The Sticky Sessions feature, also known as Session Affinity, changes the routing configuration of your application. It associates all HTTP requests from an end-user to a single web application container.

Description

By default the routing of requests is done using the Round Robin algorithm, learn more about it in our dedicated page. As an example: if you have 2 web containers, and a single user is making 2 requests to your application, each container will receive one request.

Once Sticky Sessions have been enabled, each end-user will be associated to a container and consecutive requests from this entity will all target the same web containers. Round Robin is still used but per end-user and not per request.

Technical details

Sticky Session Cookie

Any request done to the application having Sticky Sessions enabled, will receive a sc-sticky-session cookie in the request response. This cookie has a 24 hours duration. At each client’s request, a new sticky session cookie is returned, with a duration of 24 hours.

└> curl my-app.osc-fr1.scalingo.io -I
HTTP/2 200 OK
Date: Wed, 21 Mar 2018 17:51:09 GMT
Content-Type: text/html; charset=utf-8
Content-Length: 13
Connection: keep-alive
X-Request-ID: d2df9e31-7415-46bf-beff-8ba31623a817
Set-Cookie: sc-sticky-session=GCLlOQs/Ttbrq5u1Y76yc4mI0uJjTE8vpoOUS/RG5jo=; Expires=Thu, 22-Mar-18 17:51:09 GMT; Max-Age=86400; Domain=my-app.osc-fr1.scalingo.io; Path=/; HttpOnly

If the cookie is not correctly sent back, the request may be routed to another backend web container.

Sticky Session Cookie Invalidation

• If one of the container becomes unhealthy, because it has crashed or because it is overloaded, our routers will quarantine it and the sticky session cookie will be invalidated.

• Each operation changing the routing rules of the application will invalidate current cookies and end-users will receive a new version of the cookie and will be routed to a new instance. More precisely, those operations made on your app will invalidate current cookies:
  • Deployment
  • Restart
  • Scaling

• Applications hosted on the platform can be cycled at any time for internal infrastructure management purposes (like dynamic load balancing of the server), when such event happens, the sticky sessions are also invalidated.

• If the sticky session cookie expires, the client’s request is directed toward a randomly chosen container, and a new sticky session cookie is returned.

The case of Meteor Framework

For Meteor applications, Sticky Sessions are enabled by default because it is required to scale properly.