Server connection queuing for REST and WebSocket APIs
API Security Enforcer (ASE) can queue server connection requests when the backend application programming interface (API) servers are busy. When enabled, server connection queuing applies to both REST and WebSocket APIs and is configured in the API JavaScript Object Notation (JSON) file.
Connection queuing for stateless connections
Stateless connections are connections without cookies. Before enabling connection queuing, configure connection quota values for the backend API servers. After both connection quota and connection queuing are set, the requests are routed based on the following weightage formula:
Where Q i is the server connection quota for servers from i=1 to i=n
For example, if two backend servers have connection quota set as 20,000 and 40,000 connections, then the connections are served in a ratio of 20000/ (20000+40000) and 40000/ (20000+40000), that is, in the ratio of 1/3 and 2/3 for the respective servers.
When queuing is enabled and the backend servers are occupied, the connections are queued for a period. The connections are forwarded to the next available backend server during the queuing period based on the weighted ratio of server connection quota.
Connection queueing for stateful connections
Stateful connections are connections with cookies. In this mode, cookies are used to establish sticky connections between the client and the server. Before enabling connection queuing, configure connection quota values for the backend API servers. After both connection quota and connection queuing are set, the requests are routed based on the following formula:
Where Q i is the server connection quota for servers from i=1 to i=n
For example, if two backend servers have connection quota set as 20,000 and 40,000 connections, then the connections are served in a ratio of 20000/ (20000+40000) and 40000/ (20000+40000), that is, in the ratio of 1/3 and 2/3 for the respective servers. The weighted ratio of connection distribution is reached when the server connection quota is reached for all backend servers. Stateful connection distribution considers cookie stickiness with backend servers.
When queuing is enabled and the backend servers are occupied, the connections are queued for a period. Stateful connections are attempted with the same backend server. If the server becomes available during the queuing period, the connections are served. If the backend server is not available, the connections are dropped.