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 638         <h1 class="page_title">Transaction Isolation in App Engine</h1>
 639
 640
 641 <i>Max Ross, Software Engineer</i><br>
 642 <i>May 13, 2008</i>
 643 <i>Revised May 2009</i>
 644
 645 <div id="jd-content">
 646 <div class="jd-descr">
 647
 648 <p class="note" align="center">This is one of a <a href="datastore/overview.html">series</a> of in-depth articles discussing App Engine's datastore. To see the other articles in the series, see <a href="#related">Related links</a>.</p>
 649
 650 <p>According to <a href="http://en.wikipedia.org/w/index.php?title=Isolation_%28database_systems%29&oldid=208946883">Wikipedia</a>,
 651 the isolation level of a database management system "defines how/when the
 652 changes made by one operation become visible to other concurrent operations."
 653 The goal of this article is to explain
 654 <a href="../docs/python/datastore/queriesandindexes.html">query</a> and
 655 <a href="../docs/python/datastore/transactions.html">transaction</a>
 656 isolation in the Google App Engine Datastore. After reading this article
 657 you should have a better understanding of how concurrent reads and writes
 658 behave, both in and out of transactions.</p>
 659
 660 <h2>Inside transactions: Serializable</h2>
 661 <p>In order from strongest to weakest, the four isolation levels are
 662 Serializable, Repeatable Read, Read Committed, and Read Uncommitted. Datastore
 663 transactions satisfy the Serializable isolation level. Each transaction is
 664 completely isolated from all other datastore transactions and operations.
 665 Transactions on a given entity group are executed serially, one after
 666 another.</p>
 667
 668 <p>Furthermore, queries and <code>get()</code>s inside transactions see a
 669 single, consistent snapshot of the datastore that lasts for the duration of
 670 the transaction. See the
 671 <a href="../docs/python/datastore/transactions.html#Isolation_and_Consistency">Isolation
 672 and Consistency</a>
 673 section of the transaction documentation for more information, as well as
 674 <a href="http://en.wikipedia.org/wiki/Snapshot_isolation">Wikipedia's snapshot
 675 isolation article</a>.</p>
 676
 677 <h2>Outside transactions: Read Committed</h2>
 678 <p>Datastore operations <i>outside</i> transactions most closely resemble the
 679 Read Committed isolation level. Entities retrieved from the datastore by
 680 queries or <code>get()</code>s will consist only of committed data. A
 681 retrieved entity will never have partially committed data, ie some from before
 682 a commit and some from after. The interaction between queries and transactions
 683 is a bit more subtle, though, and in order to understand it we need to look at
 684 the <code>commit()</code> process in more depth.</p>
 685
 686 <h2>The commit() process</h2>
 687 <p>A <code>commit()</code> consists of two milestones - the point at which changes to an
 688 entity have been applied and the point at which changes to indices for that
 689 entity have been applied.  Let's call the first point Milestone A, and the
 690 second point, when <code>commit()</code> returns, Milestone B.  By the time we reach
 691 Milestone A, all changes to the entity have been applied.  By the time we
 692 reach Milestone B, the changes to the entity's indices have been applied.</p>
 693 <br><br>
 694
 695 <img src="transaction_iso.png"/>
 696
 697 <p>A request that looks up an updated entity by its key at a time after
 698 Milestone A is guaranteed to see the latest version of that entity.
 699 However, if a concurrent request executes a query whose predicate
 700 (the 'where clause' for you SQL/GQL fans out there) is not satisfied
 701 by the pre-update entity but is satisfied by the post-update entity,
 702 the entity will only be part of the result set if the query executes
 703 after the <code>commit()</code> operation has reached Milestone B.  In other words,
 704 during brief windows it is possible for a result set to not include an
 705 entity whose properties, according to the result of a lookup by key,
 706 satisfy the query predicate, and it is also possible for a result set
 707 to include an entity whose properties, again according to the result
 708 of a lookup by key, fail to satisfy the query predicate.  Note that
 709 while a query cannot take into account modifications that are in between
 710 Milestone A and Milestone B when deciding which entities to return,
 711 once a query decides to return a specific entity it will always return
 712 the Milestone A version of the entity.</p>
 713
 714
 715 <h2>Examples</h2>
 716 <p>We've provided a general explanation of how concurrent updates and queries
 717 interact, but if you're like me you typically find it easier to get your
 718 head around these concepts by working through concrete examples.  Let's
 719 walk through a few.  We'll start with some simple examples and then
 720 finish up with the more interesting ones.</p>
 721
 722 <p>Let's say we have an application that stores Person entities.  A Person
 723 has the following attributes:</p>
 724 <ul>
 725   <li>Name</li>
 726   <li>Height</li>
 727 </ul>
 728
 729 <p>This application supports the following operations:</p>
 730 <ul>
 731   <li><code>updatePerson()</code></li>
 732   <li><code>getTallPeople()</code>, which returns all people over 72 inches tall.</li>
 733 </ul>
 734
 735 <p>We have 2 Person entities in the datastore:</p>
 736 <ul>
 737   <li>Adam, who is 68 inches tall.</li>
 738   <li>Bob, who is 73 inches tall.</li>
 739 </ul>
 740
 741 <h2>Example 1 - Making Adam Taller</h2>
 742 <p>Suppose an application receives two requests at essentially the same time.
 743 The first request updates the height of Adam from 68 inches to 74 inches.
 744 A growth spurt!  The second request calls getTallPeople().
 745 What does getTallPeople() return?</p>
 746
 747 <p>The answer depends on the relationship between the two commit() milestones
 748 triggered by Request 1 and the getTallPeople() query executed by Request 2.
