Load-Balancing

A held connection steers new work elsewhere; weights change the comparison between peers.

Least connections measures occupied connections, not every kind of load

Least-connections balancing chooses a peer using active connection counts, with the implementation’s weighting and tie rules. In nginx, least_conn accounts for server weights and uses weighted round robin among equally suitable peers. This can help when requests occupy connections for different durations. It is still a connection-based measurement, not direct knowledge of CPU, memory pressure, queued jobs, or database contention. ...

Equal counts, weighted shares, and stable affinity express different routing policies.

Choose an nginx balancing method by the decision it actually makes

An upstream group names candidate backend endpoints, and its balancing method chooses among eligible peers. Round robin distributes selections by weight; least connections considers active connections and weights; hash methods use a selected key for affinity. These decisions describe routing policy. They do not automatically measure CPU utilization, guarantee equal response times, or make local session state durable when a peer disappears. ...

Configured failed attempts exclude a peer temporarily; active health probes are a separate capability.

Passive upstream health follows failed traffic, not a probe timer

nginx passive upstream health uses outcomes of real requests to decide whether a peer should temporarily stop receiving traffic. max_fails sets an unsuccessful-attempt threshold, while fail_timeout supplies both the counting interval and the unavailability period. These settings do not schedule periodic application probes. Active checks require a supported checking capability and configuration beyond this passive request-outcome policy. ...

One HTTP connection can visit two routed backends, while a TCP relay keeps that connection on one peer.

Transport balancing keeps a connection together; HTTP routing can split its requests

The useful distinction between transport and HTTP balancing is where a routing decision can be made. A TCP relay can select a backend for a connection and forward its byte stream without interpreting HTTP requests. An HTTP proxy parses requests and can choose an upstream using their paths or headers. Several requests on one client connection can therefore have different routing outcomes. ...