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re: What would you use as a sortable, globally unique, ID? VIEW POST

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re: I had the idea that I wanted to construct a scalable distributed event store on top of AWS and ran into similar questions (ordering in a distribute...
 

Thanks for the detailed explanation and I can see how complicating the architecture to allow ordering is not worth it in your case, especially because you have what I believe is an "event sourced" architecture, with multiple levels of granularity of events.

It is true indeed that not all listeners, especially those that are merely external consumers, are interested in that detail, in addition to the ability to replay events for those external consumers.

Our case is very limited in scope, which ultimately will become a very simple pub/sub system. The topic of what sort of "universal" IDs to choose for those events transiting the inside of the app and to be sent outside arose and thus, I opened this discuss thread.

I don't foresee any real drawbacks in choosing a guid that's also sortable right from the start, what do you think? By having IDs that are inherently sortable the consumer can use that property or ignore it as they wish

Hope this makes sense

 

The only real downside is that making them sortable will encourage you to use and depend on that feature. And when clock skew comes into play, then the code that depends on sortability will probably behave unexpectedly. When your code processes the data out of order you could observe strange things like issuing updates for data that hasn't been inserted yet. How: On a cloud provider, they may restart your code on a different node (for failure or maintenance or no reason) whose clock may be skewed from the previous node. They usually do have time sync but it is best effort -- no guarantees about clock accuracy between servers. The margin of error should be small enough that you don't have a problem normally. But if you ever do have the issue it will be hard to diagnose. Also note that if a hardware clock does fail, it is common for it to reset to zero. That would be a little easier to diagnose, but either way recovery (changing the IDs? mapping them to new IDs?) could be painful.

For an event store I use a big integer called Position (not an auto increment) to have total ordering for that node. It guarantees that whatever happens later has a larger Position than whatever happens before. But it doesn't provide a global perspective. You can get a global order by reading from nodes in Position order and then choosing the lowest timestamp among those events. When you do it that way, you know it is a "best effort" ordering, and you may be able to account for some known skewed timestamps. Even if timestamps are off, you are guaranteed that events from the same node are in order. I haven't actually had to do a global order across nodes yet, but there is one on the horizon to merge different event stores.

Forgot to mention. Random UUIDs still used to identify entities on top of the node-wide Position and event timestamps.

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