ratelimit-server modified.

TODO (on my part): monitoring (can't quite remember how that works) and nginx mail to ruaok.

Note to self: "Don't remove the nginx block just yet; email ruaok explaining how to remove it."

Changes needed to implement this:

musicbrainz-server

In the 2 (?) places where we currently apply the ws rate limit, which looks
like this:

• check "ws ip=x.x.x.x", reject with a message if failed
• check "ws global", reject with a message if failed

Then add an extra check to precede the existing two:

• check "ws ua=$user_agent", reject with a message (exact message tbc)

ratelimit-server

• munge "ws ua=$user_agent" (where $user_agent != python-musicbrainz/0.7.3)
  to "ws ua=generic-bad-ua"
• apply a leaky 500/10s limit to "ws ua=python-musicbrainz/0.7.3"
• apply a leaky 500/10s limit to "ws ua=generic-bad-ua"

other hosting

• ensure we have rrd graphing etc of "ws ua=generic-bad-ua"
• remove the nginx outright block on bad UAs

Q3: As decided in IRC:

djce: an aggregate block, and report on the aggregate.
djce: one limit/report for py73, and one limit/report for "other bad uas"

A4: On leaky vs strict: Lets go with leaky.

A2:
per ip: 503 Your ip is over limit. Stop it.
per UA: 503 your user agent string has been throttled. See wiki page, blog, sky for details.
global: 503 we're over capacity. not your fault, sorry.

A1: We want throttling on a per UA string. We want to throttle python-musicbrainz/0.7.3 differently from YourMom/8.3. We should check in this order: ua -> ip -> global

Q1: What kind of throttling? For example, is it a single throttle shared by all bad UAs, or one throttle for python-musicbrainz/0.7.3 and one for the rest, or something else? Or some sort of per-IP throttling?

Q2: When over the limit, what response should be served? Do we know what the behaviour of the bad-UA clients is for that response? For example we might find that python-musicbrainz/0.7.3 responds well to 403 but not to 503, so maybe that UA should get a 403 when over the limit not a 503. But perhaps other bad UAs should get a 503. etc.

Q3: What monitoring is required?