ATS Summit Fall 2019

The summit was held in Building B at Yahoo! on 8 Oct to 10 Oct 2019.

These notes are my personal views of the summit, things I found of interest that were not obvious in the slides. I have tried to not duplicate too much slide information.

New Committers

Aaron Canary, Peter Chou.


  • Need more cross company PR reviews. This will require other companies to spend time on this, the current cabal can’t make this happen.

  • Switched over to Slack. Anyone in the channel can invite others, just need to find anyone already there.

  • Decisions from last summit

    • Move to YAML for configurations.

    • Remove LuaJIT from the core.

  • ATS 9

    • HTTP/3 QUIC - alpha. Based on draft-20.

    • Minimum OpenSSL is now 1.0.2.

    • Tentatively drop support for Solaris - no support, no way to even build it.

    • Blockers for ATS 9 release should be added to issue 5932.

Long discussion about live testing and configuration validation on production systems. Apple uses an internal tool called “CDN Checker” to do this. This is embedded tightly with Voluspa and wouldn’t be easily separated. There were requests for more common tools to do this kind of thing. I think this is a perfect application for Http Replay, but despite all the talks about it at summits no one else seems to be aware of it. I’ve also presented on using AuTest for live testing, but that was apparently forgotten.

For ATS 10 and onwards, we want to move to having the release notes in the documentation. PRs that make non-compatible changes should also contain updates to the release notes.

Some discussion about Layer 7 Routing and A/B testing. Sudheer mentioned wanting to do ramping load balancing where load is gradually shifted between sets of upstreams.

Outbound Refactoring

Work is on feature branch “h1-outbound”. Discussion about how much of the renaming should be on master and ATS 9.

Kit asked about better manipulation of HTTP/2 with regard to gRPC proxying and handling trailing headers.

Sudheer mentioned there is a need to be able to do session level property setting for outbound sessions, which currently is done in a poor way by using transaction based hooks. A cleaner way is needed for this. It seems this can mostly be done if there are outbound session hooks.


QUIC is a generic transport layer, and HTTP/3 is really HTTP over QUIC.

Experimental support is available in ATS 9. This requires a special version of OpenSSL. There is still a feature branch, quic-latest. This is intended to be merged to master after every successful InterOp.

  • QUIC : draft-20

  • HTTP/3 : draft-20

  • QUACK : draft-03

There are a number of missing features and not suitable for production use. It is a a good base for people interested in working on HTTP/3. InterOP tests are done using an internal client traffic_quic which is based on the ATS QUIC libraries.

Should also take care to not break BoringSLL support when working on OpenSSL code.

Unified Routing

Current routing is basically “remap.config” and a few plugins such as “regex_remap”. Mostly host or path based. Want “ramping” or “A/B testing” to have partial routing between distinct groups of upstreams.

Very similar to TxnBox - have a set of routing criteria (“features”) and then apply actions (“directives”) based on the criteria.

Key ordering is predetermined by the code. Longest / most specific rules are matched first.

Rules can be conditional based on internal tracking of upstream results. Rules can be linked to “cascade” selection.

Can be configured via an external API. Rules can be created and listed. Rules have an index and a hash key which is computed from properties of the key.

Serving Wikipedia with ATS

Spent 12 months work on using ATS for Wikipedia. Was served by Varnish, now a mix of ATS and Varnish. This means all of the Wikipedia foundation (more than just Wikipedia).

Run their own CDN for general autonomy. All of the software and tools are open source and available on github.

Old split was two proxy instances, one ram only and the other disk only. Replacing the disk only proxies with ATS. Will be continuing with conversions of other proxies to ATS. Looks like the Varnish killer was the lack of TLS support.

Found several issues: #4466, #4635, #5179, #5787.

Using ATS 8.0.5.

Using Lua plugin

  • path normalization, RFC 3986

  • Tweak headers for MediaWiki processing.

  • Cache control.

Quite a bit of work on determining the logic for whether a response is cached, using system tap to track the decisions. Having problems with dealing with requests that are non cacheable based on server response, and not determinable at request time. Varnish can make a note of the non-cacheablility. I think this is distinct from negative caching.

Using named pipe for logging. Pondered writing to Kafka, not done yet.

Note all of the code and even performance graphs are publically available. Lots of interesting Puppet scripts.

Nexthop selection strategies

This is primarily a reworking of parent selection. Checks for remap, and if not remapped uses parent selection.

Single config file that contains multiple strategies. Updated “remap.config” parsing to enable a “strategy” option that specifies which strategy from the configuration should be used. Synchronization between the two files is done by always reloading the strategy config file when the “remap.config” is reloaded. Configuration files are linked via a special #include mechanism.

Planning on having plugins at some point.

AMC Tech Corner

Still hard push for not supporting old and new configuration instances.

Cache config YAML accepted, no real objects.

For down server handling, in the “service not available” case make sure cache “serve while stale” works.

No comments on working on Cache Tool outside of ATS repo.

For disabling ram cache - per volume is OK. JvD wants to look at remap level of preventing specific objects from being promoted to ram cache.

Reloadable Remap Plugins

Currently on master. This works for remap plugins. May be extended to L7R plugins.

Mostly a demo, showing how remap plugins get notified. The big change is that the dynamic libraries get unloaded and reloaded. Continuations created by the remap plugins is tracked and the library kept loaded until all the Continuations are destroyed.

Http Replay

Using for performance and testing.


Use for TS plugins.

