package chain
+
+import (
+	"errors"
+	"fmt"
+
+	"codeberg.org/lindenii/furgit/objectid"
+	"codeberg.org/lindenii/furgit/objectstore"
+)
+
+// ReadSize reads object content length from the first backend that has it.
+func (chain *Chain) ReadSize(id objectid.ObjectID) (int64, error) {
+	for i, backend := range chain.backends {
+		if backend == nil {
+			continue
+		}
+
+		size, err := backend.ReadSize(id)
+		if err == nil {
+			return size, nil
+		}
+
+		if errors.Is(err, objectstore.ErrObjectNotFound) {
+			continue
+		}
+
+		return 0, fmt.Errorf("objectstore: backend %d read size: %w", i, err)
+	}
+
+	return 0, objectstore.ErrObjectNotFound
+}