Bug 1477685

+++ This bug was initially created as a clone of Bug #1470350 +++

The algorithm for setting weight can result in an unexpected A/B balance when service weight is distributed over too many pods. The minimum service weight is 1 which means that all endpoints on the service will get 1. For example when the service weight is 2 and there are 4 pods each pod will get 1 and the total service weight becomes 4. The service ends up getting too much weight. At present this is documented suggesting that the deployment be scaled to achieve the desired weight.

This change distributes the service weight to pods until the total is used. The remaining pods in the service, if any, will get weight 0. This will preserve the total desired service weight. In the above example, 2 pods will get 1 and 2 pods will get 0 for a service total of 2.

----------------------  

While doing an OpenShift training session, I set up two services with different static HTML served. We set traffic to route across 2 services 50%/50%. I changed the number of pod replicas for service A (dneary/v3simple-spatial) to 4, and set the number of pod replicas for service B (dneary/green) to 1.

I ran the following script:

for i in `seq 1 50`; do 
  curl "http://v3simple-spatial-dneary.apps.class.molw.io";
  echo;
done

What I expected: I expected to get a 50/50 split of old text ("Hello OpenShift Ninja without a DB?") and new text ("Hello there. Have you considered OpenShift?") - either alternating or randomly across the 2 services.

What I observed: I got multiple copies of old text, with 1 copy of new text. It appears that the application is round-robin distributing load across all of the pods (4 in service A, 1 in service B). This appears to be incompatible with a 50/50 split across 2 services.


Output of command:
for i in `seq 1 50`; do
  curl "http://v3simple-spatial-dneary.apps.class.molw.io";
  echo;
done
<h1>Hello there. Have you considered OpenShift?</h1>
<h1>Hello OpenShift Ninja without a DB?</h1>
<h1>Hello OpenShift Ninja without a DB?</h1>
<h1>Hello OpenShift Ninja without a DB?</h1>
<h1>Hello OpenShift Ninja without a DB?</h1>
<h1>Hello there. Have you considered OpenShift?</h1>
<h1>Hello OpenShift Ninja without a DB?</h1>
<h1>Hello OpenShift Ninja without a DB?</h1>
<h1>Hello OpenShift Ninja without a DB?</h1>
<h1>Hello OpenShift Ninja without a DB?</h1>
<h1>Hello there. Have you considered OpenShift?</h1>
<h1>Hello OpenShift Ninja without a DB?</h1>
<h1>Hello OpenShift Ninja without a DB?</h1>
<h1>Hello OpenShift Ninja without a DB?</h1>
<h1>Hello OpenShift Ninja without a DB?</h1>
<h1>Hello there. Have you considered OpenShift?</h1>
<h1>Hello OpenShift Ninja without a DB?</h1>
<h1>Hello OpenShift Ninja without a DB?</h1>
<h1>Hello OpenShift Ninja without a DB?</h1>
<h1>Hello OpenShift Ninja without a DB?</h1>
<h1>Hello there. Have you considered OpenShift?</h1>
...

--- Additional comment from Eric Paris on 2017-07-13 00:24:45 EDT ---

Wasn't there a requirement that you had to set an annotation on the route
haproxy.router.openshift.io/balance = leastconn
or something like that?

You can try that, and if it does not work I assume we are going to want to see the yaml/json for the route in question.

--- Additional comment from Yan Du on 2017-07-13 02:05:04 EDT ---

I could reproduce the issue with latest OCP env
openshift v3.6.143
kubernetes v1.6.1+5115d708d7

$ oc set route-backends route1
NAME           KIND     TO                  WEIGHT
routes/route1  Service  service-unsecure    50 (50%)
routes/route1  Service  service-unsecure-2  50 (50%)

$ oc scale rc test-rc-1 --replicas=4
replicationcontroller "test-rc-1" scaled
$ oc get pod -w
NAME              READY     STATUS    RESTARTS   AGE
test-rc-1-33rhr   1/1       Running   0          11s
test-rc-1-mfjw6   1/1       Running   0         12m
test-rc-1-tnn5g   1/1       Running   0         11s
test-rc-1-w5gh1   1/1       Running   0         11s
test-rc-2-mmf4r   1/1       Running   0         12m

