Ottomata added a comment.

  This happpens when an IP opens up too many simultaneous connections to the 
service.  The client IP is passed on to the eventstreams service from the 
Varnish frontend http servers, and then a running count of connections per 
eventstreams application instance is kept 
<https://github.com/wikimedia/mediawiki-services-eventstreams/blob/master/routes/stream.js#L338-L347>.
 If too many connections are made to that application instance, a 429 is 
returned.  The current limit is connections 2 per application instance, of 
which there are 8 in each DC.  So you should be able to open a total of 16 
connections from your IP to a single DC 
<https://gerrit.wikimedia.org/r/plugins/gitiles/operations/deployment-charts/+/refs/heads/master/helmfile.d/services/eventstreams/values.yaml#39>
 (you will be routed to one of the two automatically based on your location).  
If you get this error, and are opening fewer than 16 connections, I'd expect a 
reconnect to route eventually route you to an application instance where you 
have fewer than 2 connections open.
  
  I suppose, the more connections you have open at once, the more likely this 
is to happen.

TASK DETAIL
  https://phabricator.wikimedia.org/T308931

EMAIL PREFERENCES
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To: Ottomata
Cc: Aklapper, pywikibot-bugs-list, Ottomata, Xqt, Jyoo1011, RBrounley_WMF, 
JohnsonLee01, SHEKH, Dijkstra, Khutuck, Zkhalido, Viztor, Wenyi, Xinbenlv, 
Vacio, Tbscho, MayS, Mdupont, JJMC89, Dvorapa, Altostratus, Avicennasis, 
Nirmos, mys_721tx, jayvdb, Masti, Alchimista, Krenair
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