The test for uniform distribution is failing: backend[0] expected 5 calls, received 10; backend[1] expected 5 calls, received 0.

An error with uneven distribution of calls between backends is usually related to load balancing issues or errors in the distribution logic.

Possible causes:

• Incorrect implementation of the balancing algorithm: for example, round-robin may fail if the counter is not updated correctly.
• Caching or sticky sessions, which cause all requests to go to one server.
• Concurrency issues: if call counters are updated without synchronization, race conditions may occur.
• Test errors: perhaps the test incorrectly counts calls or does not account for asynchrony.

For diagnostics:

• Check the request distribution logic, ensure counters are updated atomically.
• Use profiling and logging tools to track where requests are actually going.
• Check the load balancer settings (if any) and ensure there are no sticky sessions.

Example of simple round-robin load balancing with an atomic counter in Go:

var counter uint64
backends := []string{"backend0", "backend1"}

func getBackend() string {
    idx := atomic.AddUint64(&counter, 1)
    return backends[idx%uint64(len(backends))]
}

If the counter is not atomic, distribution errors may occur.