 749 Suppose it looks like this:</p>
 750 <ul>
 751   <li>Request 1, <code>put()</code></li>
 752   <li>Request 2, <code>getTallPeople()</code></li>
 753   <li>Request 1, <code>put()</code>--><code>commit()</code></li>
 754   <li>Request 1, <code>put()</code>--><code>commit()</code>-->Milestone A</li>
 755   <li>Request 1, <code>put()</code>--><code>commit()</code>-->Milestone B</li>
 756 </ul>
 757
 758 <p>In this scenario <code>getTallPeople()</code> will only return Bob.  Why?  Because the
 759 update to Adam that increases his height has not yet been committed, so
 760 the change is not yet visible to the query we issue in Request 2.</p>
 761
 762 <p>Now suppose it looks like this:</p>
 763 <ul>
 764   <li>Request 1, <code>put()</code></li>
 765   <li>Request 1, <code>put()</code>--><code>commit()</code></li>
 766   <li>Request 1, <code>put()</code>--><code>commit()</code>-->Milestone A</li>
 767   <li>Request 2, <code>getTallPeople()</code></code></li>
 768   <li>Request 1, <code>put()</code>--><code>commit()</code>-->Milestone B</li>
 769 </ul>
 770
 771 <p>In this scenario the query executes before Request 1 reaches Milestone B,
 772 so the updates to the Person indices have not yet been applied.  As a result,
 773 getTallPeople() only returns Bob.  This is an example of a result set that
 774 excludes an entity whose properties satisfy the query predicate.</p>
 775
 776 <h2>Example 2 - Making Bob Smaller (sorry Bob)</h2>
 777 <p>In this example we'll have Request 1 do something different.  Instead of
 778 increasing Adam's height from 68 inches to 74 inches, it will reduce Bob's
 779 height from 73 inches to 65 inches.  Once again, what does
 780 <code>getTallPeople()</code></p>
 781
 782 <ul>
 783   <li>Request 1, <code>put()</code></li>
 784   <li>Request 2, <code>getTallPeople()</code></li>
 785   <li>Request 1, <code>put()</code>--><code>commit()</code></li>
 786   <li>Request 1, <code>put()</code>--><code>commit()</code>-->Milestone A</li>
 787   <li>Request 1, <code>put()</code>--><code>commit()</code>-->Milestone B</li>
 788 </ul>
 789
 790 <p>In this scenario <code>getTallPeople()</code> will return only Bob.  Why?  Because
 791 the update to Bob that decreases his height has not yet been committed,
 792 so the change is not yet visible to the query we issue in Request 2.</p>
 793
 794 <p>Now suppose it looks like this:</p>
 795 <ul>
 796   <li>Request 1, <code>put()</code></li>
 797   <li>Request 1, <code>put()</code>--><code>commit()</code></li>
 798   <li>Request 1, <code>put()</code>--><code>commit()</code>-->Milestone A</li>
 799   <li>Request 1, <code>put()</code>--><code>commit()</code>-->Milestone B</li>
 800   <li>Request 2, <code>getTallPeople()</code></li>
 801 </ul>
 802
 803 <p>
 804 In this scenario <code>getTallPeople()</code> will return no one.  Why?  Because the
 805 update to Bob that decreases his height has been committed by the time
 806 we issue our query in Request 2.</p>
 807
 808 <p>Now suppose it looks like this:</p>
 809 <ul>
 810   <li>Request 1, <code>put()</code></li>
 811   <li>Request 1, <code>put()</code>--><code>commit()</code></li>
 812   <li>Request 1, <code>put()</code>--><code>commit()</code>-->Milestone A</li>
 813   <li>Request 2, <code>getTallPeople()</code></li>
 814   <li>Request 1, <code>put()</code>--><code>commit()</code>-->Milestone B</li>
 815 </ul>
 816
 817 <p>In this scenario the query executes before Milestone B, so the updates
 818 to the Person indices have not yet been applied.  As a result,
 819 <code>getTallPeople()</code> still returns Bob, but the height property of the Person
 820 entity that comes back is the updated value: 65.  This is an example of
 821 a result set that includes an entity whose properties fail to satisfy
 822 the query predicate.</p>
 823
 824 <h2>Conclusion</h2>
 825 <p>As you can see from the above examples, the transaction isolation level
 826 of the Google App Engine Datastore is pretty close to Read Committed.
 827 There are of course meaningful differences, but now that you understand
 828 these differences and the reasons behind them you should be in a better
 829 position to make intelligent, datastore-related design decisions in your
 830 applications.</p>
 831
 832 <h2 id="related">Related links</h2>
 833
 834 <ul>
 835   <li><a href="datastore/overview.html">Series Overview</a></li>
 836   <li><a href="life_of_write.html">Life of a Datastore Write</a></li>
 837   <li><a href="transaction_isolation.html">Transaction Isolation in App Engine</a></li>
 838   <li><a href="index_building.html">How Index Building Works</a></li>
 839   <li><a href="storage_breakdown.html">How Entities and Indexes are Stored</a></li>
 840   <li><a href="modeling.html">Modeling Entity Relationships</a></li>
 841   <li><a href="update_schema.html">Updating Your Model's Schema</a></li>
 842   <li><a href="handling_datastore_errors.html">Handling Datastore Errors</a></li>
 843 </ul>
 844
 845
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