NIOConnectionPool connectionPoolExhausted: Immediate Failure Versus Back-Pressure Semantics
SwiftNIO's NIOConnectionPool throws ConnectionPoolError.connectionPoolExhausted when all pooled connections are in use and no idle connection is available for a new request. The error is surfaced through the ChannelFuture 's failure handler, allowing developers to detect intermittent exhaustion under load. Because the pool size is fixed at initialization and
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