Reloading - can reload libraries, but that’s not the core problem, which is dangling Continuations.

Trying to make an ABI that makes it possible to use the same plugins on different proxies.

Multiple plugins done by linking them into a single VM. A framework does the function call routing.

VMs have API to get functions (calls to VM from proxy) and callback registration (calls from VM to proxy), plus memory copy (in and out).

Not clear on the state model. There are context objects, perhaps those are the equivalent of Lua states or “light weight threads” sort of thing. That is, “context switching” for different transactions is done by selecting a corresopnding context object. (Appears to be correct). The internal context has both state data and the methods / functions available in the context. The functions are represented as methods on an object and extracted during compilation.

In essence, to get other language support, we would need a “WASM API” kind of like the C++ API which wraps the C plugin API. This API is a concrete implementation of an abstract “proxy VM” defined by WASM. Once this works, then WASM takes care of interfacing from other languages to the concrete proxy VM.

Analytics with ATS Logs

Primary goal was to figure out how well the CDN was serving. Wanted to distinguish between regions and / or networks to be able to track down issues. Ended up using ATS logs along with other logs and beacon measurements.

Generates reports via Hadoop cluster, then uses Druid / Pivot to view the reports and graphs. Doing this with the network team enables tracking down real network problems. Has resulted in many actual fixes for problems.

Want to do real time streaming in the future.

ATS Ingress Controller

(Originally listed as “ATS and Kubernetes”)

Uses “Ingress Controller” process in a pod to control ATS, which in turns proxies access to the services in other pods. Seems like another good place for a better RPC for updating configuration and retrieving status information.

Ingress Controller dynamically adjusts the proxy state to handle changes in the service mesh. New instances, new services, services or instances removed, etc.

Need to implement more control of routing.

Network and HTTP/2 Performance Issues

Initially some throttling problems, have fixes for this in HostDB and accept logic.

An issue is transaction processing happens on the ET_NET threads and if HTTP/2 processing is expensive, this can cause cross talk latency.

In general, a number of minor issues that add up. For instance, inefficient memory handling in HPACK using HdrHeap.

Discussion of issue with accepting connections, dedicated threads vs. ET_NET acceptance. Lots of discussion, probably need to do something in this area.

There is other work going with regard to HTTP/2, mostly by Susan and Masaori.

TLS 1.3 0-RTT

Combine initial client data with handshake. This requires quite a lot of changes in ATS.

Biggest problem was figuring out what the OpenSSL options for early data are and how they work.

Early data is put in to a specific sidecar buffer.

Need some specific replay protections as the built in ones don’t work in our use case.

Part of the replay protection is limiting early requests to specific methods. Request for being able to restrict methods on a per domain basis, or look at being able to do this in “remap.config”.

Low Latency HLS

Changes to the basic HLS spec to enable much lower latency for live video. Goal is to compete in latency with standard satellite broadcasts. The biggest problems seemed to me to be the 6 second chunk, which implies a minimum 6 second delay. The official list is

  • Shrink the segment size from 6 seconds to much smaller.

  • Do long polling from user agent to upstream to get the next update of the index file.

  • User agent can ask for a push of the next segment when fetching the index file.

  • Delta for index files.

  • Index files can specify the alternate rate files to make adaptive switching faster.

Official Talk.

Delain Concert


Lightning Talks


Provides hosting services to developers. Host over 174K distinct domains. L7 routing is done by mapping everything to service on a localhost address / port.

Demonstrated how basically a random user can create a github repository which can then be linked and distributed by Netlify.

Can set up alternate sites based on pull requuests to the main website branch.

Supports redirects (really remap) rules to map paths to other paths. Can have generic redirects that allow arbitrary URLS to map to a fixed page, but files are exempted therefore the remapping must be able to detect files. This is done during deployment of the website.

Original remapping service was written in Go, and so not really suitable for a plugin. Working on converting to plugin.

SystemTap in ATS

Used system tap to tap into state_cache_open_read to detect how many cache write lock retries have done in order to tune that parameters.

Problem is the taps require intimate knowledge of ATS internals and even specific line numbers. It is possible to define tap points in the code. These can have arguments as well, which provide data to the tapper. Man page claims this is assembled to a NOP and only data about how to find the arguments is stored in a side file.

This is similar to gdb macros.

Might be reasonable to tweak the DEBUG macro to provide tap points. If it’s interesting for debugging by logs, it should be interesting for tapping. Would need to have another argument for the tap point name. Each tap point is distinguished by a provider/name pair which must be globally unique.

Example Files.

Bryan Call then spoke gdb macros for examing data in cores and live in production instances of traffic_server. Supports conditional break and resume. To avoid problems with dropped connections causing problems you need

handle SIGPIPE nostop
handle SIGPIPE noprint

Session Logging

In most cases, there is a lot of transaction logging data that is idnetical across all transactions for a given sessions. It would be handy if there was as session log and have a key per entry which can be logged with a transaction.

One issue would be the ability to have IDs that distinguish inbound vs. outbound sessions. This doesn’t seem a large hurdle. All the data needed is already there.

Some of the motivations are

  • Potentially save a lot of log space by not duplicating session level data.

  • Session logs are useful on their own - much network analysis is session based.

  • Overall, we’re moving into a state where sessions are becoming more central, particularly with regard to hooks and overridable configurations. This is just part of the same issue.

With regard to outbound vs. inbound sessions, there are some synchronization issues with when data is accessed for transaction level logging.

We should move session based data that needs log caching (e.g.