Monthly Archives: June 2021

BullFrog: Online Schema Migration, On Demand

I did my PhD on a topic I called dynamic software updating (DSU), a process by which a running application is updated with new functionality, whether to add features or fix bugs, without shutting it down. As a faculty member, I supervised several PhD students on DSU projects. These considered the semantics of DSU and ways of reasoning about and/or testing a dynamic update’s correctness (including while it’s deployed), and ways of implementing DSU, using compilation, libraries, and/or code rewriting. All of this work resulted in what are, as far as I’m aware, still the most full-featured and efficient implementations of DSU for C and Java, to date.

While DSU handles the update of long-running, single-process applications, many long-running applications also involve a database management system (DBMS) to store persistent application data. For example, an online market will have a front end to present the user interface, but the market’s inventory, purchase log, user reviews, etc. will be stored in the back-end database. As such, a single logical change to an application could well involve individual changes to both the front-end code and the contents and format of the database. Maybe our upgraded market now provides access to an item’s price history, which is implemented by extending the DB schema and by adding front-end functionality to query/access this information. To realize this upgrade dynamically, we need to change the application and the database, in one logical step. 

In a paper presented at SIGMOD this month, we describe Bullfrog, a new DBMS that supports online schema updates in a way that enables whole-application upgrades. An application change can be applied by DSU for the front-end instances and by Bullfrog for the back-end DB. A key feature of Bullfrog is that on the one hand, the schema change is immediate, which simplifies front-end/back-end coordination of the update, especially when schema changes are backward incompatible. On the other hand, data migration to the new schema is lazy, as the application demands it. Lazy migration avoids a potentially lengthy update-time pause, which would result in loss of availability, defeating the
whole point of DSU. There were lots of challenges to realizing the lazy updating model. I give a flavor of the approach here; the paper has the details.
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Filed under Research, Systems