Add transaction buffer filter.

filter/txnBuf/doc.go

Index: filter/txnBuf/doc.go
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+// Copyright 2015 The Chromium 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 txnBuf contains a transaction buffer filter for the datastore
+// service.
+//
+// By default, datastore transactions take a snapshot of the entity group as
+// soon as you Get or Put into it. All subsequent Get (and query) operations
+// reflect the state of the ORIGINAL transaction snapshot, regardless of any
+// Put/Delete operations you've done since the beginning of the transaction.
+//
+// If you've installed this transaction buffer, then:
+//   - All mutations will be reflected in all read operations (see LIMITATIONS).
+//     Without the buffer, read operations always observe the state of the
+//     entity group(s) at the time that the transaction started.
+//
+//   - All mutation operations will be buffered until the close of the
+//     transaction. This can help reduce the transaction size, and thus avoid
+//     the transaction size limit (currently 10MB). Multiple puts to the same
+//     entity will not increase the transaction size multiple times.
+//
+//   - Transactions inside of an existing transaction will add to their outer
+//     transaction if they don't cause the outer transaction to exceed its
+//     size budget.
+//
+//   - If an inner transaction would cause the OUTERMOST transaction to exceed
+//     the appengine-imposed 10MB transaction size limit, an error will be
+//     returned from the inner transaction, instead of adding it into the
+//     outermost transaction. This only applies to the first level of inner
+//     transactions, and does not apply to recursive transactions. The reason
+//     for this is that it's entirely feasible for inner transactions to
+//     temporarially exceed the limit, but still only commit an outer
+//     transaction which is under the limit. An example of this would be having
+//     one inner-inner transaction add a lot of large entities and then having
+//     a subsequent inner-inner transaction delete some of those entities.
+//
+// LIMITATIONS (only inside of a transaction)
+//   - KeysOnly/Projection/Count queries are supported, but may incur additional
+//     costs.
+//
+//     These query types are implemented via projection queries, but will
+//     project all order-by fields in addition to any specified in the original
+//     query.
+//
+//   - Distinct Projection queries do all 'distinct' deduplication in-memory.
+//     This could make them substantially more expensive than their native
+//     equivalent.
+//
+//   - Metadata entities (e.g. `__entity_group__`) will reflect their values as
+//     they were at the beginning of the transaction, and will not increment
+//     as you write inside of the transaction.
+//
+//   - Query cursors are not supported. Since the cursor format for the
+//     in-memory datastore implementation isn't compatible with the production
+//     cursors, it would be pretty tricky to make it so that cursors were
+//     viable outside the transaction as well as inside of it while also having
+//     it accurately reflect the 'merged' query results.
+//
+//   - No parallel access to datastore while in a transaction; all nested
+//     transactions are serialized. This is done for simplicity and correctness.
+//
+//   - The changing of namespace inside of a transaction is undefined... This is
+//     just generally a terrible idea anyway, but I thought it was worth
+//     mentioning.
+package txnBuf