> Checkpointers do not block writers, and writers do not block checkpointers. Th...

jacoblambda · 2024-01-15T13:55:57 1705326957

> I don't get how wal2 fixes the long-running reader problem though. Maybe they were just referring to the former problem?

Because with a single wal file you can't checkpoint it during a read since said file may change out from under you.

With two wal files, the one you are actively appending to can be treated like in wal1 mode but the one that isn't being appended to is immutable for the time being just like the main database.

This means you can treat the actual db file and the immutable wal file together as one immutable database file with some underlying abstraction. That abstraction then allows you to perform the checkpoint operation since the abstraction can keep all that immutable data accessible in some form or another while reworking the data structure of the db file.

Then once the checkpoint is complete, the abstraction can clear the now redundant immutable wal file, become transparent, and just present the underling single DB file.

And now once the wal file you are actively appending to reaches a sufficient size, you "lock" that one, rendering it immutable, and switch over to appending to the cleared wal file you were previously checkpointing. With this you can now checkpoint again without blocking reads or writes.

liuliu · 2024-01-15T15:14:50 1705331690

But Wal also keeps "history" so one reader transaction can see x0 from wal1, another reader can see x1 from Wal1. Wal1 cannot be merged into main db otherwise x0 is lost?

jacoblambda · 2024-01-15T17:23:00 1705339380

Yes that is correct. You can't checkpoint data after the oldest snapshot. But to my knowledge there's no way to force a read snapshot to continue to exist.

So while you can make multiple back to back reads that use the same snapshot, I believe there's no guarantee that the snapshot will still exist when the next read is opened unless the previous read is also still open (in which case an error is returned).

That seems to set an upper bound on how long a reader can block a checkpoint (unless the reader is intentionally staging reads to block the checkpoint).

Theoretically you could implement checkpoints that flatten everything between snapshots into single commits but the complexity and overhead probably isn't worth it given that the only real blocker for wal2 is an edge case that is nigh impossible to encounter unless you intentionally try to trigger it.

liuliu · 2024-01-15T18:02:41 1705341761

Open an BEGIN transaction forces read from a particular snapshot in SQLite. There are some complications around lock upgrade in that case: BEGIN a, read, BEGIN b, read, write from a transaction fail because cannot upgrade read lock to write lock. Other than that, in WAL mode, it will work exactly as expected:

BEGIN a, read x0 BEGIN b, write x1, END b BEGIN c, read will return x1 Back to a transaction, read again, return x0 still.

willvarfar · 2024-01-15T14:56:57 1705330617

> once the wal file you are actively appending to reaches a sufficient size, you "lock" that one

so there might eventually be wal3 and wal4 files and so on?

jacoblambda · 2024-01-15T15:13:28 1705331608

nope. since if the wal you are checkpointing isn't done yet, you just wait to "lock" and switch files until that operation is complete.

Checkpointing can be considered "lock free" since the operation will always eventually complete. How long it takes will depend on the wal file being checkpointed into the db but it'll eventually complete in some finite amount of time.

Because you know that any given checkpointing operation has to eventually complete, you can simply keep appending to the current "append" wal file and then tackle those changes when you finish the current checkpoint op (at which point the wal file you just finished checkpointing is free to take the appends).

athoscouto · 2024-01-15T15:22:33 1705332153

Wouldn't that require the reader to finish? Making the reader block the checkpoint from wrapping up?

jacoblambda · 2024-01-15T16:15:10 1705335310

Not if you abstract those reads. It's not like the application is directly opening a file handle. Instead it goes through the sqlite library which accesses not only the db and the wal file but also a non-persistent shm (shared memory) file used for maintaining a consistent view of the write ahead log.

When a reader is reading, it puts a shared lock on the specific data it is reading in the shm file. The checkpointer respects that lock and may (potentially) continue working elsewhere in the db file, slowly updating the indices for checkpointed data in the shm file.

The checkpointer won't change the underlying data that the reader has a lock on but they may have created a new location for it. When the reader is finally done reading, the checkpointer can quickly grab an exclusive lock and update the header in the shm for that data to point to the new destination (and then release said lock). Since the checkpointer never holds this lock for very long, the reader can either block when trying to get a shared lock or it can retry the lock a few moments later. Now that the header in the shm only points to the new location, the checkpointer can safely do whatever it needs to with the data in the old location.

Slowly rinse repeat this until the checkpointer has gotten through the entire write ahead log. At that point there should be no remaining references in the shm to data within the wal file.

Now the wal file can be "unlocked" and if the other wal file is large enough, it can be locked, writes switch over to the other wal, and the cycle repeats anew.

Edit: Importantly, this requires that all readers be on a snapshot that includes at least one commit from the "new" wal file. So compared with wal1, wal2 allows you to have long running readers as long as they start past the last commit of the "previous" wal file.

riwsky · 2024-01-15T15:18:02 1705331882

Think of them more as “walblue” and “walgreen”

stavros · 2024-01-15T13:39:00 1705325940

I suspect that having two WAL files means you can freely checkpoint the one that isn't currently being written to, but that's just a guess.