Want more in-depth information about how Search works? Read our Advanced guide to how Google Search works. Except as otherwise noted, the content of this page is licensed under the Creative Commons Attribution 4. For details, see the Google Developers Site Policies. Documentation Not much time? Beginner SEO Get started. Establish your business details with Google. The disadvantage of session affinity is that your load might be less evenly distributed.
Session affinity operates on a best-effort basis to deliver requests to the same backend that served the initial request. Without session affinity enabled, load balancers distribute new requests based on a 5-tuple hash, as follows:. For pass-through load balancers, if a backend instance or endpoint is healthy, subsequent requests go to the same backend VM or endpoint.
For proxy-based load balancers, if a backend instance or endpoint is healthy and isn't at capacity subsequent requests go to the same backend VM or endpoint. The balancing mode determines when the backend is at capacity. Target pool-based network load balancers don't use backend services. Instead, you set session affinity for network load balancers through the sessionAffinity parameter in Target Pools. Do not rely on session affinity for authentication or security purposes.
Session affinity is designed to break when a backend is at or above capacity or if it becomes unhealthy. Google Cloud load balancers provide session affinity on a best-effort basis. Factors such as changing backend health check states or changes to backend fullness, as measured by the balancing mode, can break session affinity. This is because changes in the instance utilization can cause the load balancing service to direct new requests or connections to backend VMs that are less full.
This breaks session affinity. When load balancers have session affinity enabled, they load balance well when there is a reasonably large distribution of unique sessions. Reasonably large means at least several times the number of backend instances in the instance group.
When you test a load balancer with a small number of sessions, traffic isn't evenly distributed. For external and internal HTTP S load balancers, session affinity might be broken when the intended endpoint or instance exceeds its balancing mode's target maximum.
Consider the following example:. Client IP affinity directs requests from the same client IP address to the same backend instance. Client IP affinity is an option for every Google Cloud load balancer that uses backend services. Client IP affinity is a two-tuple hash consisting of the client's IP address and the IP address of the load balancer's forwarding rule that the client contacts.
The client IP address as seen by the load balancer might not be the originating client if it is behind NAT or makes requests through a proxy. This can cause incoming traffic to clump unnecessarily onto the same backend instances. If a client moves from one network to another, its IP address changes, resulting in broken affinity. When you set generated cookie affinity, the load balancer issues a cookie on the first request. For each subsequent request with the same cookie, the load balancer directs the request to the same backend VM or endpoint.
Cookie-based affinity can more accurately identify a client to a load balancer, compared to client IP-based affinity. For example:. With cookie-based affinity, the load balancer can uniquely identify two or more client systems that share the same source IP address. Using client IP-based affinity, the load balancer treats all connections from the same source IP address as if they were from the same client system. If a client changes its IP address, cookie-based affinity lets the load balancer recognize subsequent connections from that client instead of treating the connection as new.
An example of when a client changes its IP address is when a mobile device moves from one network another. If the URL map's path matcher has multiple backend service for a host name, all backend services share the same session cookie.
The lifetime of the HTTP cookie generated by the load balancer is configurable. You can set it to 0 default , which means the cookie is only a session cookie. Or you can set the lifetime of the cookie to a value from 1 to seconds 24 hours inclusive.
If the client does not provide the cookie, the proxy generates the cookie and returns it to the client in a Set-Cookie header. Regardless of the type of affinity chosen, a client can lose affinity with a backend in the following situations:.
Most Google Cloud load balancers have a backend service timeout. The default value is 30 seconds. The full range of timeout values allowed is 1 - 2,,, seconds. This is the amount of time that the load balancer waits for a backend to return a full response to a request. For example, if the value of the backend service timeout is the default value of 30 seconds, the backends have 30 seconds to deliver a complete response to requests.
The load balancer retries the HTTP GET request once if the backend closes the connection or times out before sending response headers to the load balancer. If the backend sends response headers even if the response body is otherwise incomplete or if the request sent to the backend is not an HTTP GET request, the load balancer does not retry.
If the backend does not reply at all, the load balancer returns an HTTP 5xx response to the client. To change the allotted time for backends to respond to requests, change the timeout value. For HTTP traffic, the maximum amount of time for the client to complete sending its request is equal to the backend service timeout. If the problem is because of clients that are experiencing performance issues, you can resolve this issue by increasing the backend service timeout.
If the HTTP connection is upgraded to a WebSocket, the backend service timeout defines the maximum amount of time that a WebSocket can be open, whether idle or not.
To allow more or less time before the connection is deleted, change the timeout value. This idle timeout is also used for WebSocket connections. Backend service timeout has no meaning for these pass-through load balancers. For Traffic Director , the backend service timeout field specified using timeoutSec is not supported with proxyless gRPC services.
For such services, configure the backend service timeout using the maxStreamDuration field. This is because gRPC does not support the semantics of timeoutSec that specifies the amount of time to wait for a backend to return a full response after the request is sent. Each backend service whose backends are instance groups or zonal NEGs must have an associated health check. When you create a load balancer using the Google Cloud Console, you can create the health check, if it is required, when you create the load balancer, or you can reference an existing health check.
When you create a backend service using either instance group or zonal NEG backends using the gcloud command-line tool or the API, you must reference an existing health check. Refer to the load balancer guide in the Health Checks Overview for details about the type and scope of health check required. For related documentation and information about how backend services are used in load balancing, review the following:.
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