This document discusses cache-aware server push in H2O version 1.5. It describes calculating a fingerprint of cached assets using a Golomb compressed set to identify what assets need to be pushed from the server. It also discusses implementing this fingerprint using a cookie or service worker. The hybrid approach stores responses in the service worker cache and updates the cookie fingerprint. H2O 1.5 implements cookie-based fingerprints to cancel push indications for cached assets, potentially improving page load speeds.

1. Copyright (C) 2015 DeNA Co.,Ltd. All Rights Reserved.
Cache-aware Server Push
in H2O version 1.5
Kazuho Oku
DeNA Co., Ltd.
1Cache-aware Server Push in H2O version 1.5

2. Copyright (C) 2015 DeNA Co.,Ltd. All Rights Reserved.
Finding out what is NOT cached
2Cache-aware Server Push in H2O version 1.5

3. Copyright (C) 2015 DeNA Co.,Ltd. All Rights Reserved.
Cache Fingerprinting
 include in every HTTP request a fingerprint of
the cached blocking assets (e.g. CSS and script files)
belonging to the same origin
⁃ server uses the fingerprint to determine what
should be pushed
 algorithm: Golomb compressed sets
⁃ compressed form of Bloom filter
⁃ key(asset) = sha1(asset_URL + asset_etag)
⁃ estimated size: 1.x * N * log2(1/P) bits
⁃ N: # of entries, P: possibility of false positive
⁃ e.g. 800 bits for 100 asset files, 1% false positive
3Cache-aware Server Push in H2O version 1.5

4. Copyright (C) 2015 DeNA Co.,Ltd. All Rights Reserved.
Calculating the Fingerprint
 ServiceWorker
⁃ iterate though the SW cache and calculate
⁃ the issue: SW may not be running
• esp. for first visit or returning user (after some time),
in which case we need to push things
 Cookie
⁃ calculate in server, and set it as a Cookie
⁃ can always be used
⁃ the issue: can't update when a cache entry gets
purged
• means there might be false positives
4Cache-aware Server Push in H2O version 1.5

5. Copyright (C) 2015 DeNA Co.,Ltd. All Rights Reserved.
Calculating the Fingerprint
 hybrid approach
⁃ use Cookie header to send the fingerprint
⁃ store response in SW cache whenever possible
• even if SW is not activated (yet)
⁃ in SW, always recalculate the fingerprint and
update the Cookie
• so that the values becomes precise
5Cache-aware Server Push in H2O version 1.5

6. Copyright (C) 2015 DeNA Co.,Ltd. All Rights Reserved.
Cookie-based impl. in H2O version 1.5
 tracking issue: github.com/h2o/h2o/issues/421
⁃ thanks to Jxck, Ilya, Tatsuhiro for the feedbacks!
 H2O cancels push indications sent from backend, if
client is likely to already have the data in cache
 hybrid impl. in the works
6Cache-aware Server Push in H2O version 1.5

7. Copyright (C) 2015 DeNA Co.,Ltd. All Rights Reserved.
Configuration (H2O 1.5)
mruby.handler: |
lambda do |env|
push_paths = []
if /(/|.html)$/.match(env["PATH_INFO"])
push_paths << "/search/jquery-1.9.1.min.js"
push_paths << "/search/oktavia-jquery-ui.js"
...
end
return [
399, # delegate the request to next handler
push_paths.empty? ? {}
: {"link" =>
push_paths.map{|p| "<#{p}>; rel=preload"}.join("n")},
[]
]
end
file.dir: /path/to/doc-root
7Cache-aware Server Push in H2O version 1.5

8. Copyright (C) 2015 DeNA Co.,Ltd. All Rights Reserved.
Demo
 Web server (H2O) tracks the cache fingerprint
⁃ and cancels the push if the client has the asset
• or, we might push 304
⁃ or actually does the push if otherwise
8Cache-aware Server Push in H2O version 1.5

9. Copyright (C) 2015 DeNA Co.,Ltd. All Rights Reserved.
Benchmark scores
 first-paint does not change
⁃ reason: no dependency chain bet. assets,
packets arriving in batch for every RTT
 unload is delayed (since assets arrive before HTML)
9Cache-aware Server Push in H2O version 1.5