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diff --git a/sumdb/internal/tkv/tkv.go b/sumdb/internal/tkv/tkv.go
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+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package tkv defines a simple interface to a
+// transactional key-value storage system.
+package tkv
+
+import "context"
+
+// A Storage is a transaction key-value storage system.
+type Storage interface {
+	// ReadOnly runs f in a read-only transaction.
+	// It is equivalent to ReadWrite except that the
+	// transaction's BufferWrite method will fail unconditionally.
+	// (The implementation may be able to optimize the
+	// transaction if it knows at the start that no writes will happen.)
+	ReadOnly(ctx context.Context, f func(context.Context, Transaction) error) error
+
+	// ReadWrite runs f in a read-write transaction.
+	// If f returns an error, the transaction aborts and returns that error.
+	// If f returns nil, the transaction attempts to commit and then then return nil.
+	// Otherwise it tries again. Note that f may be called multiple times and that
+	// the result only describes the effect of the final call to f.
+	// The caller must take care not to use any state computed during
+	// earlier calls to f, or even the last call to f when an error is returned.
+	ReadWrite(ctx context.Context, f func(context.Context, Transaction) error) error
+}
+
+// A Transaction provides read and write operations within a transaction,
+// as executed by Storage's ReadOnly or ReadWrite methods.
+type Transaction interface {
+	// ReadValue reads the value associated with a single key.
+	// If there is no value associated with that key, ReadKey returns an empty value.
+	// An error is only returned for problems accessing the storage.
+	ReadValue(ctx context.Context, key string) (value string, err error)
+
+	// ReadValues reads the values associated with the given keys.
+	// If there is no value stored for a given key, ReadValues returns an empty value for that key.
+	// An error is only returned for problems accessing the storage.
+	ReadValues(ctx context.Context, keys []string) (values []string, err error)
+
+	// BufferWrites buffers the given writes,
+	// to be applied at the end of the transaction.
+	// BufferWrites panics if this is a ReadOnly transaction.
+	// It returns an error if it detects any other problems.
+	// The behavior of multiple writes buffered using the same key
+	// is undefined: it may return an error or not.
+	BufferWrites(writes []Write) error
+}
+
+// A Write is a single change to be applied at the end of a read-write transaction.
+// A Write with an empty value deletes the value associated with the given key.
+type Write struct {
+	Key   string
+	Value string
+}