88 lines
3.2 KiB
Go
88 lines
3.2 KiB
Go
// Copyright 2011 Google Inc. All rights reserved.
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// Use of this source code is governed by the Apache 2.0
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// license that can be found in the LICENSE file.
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package datastore
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import (
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"errors"
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"golang.org/x/net/context"
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"google.golang.org/appengine/internal"
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pb "google.golang.org/appengine/internal/datastore"
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)
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func init() {
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internal.RegisterTransactionSetter(func(x *pb.Query, t *pb.Transaction) {
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x.Transaction = t
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})
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internal.RegisterTransactionSetter(func(x *pb.GetRequest, t *pb.Transaction) {
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x.Transaction = t
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})
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internal.RegisterTransactionSetter(func(x *pb.PutRequest, t *pb.Transaction) {
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x.Transaction = t
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})
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internal.RegisterTransactionSetter(func(x *pb.DeleteRequest, t *pb.Transaction) {
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x.Transaction = t
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})
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}
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// ErrConcurrentTransaction is returned when a transaction is rolled back due
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// to a conflict with a concurrent transaction.
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var ErrConcurrentTransaction = errors.New("datastore: concurrent transaction")
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// RunInTransaction runs f in a transaction. It calls f with a transaction
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// context tc that f should use for all App Engine operations.
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//
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// If f returns nil, RunInTransaction attempts to commit the transaction,
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// returning nil if it succeeds. If the commit fails due to a conflicting
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// transaction, RunInTransaction retries f, each time with a new transaction
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// context. It gives up and returns ErrConcurrentTransaction after three
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// failed attempts. The number of attempts can be configured by specifying
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// TransactionOptions.Attempts.
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//
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// If f returns non-nil, then any datastore changes will not be applied and
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// RunInTransaction returns that same error. The function f is not retried.
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//
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// Note that when f returns, the transaction is not yet committed. Calling code
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// must be careful not to assume that any of f's changes have been committed
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// until RunInTransaction returns nil.
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//
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// Since f may be called multiple times, f should usually be idempotent.
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// datastore.Get is not idempotent when unmarshaling slice fields.
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//
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// Nested transactions are not supported; c may not be a transaction context.
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func RunInTransaction(c context.Context, f func(tc context.Context) error, opts *TransactionOptions) error {
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xg := false
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if opts != nil {
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xg = opts.XG
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}
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attempts := 3
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if opts != nil && opts.Attempts > 0 {
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attempts = opts.Attempts
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}
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for i := 0; i < attempts; i++ {
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if err := internal.RunTransactionOnce(c, f, xg); err != internal.ErrConcurrentTransaction {
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return err
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}
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}
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return ErrConcurrentTransaction
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}
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// TransactionOptions are the options for running a transaction.
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type TransactionOptions struct {
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// XG is whether the transaction can cross multiple entity groups. In
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// comparison, a single group transaction is one where all datastore keys
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// used have the same root key. Note that cross group transactions do not
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// have the same behavior as single group transactions. In particular, it
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// is much more likely to see partially applied transactions in different
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// entity groups, in global queries.
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// It is valid to set XG to true even if the transaction is within a
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// single entity group.
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XG bool
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// Attempts controls the number of retries to perform when commits fail
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// due to a conflicting transaction. If omitted, it defaults to 3.
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Attempts int
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}
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