Boundaries of Eventual Consistency in Java

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Boundaries of Eventual Consistency
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For many applications, particularly applications where each of the data items is owned by exactly one user, consistency becomes a more narrow concern For example, consider an application that maintains a set of to-do lists for users If all of the lists are private, consistency involves how each user sees his or her own data, and it makes no difference to Jill whether Jack s data is consistent This narrowing holds even in cases where the data is made public Pretend that these are now wish lists and not to-do lists If Jack adds a new pail to his wish list, it is important that the publicly visible list be updated, but if that update is delayed by a full second, the delay is probably not even noticeable However, it is quite important for Jack to see a consistent view of his own list If he adds an item and then finds it immediately absent from the list, it will look like the update failedThis level of consistency, where each read reflects the results of every prior write by the same client, is called read-your-writes Allowing a client to see an always-consistent view of the data he is changing is a level of consistency that is useful in many situationsA relational database will give you this level of consistency automatically by virtue of the strong consistency guaranteeA relational database cluster, on the other hand, may not be able to guarantee read-your-writes when a slave database services client reads SimpleDB does not have the notion of a connected client, however, and does not guarantee read-your-writes consistency Each request is independent, and there is neither a persistent connection nor a session layerThis is not really a concern for batch-processing situations like indexing or data import; this is the concern when handling requests for connected users who are submitting updatesThis applies equally to application users for a website, a desktop app, or a mobile app It is important to realize that it is easy to build a system with guaranteed read-yourwrites consistency (or stronger) on top of SimpleDB by using consistent reads but that it comes by giving back some of the availability Having said that, it is possible to build a system that provides read-your-writes most of the timeThere is a situation that you need to be aware of when using SimpleDB during an outage, you may be unable to achieve read-your-writes for some users until the problem is resolved Knowing that, the remaining issue is the implementation of a system that can give you that consistency most of the time while still offering (potentially degraded) service during an outage
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Implementing a Consistent View
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When it comes down to the practical matter of how to deliver a consistent view to your users when consistent reads are failing, there are two types of strategy you can use: data access patterns and cachingThese techniques are independent and can be used together to achieve the best outcome Optimizing Data Access Patterns Many of the data access needs of an application are dictated by the application s requirements However, the database access patterns are frequently a consequence of application design Modern technologies, particularly web technologies, make it possible to sculpt
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6 Working Beyond the Boundaries
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the application s data access to achieve a quicker and more responsive user experience These same technologies can also help you work within the SimpleDB eventual consistency window There is one primary data access pattern that causes eventual consistency issues: a write immediately followed by a read In most cases, this means a call to PutAttributes for a specific item and then a query whose results should reflect the changes to that item It could also be a GetAttributes request for the same item that follows the PutAttributes As an example of the query scenario, consider the way forums typically work on the InternetYou view a thread of comments and you decide to post your own comment, so you click on the reply linkAfter typing in your comment, you click on the submit button; the server saves your comment to the database (subject to any rules) and then redirects your browser to the thread page, where your new comment is posted at the end In a hypothetical forum application based on SimpleDB, the place where things could go wrong is in the redirect after the new comment is saved If the part of the application that saves the post is completely separate from the part that subsequently displays the post, some trade-offs are being madeThis pattern is not inherently bad, but with an eventually consistent database, the new comment could fail to turn up in the subsequent query because it is performed so quickly after saving the data Using a consistent read solves this problem much of the time but if one of the replicas storing your domain data fails, it can cause consistent reads to return an error while normal reads continue to work with eventual consistency The real problem with this situation is that the necessary data is available in memory but is being dumpedThe solutions to this problem all involve holding on to that data rather than dumping it One option is to use a dispatch mechanism rather than a redirect for populating the follow-up query Rather than round tripping to the user s web browser, a dispatch hands off the request to a different handler on the serverThe unnecessary Internet latency is avoided, and because dispatch allows data passing, the important data is not discarded In this scenario, the handler that saves the comment to the database passes that comment to the handler responsible for displaying a threadThe thread display handler needs one extra capability: merging the passed data into the normal query results This technique, and others like it, works well with SimpleDB because consistency is normally reached before the user can make another requestWhen looking at the data access patterns in an application, whenever you see a write immediately followed by a read, it is OK to use a consistent read but it is also wise to add error handling that either handle it in the same request if the immediacy is required, or place a user action between them if it can waitThis allows you to benefit from the high availability built into SimpleDB Another example of this principle is the display of user profile information Consider a web application where the user has a profile page and can change some aspects of how it appears In the use case where the user saves a profile change and automatically receives a redirect to the profile page, the same data access pattern emerges as with posting forum comments In this case, the follow-up action is a lookup rather than a query, but the im-
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