[keycloak-dev] Sticky sessions in backchannel requests

[Marek Posolda]

I was thinking about possibilities how to solve the issue with sticky 
sessions in backchannel requests. It seems that we need to support the 
scenarios, when backchannel requests won't be able to share the sticky 
session cookie with the browser. However in many cases it may be 
possible that backchannel requests can participate in the "sticky 
session", which will have great advantage for performance. Some thoughts:

- OAuth code (the one used in code-to-token backchannel request) can be 
JWT signed by realm HMAC key. One of the claims in the code could be 
"route-id"

- Refresh tokens will also contain "route-id" claim

- Keycloak OIDC adapters will be able to read the "route-id" from the 
code and they will attach sticky session information to the single 
backchannel request. Hence the backchannel request will be able to 
participate in sticky session and will be properly routed by 
loadbalancer to the correct node. Same for requests using refresh token.

- It seems we will need some flexibility how is the "sticky session" 
added to the request to support various loadbalancers (cookie, 
path-parameters etc). Maybe we need some SPI on adapter side? Or just 
some kind of expressions/tokens, which will help to configure how 
exactly will cookie/path parameter look like?

- Keycloak.js adapter seems to be working already. Ajax requests are 
re-sending the browser cookies and they are available in backchannel 
endpoint on server-side. This is true even if AUTH_SESSION_ID cookie is 
httpOnly. I've tested with Firefox, Chrome, my mobile phone. Not sure if 
this is different in other browsers/devices (cordova etc)?

- For SAML backchannel requests (backchannel logout), we can add the 
same to our own SAML adapters though.

- If people use 3rd party adapters, they won't have this. However we can 
also have support for some loadbalancers to route the requests directly 
based on the content of the route-id inside code (or refresh token). In 
many cases, customers will already have established loadbalancer in 
their deployments. However some others might be starting their 
deployment from the scratch and we can suggest them to use some of our 
preferred loadbalancers though.

I've checked that if wildfly+mod_cluster is used as loadbalancer, we 
have the flexibility to inject custom undertow handler, which will be 
able to look into JWT and add the route info to the HttpServerExchange, 
from where mod_cluster will read it. Maybe we can investigate this 
possibility in the other popular loadbalancers (nginx etc) ?

So in summary: Backchannel requests won't be able to participate in 
sticky session just if: 3rd party adapter is used AND customer can't use 
our preferred loadbalancer.

WDYT?

Marek