Fastlane broadcasts

[palkan]

Context

Action Cable decodes and re-encodes every broadcasted message for every client which is unnecessary in most cases. This PR provides a fastlane_broadcasts configuration option to skip double encoding and transmit broadcast messages as is to clients not using custom stream coders or user-defined callbacks.

This is a continuation of the previous work: rails/rails#26999

Benchmarks

The cost of excess encoding heavily depends on the broadcasting payload size and nature. We consider three examples: a small JSON, a larger JSON, and a Turbo Stream "append" action.

Here are the results:

$ be ruby --yjit  benchmarks/broadcasting.rb

Running benchmark with N=10, M=100, adapter=async, fastlane_broadcasts_enabled=false, worker_pool_size=4

ruby 3.4.1 (2024-12-25 revision 48d4efcb85) +YJIT +PRISM [aarch64-linux]
Warming up --------------------------------------
small json (19 Bytes)
                        10.000 i/100ms
large json (80.5 KB)     1.000 i/100ms
turbo stream (2.95 KB)
                         1.000 i/100ms
Calculating -------------------------------------
small json (19 Bytes)
                        101.991 (±19.6%) i/s    (9.80 ms/i) -    980.000 in  10.053077s
large json (80.5 KB)      0.557 (± 0.0%) i/s     (1.80 s/i) -      6.000 in  10.786842s
turbo stream (2.95 KB)
                         16.618 (±24.1%) i/s   (60.18 ms/i) -    156.000 in  10.068446s

$ FASTLANE_BROADCASTS=1 be ruby --yjit  benchmarks/broadcasting.rb

Running benchmark with N=10, M=100, adapter=async, fastlane_broadcasts_enabled=true, worker_pool_size=4

ruby 3.4.1 (2024-12-25 revision 48d4efcb85) +YJIT +PRISM [aarch64-linux]
Warming up --------------------------------------
small json (19 Bytes)
                        38.000 i/100ms
large json (80.5 KB)     1.000 i/100ms
turbo stream (2.95 KB)
                         9.000 i/100ms
Calculating -------------------------------------
small json (19 Bytes)
                        327.354 (±25.0%) i/s    (3.05 ms/i) -      3.116k in  10.076621s
large json (80.5 KB)      3.826 (± 0.0%) i/s  (261.36 ms/i) -     38.000 in  10.019392s
turbo stream (2.95 KB)
                         85.029 (±31.8%) i/s   (11.76 ms/i) -    765.000 in  10.078424s

So, for small JSON payloads the fastlane version is ~3x faster, for larger JSON — ~7x faster, for HTML/Hotwire payloads — ~5x faster.

Here are Vernier profile screenshots:

• Fastlane implementation:

• Current implementation: