async-flumelog

Async flumelog

This module is heavily inspired by flumelog-aligned-offset. It is an attempt to write something simpler that is easier to reason about. Flumelog is the lowest part of the SSB stack, so it should extremly stable while still maintaining good performance.

An async flumelog consists of a number of blocks, that contain a number of records. The records are simply length + data. A record must be in one and only one block, which means there probably will be some empty space at the end of a block. Blocks are always written in full.

<block>
  <record
    <record.length: UInt16LE>
    <record.data>
  </record>*
</block>*

Contrasting to flumelog-aligned-offset there is no length after the data in a record and no pointer at the end of a block. These were to be able to run the log in reverse, but I have never seen the need for that.

Writing to the log is always async. Note this is different from flumelog-offset and [flumelog-aligned-offfset]. The since observable will be updated once the data is written. onDrain can be used to know when data has been written if needed. Streaming will only emit values that have been written to storage. This is to ensure that a view will never to ahead of the main log and thus end up in a bad state if the system crashes before data is written. get will return values that have not been written to disk yet.

Benchmarks

Running bench-flumelog reveals the following numbers. Async flumelog is faster in all tests except random, where flumelog aligned offset is a lot faster. The append test is very slow for flumelog-aligned-offset because it writes every message synchronously. The most important numbers are append (used for onboarding) and stream (used for building indexes).

async flumelog:

name, ops/second, mb/second, ops, total-mb, seconds
append, 672175.964, 100.417, 3361552, 502.188, 5.001
stream, 1276215.641, 190.656, 3361552, 502.188, 2.634
stream no cache, 1355464.516, 202.495, 3361552, 502.188, 2.48
stream10, 2046797.202, 305.777, 10244220, 1530.418, 5.005
random, 21936.533, 3.277, 110604, 16.522, 5.042

flumelog aligned offset:

name, ops/second, mb/second, ops, total-mb, seconds
append, 769.2, 0.114, 3856, 0.576, 5.013
stream, 128533.333, 19.207, 3856, 0.576, 0.03
stream no cache, 124387.096, 18.587, 3856, 0.576, 0.031
stream10, 428444.444, 64.023, 38560, 5.762, 0.09
random, 907996.6, 135.684, 4540891, 678.556, 5.001

flumelog offset:

name, ops/second, mb/second, ops, total-mb, seconds
append, 306180.037, 45.74, 3064556, 457.817, 10.009
stream, 294511.348, 43.997, 2945408, 440.017, 10.001
stream no cache, 327724.949, 48.959, 3064556, 457.817, 9.351
stream10, 452973.302, 67.67, 4530186, 676.776, 10.001
random, 28774.712, 4.298, 287891, 43.008, 10.005

To run the benchmarks the small bench-flumelog.patch needs to be applied.