Building a Distributed Log from Scratch, Part 1: Storage Mechanics | Hacker News Reader

Building a Distributed Log from Scratch, Part 1: Storage Mechanics

bravenewgeek.com

278 pointstylertreat8 comments

This is a really neat and good article! I did a talk in Sweden the other month about how to build a distributed database, hopefully it may also be fun/useful/informative for others: https://youtu.be/5fCPRY-9hkc (it uses CRDTs instead, so is a counter-part that isn't totally ordered and isn't append-only).

I like the OP article though, cause I learned about NATS streaming, which I hadn't heard of before - just Kafka. Will have to check it out.

NATS streaming was definitely neat. And thanks for sharing your video, always looking for new distributed database resources.

I have to admit that I only recently got familiar with logs. I was designing a B+Tree[1] in Python for fun and was struggling to make it survive crashes: a single insertion in a tree often results in multiple page writes which is not atomic.

The solution to this problem is simple and elegant with a Write-Ahead Log. Every page write is appended to the log and only merged back into the tree file when it's sure that the log is safely written to storage.

SQLite has an extensive documentation of its WAL file format, which is great for learning.

[1] https://github.com/NicolasLM/bplustree

[2] https://www.sqlite.org/fileformat.html#the_write_ahead_log

If you're interested in those, there are at least two other designs you should have a look at:

- couchdb uses a single file for each db, which means the write ahead log _is_ the storage. The atomicity is guaranteed by saying that the latest root is the valid root. If writes are interrupted, everything since the last root is invalid and discarded upon restart. A simple design that just works, although it tends to be wasteful and requires frequent compactions

- lmdb (https://en.m.wikipedia.org/wiki/Lightning_Memory-Mapped_Data...) uses copy on write to make sure space is properly used, and atomicity is provided by sharing only the strict minimal pieces of information, so small in fact atomicity is guaranteed by the os. Follow the different links in the wikipedia page, there's a lot of interesting stuff

This is a good introduction, and a very useful abstraction. At Morgan Stanley we have built a PL/compiler and tools around a method of logging like this -- logging algebraic data types and live querying out of them with Haskell-like comprehensions/pattern-matching/etc:

https://github.com/Morgan-Stanley/hobbes

One question about the index file, "in Kafka, the index uses 4 bytes for storing an offset relative to the base offset and 4 bytes for storing the log position." Isn't the relative offset to the base offset already pointing to the physical location of the message in the segment file? What's the purpose of the second 4-byte field log position?

I think that the offsets are both in terms of number of messages, whereas the log position is the byte in the file that the message occurs at.

Right, I think earlier Kafka it was actually the position as well, but then they moved to the current method.

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