$ for i in {1..50}; do curl route1-sess.0713-u9a.qe.rhcloud.com ; done
Hello-OpenShift-1 http-8080
Hello-OpenShift-2 http-8080
Hello-OpenShift-1 http-8080
Hello-OpenShift-1 http-8080
Hello-OpenShift-1 http-8080
Hello-OpenShift-1 http-8080
Hello-OpenShift-2 http-8080
Hello-OpenShift-1 http-8080
Hello-OpenShift-1 http-8080
Hello-OpenShift-1 http-8080
Hello-OpenShift-1 http-8080
Hello-OpenShift-2 http-8080
Hello-OpenShift-1 http-8080
Hello-OpenShift-1 http-8080
Hello-OpenShift-1 http-8080
Hello-OpenShift-1 http-8080
Hello-OpenShift-2 http-8080
Hello-OpenShift-1 http-8080
Hello-OpenShift-1 http-8080
Hello-OpenShift-1 http-8080
Hello-OpenShift-1 http-8080
Hello-OpenShift-2 http-8080
Hello-OpenShift-1 http-8080
Hello-OpenShift-1 http-8080
Hello-OpenShift-1 http-8080
Hello-OpenShift-1 http-8080
Hello-OpenShift-2 http-8080
Hello-OpenShift-1 http-8080
Hello-OpenShift-1 http-8080
Hello-OpenShift-1 http-8080
Hello-OpenShift-1 http-8080
Hello-OpenShift-2 http-8080
Hello-OpenShift-1 http-8080
Hello-OpenShift-1 http-8080
Hello-OpenShift-1 http-8080
Hello-OpenShift-1 http-8080
Hello-OpenShift-2 http-8080
Hello-OpenShift-1 http-8080
Hello-OpenShift-1 http-8080
Hello-OpenShift-1 http-8080
Hello-OpenShift-1 http-8080
Hello-OpenShift-2 http-8080
Hello-OpenShift-1 http-8080
Hello-OpenShift-1 http-8080
Hello-OpenShift-1 http-8080
Hello-OpenShift-1 http-8080
Hello-OpenShift-2 http-8080
Hello-OpenShift-1 http-8080
Hello-OpenShift-1 http-8080
Hello-OpenShift-1 http-8080

The route balance is roundrobin by default with multiple service in route, but even I set haproxy.router.openshift.io/balance = leastconn, it doesn't work too.
For the route yaml file and the haproxy.config are in attachment.

--- Additional comment from Yan Du on 2017-07-13 02:05 EDT ---



--- Additional comment from Ben Bennett on 2017-07-13 11:02:17 EDT ---

This is actually functioning as intended... but the behavior is not properly documented (and is rather confusing anyway).

The weights apply to each backend, so if you have a route with service A at weight 1 and service B at weight 2, if the number of back-ends for each is equal, then a pod backing A will get 33% of the traffic and a pod backing B will get 66%.  BUT if there are 2 endpoints for A and 1 for B, the numbers change... an A pod will respond 50% of the time and a B pod will respond 50% of the time.

Since weighting requires round-robin, you would expect to see round-robin behavior if all weights are equal.

If you set the balance type to leastconn, weighting has no effect.

We could take a feature request to change the behavior to set the weights proportionally based on the fraction of endpoints that service has, but that is not the way this was originally designed and approved.

We really need to update the docs to make this more clear, trello card https://trello.com/c/MajuXbiV tracks the docs improvements, and I have asked our networking docs person, and a networking developer to look at improving this ASAP.

--- Additional comment from Ben Bennett on 2017-07-13 14:51:50 EDT ---

Re-opened because the networking team decided that while we made a deliberate choice of this behavior... it was not a great one and the current behavior will surprise a lot of users.

--- Additional comment from Dave Neary on 2017-07-13 15:54:22 EDT ---

Thanks for the update and the confirmation Ben, Yan - I know this surprised me (and the evangelist training us). Also, thanks for showing me up with the {1..50} shell built-in over running seq, yan ;-)

--- Additional comment from Phil Cameron on 2017-07-25 16:53:46 EDT ---

Docs PR 4847
https://github.com/openshift/openshift-docs/pull/4847

--- Additional comment from Phil Cameron on 2017-07-25 16:53:56 EDT ---

Docs PR 4847
https://github.com/openshift/openshift-docs/pull/4847

--- Additional comment from openshift-github-bot on 2017-07-27 00:43:29 EDT ---

Commit pushed to master at https://github.com/openshift/openshift-docs

https://github.com/openshift/openshift-docs/commit/0ccbe544c0db84a9436738fed7051ea1a959b3ae
Document a/b deployment

A route can front up to 4 services that handle the requests.
The load balancing strategy governs which endpoint gets each request.
When roundrobin is chosen, the portion of the requests that each
service handles is governed by the weight assigned to the service.
Each endpoint in the service gets a fraction of the service's requests

bug 1470350
https://bugzilla.redhat.com/show_bug.cgi?id=1470350

Code change is in origin PR 15309
https://github.com/openshift/origin/pull/15309

--- Additional comment from openshift-github-bot on 2017-08-02 06:29:59 EDT ---

Commits pushed to master at https://github.com/openshift/origin

https://github.com/openshift/origin/commit/b0db0b2e3bcf1db113ed61ed1083853c986b6ef9
Make A/B deployment proportional to service weight

Distribute requests among the route's services based on service weight.
The portion of the total weight that each service has is distributed
evenly among the service's endpoints.

bug 1470350
https://bugzilla.redhat.com/show_bug.cgi?id=1470350

https://github.com/openshift/origin/commit/b64c94eb239034b0a8df8abf290f5322d7600855
Merge pull request #15309 from pecameron/bz1470350

Automatic merge from submit-queue (batch tested with PRs 15533, 15414, 15584, 15529, 15309)

Make A/B deployment proportional to service weight

Distribute requests among the route's services based on service weight.
The portion of the total weight that each service has is distributed
evenly among the service's endpoints.

bug 1470350
https://bugzilla.redhat.com/show_bug.cgi?id=1470350