Planning for the Application Lifecycle in Java

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7 Planning for the Application Lifecycle
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selected with an equality operator, it is almost certainly not possible to use read-repair in this situation
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Adding new fields to items that already exist is especially easy in SimpleDBWhether you use a batch process or make the changes more slowly over time is up to youThere are no schema changes to coordinate between clients or between domainsYou are free to make just the changes as you need toThis can include the same read-repair style process as discussed previously with the format changes More simply, you may just be adding a new data field If this is the case, you can just add the data when it is available For example, if a user-tagging feature is being rolled out, you can just deploy the code responsible for storing the tagsThe tags will be stored as they are created, and there is nothing special that needs to be done In a case where you already have the full set of data, you will need to put into place a mechanism to backfill the existing items in addition to adding the new values Read-repair can work nicely for this in situations where an extended but incremental performance hit is more desirable than a short but full outageAn example is a count that is derived (and then cached) from a query, and now you want to store the actual count value in the itemAssuming it is a vote count, updating the vote count as votes are made will be part of the normal application code, but going back to update the older votes can be done as a special-case process while old-style items still existThe read-repair can be done in the same steps as with a format change, transitioning between the old way and the new way In this case, there is no need to run separate queries; it is the old vote-count query that must be run if there is no vote value in the itemTherefore, the code must be able to detect the old style items as they are read and make the change
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Using a Batch Process to Apply Updates
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Updating the format or the layout of data items in SimpleDB has the benefits of speed and simplicityThe drawback is that you will need to prevent normal application access to the domain during the course of the update If you are able to use a scheduled maintenance period to perform changes, using a batch update can be very convenientThe speed comes from the fact that you can submit updates as fast as SimpleDB will accept them without competing with normal application requests for domain bandwidthThis is how you would expect to apply an update The simplicity, compared to the read-repair approach, comes from the fact that you do not need to implement and test any special application logicThe expected application changes still have to be made surrounding the changes being applied, but nothing extra Further, those changes may be confined to a persistence layer and only touch the application code via annotations or configurationAll of the data conversion logic remains neatly contained within the update utility, which can run to completion, enabling a nice clean end to the update activities
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If your application is one where growth is steady and performance is critical, it is wise to keep an eye on database usage and growthAlthough rolling out synchronous schema updates is not typically part of SimpleDB usage, by design, some of the same issues can crop up and require some attention In particular, you may need some form of limited and homegrown schema to manage object mapping, admin tools, or reporting tools in some combination If you do, it is best to be informed about what a schema change rollout might look like
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