add non-alternatives

tsa/howto/cache.mdwn

@@ -150,13 +150,27 @@ unlimited gigabit.
 
 ## Alternatives considered
 
-Four alternatives were considered:
+Four alternatives were seriously considered:
 
  * Apache Traffic Server
  * Nginx proxying + caching
  * Varnish + stunnel
  * Fastly
 
+Other alternatives were ignored:
+
+ * [Apache HTTPD caching](https://httpd.apache.org/docs/2.4/caching.html) - performance expected to be sub-par
+ * [Envoy][] - [not designed for caching](https://github.com/envoyproxy/envoy/issues/868), [external cache support
+   planned in 2019](https://blog.getambassador.io/envoy-proxy-in-2019-security-caching-wasm-http-3-and-more-e5ba82da0197?gi=82c1a78157b8)
+ * [HAproxy](https://www.haproxy.com/) - [not designed to cache large objects](https://www.haproxy.com/documentation/aloha/9-5/traffic-management/lb-layer7/caching-small-objects/)
+ * [Nuster](https://github.com/jiangwenyuan/nuster) - new project, not packaged in Debian (based on
+   HAproxy), performance [comparable with nginx and varnish](https://github.com/jiangwenyuan/nuster/wiki/Web-cache-server-performance-benchmark:-nuster-vs-nginx-vs-varnish-vs-squid#results)
+   according to upstream, although impressive improvements
+ * [Polipo](https://en.wikipedia.org/wiki/Polipo) - not designed for production use
+ * [Squid](http://www.squid-cache.org/) - not designed as a reverse proxy
+ * [Traefik](https://traefik.io/) - [not designed for caching](https://github.com/containous/traefik/issues/878)
+
+[Envoy]: https://www.envoyproxy.io/
 ### Apache Traffic Server
 
 Pros:
@@ -240,6 +254,8 @@ Cons:
 
 ## Sources
 
+Benchmarks:
+
  * [Bizety: Nginx vs Varnish vs Apache Traffic Server - High Level
    Comparison](https://www.bizety.com/2016/01/07/nginx-vs-varnish-vs-apache-traffic-server-high-level-comparison/) - "Each proxy server has strengths and weakness"
  * [ScaleScale: Nginx vs Varnish: which one is better?](https://www.scalescale.com/tips/nginx/nginx-vs-varnish/) - nginx +
@@ -254,6 +270,11 @@ Cons:
  * [University of Oslo: Performance Evaluation of the Apache Traffic
    Server and Varnish Reverse Proxies](https://pdfs.semanticscholar.org/157b/bec8591a9fdb21e90831309f10ff6705b70d.pdf) - "Varnish seems the more
    promising reverse proxy server"
+ * [Loggly: Benchmarking 5 Popular Load Balancers: Nginx, HAProxy,
+   Envoy, Traefik, and ALB](https://www.loggly.com/blog/benchmarking-5-popular-load-balancers-nginx-haproxy-envoy-traefik-and-alb/)
+
+Tutorials and documentation:
+
  * [Apache.org: Why Apache Traffic Server](https://svn.apache.org/repos/infra/websites/production/trafficserver/content/why-ats.html) - upstream docs
  * [czerasz.com: Nginx Caching Tutorial - You Can Run Faster](https://czerasz.com/2015/03/30/nginx-caching-tutorial/) -
    tutorial