Originally posted on Opensource.com
Live demos are the bane of professional speakers everywhere. Even the
most well-prepared live demo can go wrong for unforeseeable reasons.
This is a bad thing to happen while you're up on-stage in front of 300
people. Live demos of remote web apps are so fraught with peril that
most people find other ways of presenting them. Screenshots can never
fail, and local sandboxes won't fail on overloaded conference Internet
connections. But what if we can't set up a local sandbox in time for our
talk? What if our database is huge and complex? What if our app has
animation and interactions that we can't show with screenshots?
What if we could record our use of a web application and then replay the
stored responses at the right time? Lucky for us, it's easy to proxy
HTTP, the protocol that web browsers and web servers use to communicate
with each other. This means we can put an intermediary between our
browser and the server to do whatever we want. Often caching does
content filtering (corporate filters, parental filters). But caching
data on a server closer to the user can speed up a website.
We're going to use a web proxy in a similar way: We'll cache our content
and serve that cached data to our web browser. However, we're going to
run our proxy on the same machine as our web browser. And, we're going
to set it up to cache only the things that we want. This way we can run
a live demo on an unstable connection.
Install and Configure Squid HTTP Proxy
First, we need to install and configure our proxy. I'm on a Mac, so
I was able to install the Squid HTTP Proxy
via Homebrew, a free package manager for MacOS.
For our live demo, we want to cache the application we are trying to
demo and any other content the application needs. Anything else is
unnecessary. To do this, Squid has Access Control Lists
configure an ACL with a list of domains that we should cache, and deny
everything else. For maximum coverage, we should add both the host name
and the IP addresses to the ACL. Since HTTP proxies are also used for
DNS, most of the time the proxy is looking up the DNS records. But
sometimes a browser already knows the IP and will just tell the proxy to
get on with it.
So, here's our list of domains and IPs:
acl cacheDomain dstdomain beta.cpantesters.org
acl cacheDomain dstdomain api.cpantesters.org
acl cacheDomain dstdomain www.cpantesters.org
acl cacheDomain dstdomain 220.127.116.11
acl cacheDomain dstdomain cdnjs.cloudflare.com
The first three domains are the applications that I'm running. The
fourth is the IP address for that application server: All the domains
that I depend on from CDNJS.
Once we've listed what we want to cache, we can forbid any other domains
from being cached:
cache deny !cacheDomain
Next, we should tell Squid where to put our cache and how much disk
space to use. Homebrew's Squid configuration has a
cache_dir line, commented
out. We need to enable it and increase the disk space available to
ensure that our data stays cached. When the disk space is used up, Squid
starts deleting old cached data, which we can't have during our demo.
# Uncomment and adjust the following to add a disk cache directory.
cache_dir ufs /usr/local/var/cache/squid 1024 16 256
The first number at the end of the line is the cache size in MB, which
I adjusted to 1024 (1 GB).
Finally, we should make sure that we can use Squid's management API, and
that it's only open to the local machine. This should be the default, so
look for these
http_access lines, and add them if they don't exist.
# Only allow cachemgr access from localhost
http_access allow localhost manager
http_access deny manager
After allowing cache manager access from
localhost, we should disable
the cache manager password:
cachemgr_passwd none all
Now we're done with the configuration file. Our full configuration file is located
Now that we've configured our proxy, we can start it up. Homebrew says
brew services start squid, but your platform may need something
different. This gets the proxy started and waiting for requests. Next
we need to configure our browser to use the proxy.
Configure your web browser
Configuring your web browser for an HTTP proxy depends on what browser
you use and what OS you use. If you're using Chrome or Safari on MacOS,
you can go to System Preferences to configure a proxy. However, if
you're using Firefox, you can configure the browser to use a proxy, and
leave the rest of the system alone. Other operating systems have other
ways to configure proxies, and you should check your OS's documentation.
There are some good browser plugins for managing HTTP proxies, but
unfortunately not for Safari or IE. If you're using Chrome, try Proxy
If you're using Firefox, use FoxyProxy
These plugins make it easier to manage HTTP proxies.
Run through the demo to cache your content
Once you configure your proxy, you can run through your demo to test it.
Do this on a good Internet connection. As you run through your demo,
your browser asks its proxy to fetch all the demo's data. As your proxy
does this, it caches it on disk. Since your computer is online, Squid
will follow the caching rules that the web server asks it to. This means
caching for a specific length of time, and possibly revalidating the
data to see if it changed.
