sumdb/internal/tkv: interface for transaction key-value store
This is the underlying storage interface for a Go checksum database.
Eventually the code will live outside golang.org/x/exp.
The interface is designed to be implementable by a variety
of storage options, including Google Cloud Spanner and a
single-file key-value database.
Change-Id: I7bd33711eec94b30196f998fb1003ef91e7e8d55
Reviewed-on: https://go-review.googlesource.com/c/exp/+/161662
Reviewed-by: Filippo Valsorda <filippo@golang.org>
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
+}
