Global traffic routing

This page covers our mechanisms for routing user requests through our Traffic infrastructure layers. The routing can be modified through administrative actions to improve performance and/or reliability, and/or respond to network outage conditions.
== Sites ==
There are currently seven total data centers (or "sites"). All locations can receive direct user traffic, however eqiad and codfw also host Core application services, whereas ulsfo, esams, drmrs, eqsin, and magru are limited to Edge caching.
== GeoDNS (User-to-Edge Routing) ==
The first point of entry is when the client performs a DNS request on one of our public hostnames. Our authoritative DNS servers perform GeoIP resolution and hand out one of several distinct IP addresses, sending users approximately to their nearest site. We can disable sending users directly to a particular site through DNS configuration updates. Our DNS TTLs are commonly 5 minutes long, and some rare user caches will violate specs and cache them longer. The bulk of the traffic should switch inside of 5 minutes, though, with a fairly linear progression over that window.
== Edge routing ==
Internet traffic can enter through the front edge (traffic layer) of any of the data centers. Requests that are not cache hits are then routed on to eventually reach a backend service (application layer) in a core data center.
Ideally, all of our application-layer services operate in an active/active configuration, meaning they can directly and simultaneously accept web traffic in either of the application data centers. Some application services are active/passive, meaning that they're only accepting web traffic in only the primary application data center, but not the secondary at the same time. Active/active services might be temporarily configured to use only a single one of the application data centers for various operational maintenance or outage reasons.
In the active/active application's case, global traffic is effectively split. Users whose traffic enters at eqsin, ulsfo, or codfw would reach the application service in codfw, and users whose traffic enters at drmrs, esams, or eqiad would reach the application service in eqiad.
When LVS (the entry point of the CDN) balances traffic to port 443 it uses a hash of the client IP to help with TCP Fast Open and TLS session persistence.
Within the caching layer (cp servers), the jump from HAProxy:443 to Varnish:80 is direct on the local host. However, the jump from Varnish (frontend, port 80) to Apache Traffic Server (backend, port 3128) is different: For that jump, we hash on the URL (and other req meta-data) when balancing to the backends to divide the cache space among all machines, and thus the request typically moves from one machine to another within the same cluster.
== Cache-to-application routing ==
=== Frontend routing ===
Upon entering a given data center, HTTP requests reach a cache frontend host running Varnish. At this layer, caching is controlled by either the cache::req_handling or cache::alternate_domains hiera setting. The former is used by main sites like the wikis and upload.wikimedia.org, while the latter is used by miscellaneous sites such as for example phabricator.wikimedia.org and grafana.wikimedia.org. Choosing which data structure to use depends on whether the site needs to be controlled by the regular or misc VCL, most likely misc. It is thus almost sure that additional services need to be added to cache::alternate_domains. If in doubt, contact the traffic team. The format of both data structures is:
cache::alternate_domains:
 hostname1:
 caching: 'normal'
 hostname2:
 caching: 'pass'
In Puppet terms there is a data type for those structures: Profile::Cache::Sites. The caching attribute is particularly interesting, see its type definition.
A value of normal in the caching attribute means that Varnish will cache the responses for this site unless Cache-Control says otherwise. Conversely, pass means that objects for this site are never to be cached. It would be preferable to specify normal and ensure that the origin returns Cache-Control with appropriate values for responses that should not be cached, but where this is not possible pass can be used. For sites that need to support websockets, such as Phabricator/Etherpad, use websockets. A sample of the production values for cache::alternate_domains as of July 2020 follows.
cache::alternate_domains:
 15.wikipedia.org:
 caching: 'normal'
 analytics.wikimedia.org:
 caching: 'normal'
 annual.wikimedia.org:
 caching: 'normal'
 blubberoid.wikimedia.org:
 caching: 'pass'
 bienvenida.wikimedia.org:
 caching: 'normal'
 etherpad.wikimedia.org:
 caching: 'websockets'
=== Backend routing ===
In case there is no cache hit at the frontend layer, requests are sent to a cache backend running ATS in the same DC. Backend selection is done by applying consistent hashing on the request URL. If at the backend layer there is also no cache hit, the final step is routing requests out the back edge of the Traffic caching infrastructure into the application layer. The application layer services can exist at one or both of the two application data centers (eqiad and codfw) on a case-by-case basis. This is controlled by ATS remap rules mapping the Host header to a given origin server hostname.
