Accidenal complexity: A tale of two GUIDs
For a new feature in RavenDB, I needed to associate each transaction with a source ID. The underlying idea is that I can aggregate transactions from multiple sources in a single location, but I need to be able to distinguish between transactions from A and B.Luckily, I had the foresight to reserve space in the Transaction Header, I had a whole 16 bytes available for me. Separately, each Voron database (the underlying storage engine that we use) has a unique Guid identifier. And a Guid is 16 bytes… so everything is pretty awesome.There was just one issue. I needed to be able to read transactions as part of the recovery of the database, but we stored the database ID inside the database itself. I figured out that I could also put a copy of the database ID in the global file header and was able to move forward. This is part of a much larger change, so I was going full steam ahead when I realized something pretty awful. That database Guid that I was relying on was already being used as the physical identifier of the storage as part of the way RavenDB distributes data. The reason it matters is that under certain circumstances, we may need to change that. If we change the database ID, we lose the association with the transactions for that database, leading to a whole big mess. I started sketching out a design for figuring out that the database ID has changed, re-writing all the transactions in storage, and… a colleague said: why don’t we use another ID?It hit me like a ton of bricks. I was using the existing database Guid because it was already there, so it seemed natural to want to reuse it. But there was no benefit in doing that. Instead, it added a lot more complexity because I was adding (many) additional responsibilities to the value that it didn’t have before.Creating a Guid is pretty easy, after all, and I was able to dedicate one I called Journal ID to this purpose. The existing Database ID is still there, and it is completely unrelated to it. Changing the Database ID has no impact on the Journal ID, so the problem space is radically simplified.I had to throw away heaps of complexity because of a single comment. I used the Database ID because it was there, never considering having a dedicated value for it. That single suggestion led to a better, simpler design and faster delivery. It is funny how you can sometimes be so focused on the problem at hand, when a step back will give you a much wider view and a better path to the solution.
For a new feature in RavenDB, I needed to associate each transaction with a source ID. The underlying idea is that I can aggregate transactions from multiple sources in a single location, but I need to be able to distinguish between transactions from A and B.
Luckily, I had the foresight to reserve space in the Transaction Header, I had a whole 16 bytes available for me. Separately, each Voron database (the underlying storage engine that we use) has a unique Guid identifier. And a Guid is 16 bytes… so everything is pretty awesome.
There was just one issue. I needed to be able to read transactions as part of the recovery of the database, but we stored the database ID inside the database itself. I figured out that I could also put a copy of the database ID in the global file header and was able to move forward.
This is part of a much larger change, so I was going full steam ahead when I realized something pretty awful. That database Guid that I was relying on was already being used as the physical identifier of the storage as part of the way RavenDB distributes data. The reason it matters is that under certain circumstances, we may need to change that.
If we change the database ID, we lose the association with the transactions for that database, leading to a whole big mess. I started sketching out a design for figuring out that the database ID has changed, re-writing all the transactions in storage, and… a colleague said: why don’t we use another ID?
It hit me like a ton of bricks. I was using the existing database Guid because it was already there, so it seemed natural to want to reuse it. But there was no benefit in doing that. Instead, it added a lot more complexity because I was adding (many) additional responsibilities to the value that it didn’t have before.
Creating a Guid is pretty easy, after all, and I was able to dedicate one I called Journal ID to this purpose. The existing Database ID is still there, and it is completely unrelated to it. Changing the Database ID has no impact on the Journal ID, so the problem space is radically simplified.
I had to throw away heaps of complexity because of a single comment. I used the Database ID because it was there, never considering having a dedicated value for it. That single suggestion led to a better, simpler design and faster delivery.
It is funny how you can sometimes be so focused on the problem at hand, when a step back will give you a much wider view and a better path to the solution.
