World’s Smallest No SQL Database: Persistent transaction logs
As it stand the World’s Smallest No SQL Database will last only as long as you actually have power. The moment that you have a restart, all the data is gone. The is actually how several databases are running, but for now, we are going to assume that this is not desirable. The question now becomes, how do you actually store the data on disk?
This really becomes a pretty complex question, because you need to store the data on disk in a way that is crash safe, allow updates, and doesn’t take all the disk space in the world. Before we will get to the actual on disk data structures, we need to discuss how we implement persistent logs. Persistent logs are the key way that databases gets Durability. And as it turned out, there are just two ways of doing that that I am aware of:
- Append only
- Transaction log
Append only models rely on the fact that you only append to create a safe way to ensure that the data is either all in or all out. When we write, we don’t overwrite values, we are creating new values, and then we write where the last log entry is located. In CouchDB, for example, this is done by modifying the header portion of the file to point to the new data. In LMDB this is done by updating the page listing with the new pages. In both cases, you actually commit a transaction when the on disk pointer is changed to point to the new data. If you crash midway, nothing bad happened, you didn’t update the on disk pointer, it is still pointing at the old location. Worst case scenario, you wasted some disk space, and you probably have a way to reclaim that anyway.
Transaction logs use a different way to handle this. They are also usually implemented as append only files, into which we write the new data. Afterward, we can apply those changes in memory / on disk safely. A crash would simply mean having to replay the log. This is how leveldb, for example, works. This is also the essential idea behind how SQL Server Oracle works. Although in their case I believe that the transaction log usually contain both prev/new state of the pages they changed for a specific transaction.
One thing that you have to be aware of, either way, is that you should be prepared to handle scenarios where your process crashed midway, or when your entire machine had the plug pulled out. That means using fsync, sure, but it also means that you might have to do log replay, or be ready to see if you can recover something from the append only model.
The good thing about the transaction log approach is that after you have committed the changes to the standard persistent format, you can clear it (usually by just creating a new file and deleting the old one). With the append only model, you usually have to run some sort of compaction to clear things out. Note that the transaction log model vs append only model doesn’t really mean about how the rest of the persistent data is actually stored. We will touch on that on the next post.