As we run through our demo, we should make sure that our cache is being
used. The easiest way to do that is to read Squid's log. For my
configuration, it was located at
Inside are lines that look like this:
1498020228.970 203 ::1 TCP_MISS/200 3653 GET http://beta.cpantesters.org/chart.html? - HIER_DIRECT/18.104.22.168 text/html
1498020229.523 314 ::1 TCP_REFRESH_MODIFIED/200 8130 GET http://api.cpantesters.org/v3/release/dist/Statocles - HIER_DIRECT/22.214.171.124 application/json
1498020236.187 6945 ::1 TCP_MISS/200 148284 GET http://api.cpantesters.org/v3/summary/Statocles/0.077 - HIER_DIRECT/126.96.36.199 application/json
1498020240.783 186 ::1 TCP_MISS/200 6597 GET http://people.w3.org/~dom/archives/2006/09/offline-web-cache-with-squid/ - HIER_DIRECT/188.8.131.52 text/html
The important parts of this line are the URL and the status.
TCP_MISS/200 means "this request was not in our cache, and the remote
server returned a
200 OK HTTP response".
means "this request was in our cache, but we refreshed it from the
remote server which returned a
200 OK HTTP response". This is our
cache building and refreshing itself because we're on a stable
connection. Once we have some data in our cache, we'll start seeing
things like this:
1498063273.261 0 ::1 TCP_INM_HIT/304 299 GET http://beta.cpantesters.org/chart.html - HIER_NONE/- text/html
1498063281.831 0 ::1 TCP_MEM_HIT/200 8187 GET http://api.cpantesters.org/v3/release/dist/Statocles - HIER_NONE/- application/json
1498063298.103 0 ::1 TCP_MEM_HIT/200 8187 GET http://api.cpantesters.org/v3/release/dist/Statocles - HIER_NONE/- application/json
1498063300.473 8 ::1 TCP_MEM_HIT/200 154917 GET http://api.cpantesters.org/v3/summary/Statocles/0.083 - HIER_NONE/- application/json
TCP_INM_HIT/304 means "The cache responded to this request with a
Not Modified response". The
TCP_MEM_HIT/200 means "The cache
responded to this request with a
200 OK HTTP response". These are what
we want: The cache is serving responses, not the remote server.
Run Your Demo
Now that our cache is operating well on a stable connection, we can give
our demo on an unstable one. First, we want to make sure that our cache
does not try to access the remote server (Squid's "offline" mode). To do
this, Squid has a management client called
squidclient which we can use
to toggle offline mode.
$ squidclient mgr:offline_toggle
HTTP/1.1 200 OK
Date: Tue, 04 Jul 2017 21:16:36 GMT
Expires: Tue, 04 Jul 2017 21:16:36 GMT
Last-Modified: Tue, 04 Jul 2017 21:16:36 GMT
X-Cache: MISS from gwen.local
Via: 1.1 gwen.local (squid/3.5.26)
offline_mode is now ON
Squid's offline mode minimizes attempts to get remote content. Since we
cached all our content running through our demo, this means Squid will
be serving our demo!
So now we can run our demo worry-free! All the remote content is served
by the local machine, so it doesn't matter how good the conference wi-fi
is. As long as stick to things we've already cached, our web application
This year I had one goal for CPAN Testers:
Replace the current Metabase API with
a new API that did not write to Amazon SimpleDB. The current
high-availability database that raw incoming test reports are written is
Amazon SimpleDB behind an API called Metabase.
Metabase is a highly-flexible data
storage API designed to work with massive, unstructured data sets and
still allow for sane organization and storage of data. Unfortunately,
Amazon SimpleDB is as it says on the tin: Simple. Worse, it's expensive:
Like most Amazon services, it charges for usage, so there's a huge
incentive for CPAN Testers to use it as little as possible (which made
some of the code quite obtuse).
So, I made a plan to excise the Metabase. Since we already cached every
raw test report locally in the CPAN Testers MySQL database, I planned to
write a new Metabase API that wrote directly to the cache, and then
adjust the backend processing to read from the cache. I spent the better
part of a month working through all the Metabase APIs, how the data was
stored in the database, and how to translate between a simple JSON
format and the serialized Metabase objects. However, some proper schema
design prevented me from finishing this project: A single
column could not be changed to allow nulls very easily, it being a 600GB
table. The one time where a well-designed schema was a bad thing!
But then Garu, author of
came up with an idea to make a new test report format. These new reports
would have to be stored in a new place, and I discovered that MySQL had
recently started building some rich JSON
a new JSON test report format and storing it in our new
high-availability MySQL cluster seemed like a perfect solution for
storing our raw test reports.
After a few weeks of discussion, I finally realized that it would be an
easier task to make a backwards-compatible Metabase API write to the new
test report MySQL table, even though it increased the amount of work
that needed to be done:
- Complete the new test report format schema (Garu)
- Write the new backwards-compatibility Metabase API (Me)
- Write a new test report processor that writes to the old Metabase
cache tables (Joel Berger)
- Write a migration script from the old Metabase cache tables to the new
test report JSON object (?)
With that plan, I headed for Lyon.
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