The list of mappings and transformations (via Lua plugins) is controlled by Hiera key profile::trafficserver::backend::mapping_rules. For production, these are managed in profile/trafficserver/backend.yaml.
For most services, the configuration of whether the service is active/active or active/passive is done via DNS/Discovery. The exception to this rule is services available in one primary DC only, such as pivot (eqiad-only) in the example below:
profile::trafficserver::backend::mapping_rules:
 - type: map
 target: http://15.wikipedia.org
 replacement: https://webserver-misc-apps.discovery.wmnet
 - type: map
 target: http://phabricator.wikimedia.org
 replacement: https://phabricator.discovery.wmnet
 - type: map
 target: http://pivot.wikimedia.org
 replacement: https://an-tool1007.eqiad.wmnet
Any administrative action such as depooling an application DC for active/active services, or moving an active/passive service from one application DC to the other, can be performed via DNS discovery updates.
When adding a new service to profile::trafficserver::backend::mapping_rules, ensure that the public hostname (ie: the hostname component of target) is included in the Subject Alternative Name (SAN) list of the certificate served by replacement. This is needed to ensure a successful TLS connection establishment between ATS and the origin server.
The following command provides an example for how to verify that the hostname phabricator.wikimedia.org is included in the SAN of the certificate offered by phabricator.discovery.wmnet:
$ echo | openssl s_client -connect phabricator.discovery.wmnet:443 2>&1 | openssl x509 -noout -text | grep -q DNS:phabricator.wikimedia.org && echo OK || echo KO
OK
If the above command fails, you might have to update the origin server certificate to include the public hostname. See Cergen.
To further verify that HTTPS requests are served properly by the configured origin, and everything works including the TLS handshake:
# get the IP address of phabricator.discovery.wmnet
$ host phabricator.discovery.wmnet
phabricator.discovery.wmnet is an alias for phab1001.eqiad.wmnet.
phab1001.eqiad.wmnet has address 10.64.16.8
# test an HTTPS request
$ curl -I https://phabricator.wikimedia.org --resolve phabricator.wikimedia.org:443:10.64.16.8 
HTTP/1.1 200 OK
[...]
== Management ==
=== Disabling a Site ===
To disable a site as an edge destination for user traffic in GeoDNS:
Downtime the matching site alert (if there is one) in https://icinga.wikimedia.org/cgi-bin/icinga/status.cgi?search_string=traffic+drop
In the operations/dns repo, edit the file admin_state
There are instructions inside for complex changes, but for the basic operation of completely disabling a site, the line you need to add at the bottom for e.g. disabling esams is:
 geoip/generic-map/esams => DOWN
... and then deploy the DNS change in the usual way: merge through gerrit, ssh to any one of our authdns servers (in A:dns-rec), and execute authdns-update as root.
=== Hard enforcement of GeoDNS-disabled sites ===
In the case that we need to guarantee that zero requests are flowing into the user-facing edge of a disabled site for a given cache cluster (or all clusters), we can forcibly block all traffic at the front edge. This should only be done when strictly necessary, and only long after (e.g. 24H after) making the DNS switch above, to avoid impacting those with minor trailing DNS cache update issues. To lock traffic out of the cache frontends for a given cluster in a given site, you'll need to merge and deploy a puppet hieradata update which sets the key cache::traffic_shutdown to true for the applicable cluster/site combinations.
For example, to lock all traffic out of the text cluster in eqiad, add the following line to hieradata/role/eqiad/cache/text.yaml:
cache::traffic_shutdown: true
Once the change is merged and applied to the nodes with puppet, all requests sent to eqiad will get a HTTP 403 response from the cache frontends instead of being served from cache or routed to the appropriate origin server.
== External links ==
*Geomapping of the different datacenters, as seen in the dns configuration
Category:Caching
Category:SRE Traffic