Published Jan 11, 2024
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Layer 7 load balancers also face another challenge known as stickiness or persistence. The principle is that they generally have to direct multiple subsequent requests or connections from a same origin (such as an end user) to the same target. The best known example is the shopping cart on an online store. If each click leads to a new connection, the user must always be sent to the server which holds his shopping cart. Content-awareness makes it easier to spot some elements in the request to identify the server to deliver it to, but that’s not always enough. For example if the source address is used as a key to pick a server, it can be decided that a hash-based algorithm will be used and that a given IP address will always be sent to the same server based on a divide of the address by the number of available servers. But if one server fails, the result changes and all users are suddenly sent to a different server and lose their shopping cart. The solution against this issue consists in memorizing the chosen target so that each time the same visitor is seen, he’s directed to the same server regardless of the number of available servers. The information may be stored in the load balancer’s memory, in which case it may have to be replicated to other load balancers if it’s not alone, or it may be stored in the client’s memory using various methods provided that the client is able to present this information back with every request (cookie insertion, redirection to a sub-domain, etc). This mechanism provides the extra benefit of not having to rely on unstable or unevenly distributed information (such as the source IP address). This is in fact the strongest reason to adopt a layer 7 load balancer instead of a layer 4 one.