Question
What telemetry can confirm whether intermittent Nomad client RPC failures stem from stale server connections?
Ari Echo
0 reputation · 25 May 2020, 03:14 UTC
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0 reputation · 25 May 2020, 03:14 UTC
Nomad clients maintain long-lived, multiplexed connections to Nomad servers for RPC traffic. In environments with NAT gateways, load balancers, or firewalls that silently drop idle TCP sessions, these connections can become stale without either side noticing immediately, which is one plausible cause of intermittent RPC latency or failures. The exact behavior is version-dependent and I have not verified it against a specific release.
The uncertainty is what a client actually does when a server connection goes stale: whether it detects the dead connection proactively (for example via application-level heartbeats or yamux keepalives) or only discovers it on the next RPC attempt, and whether it then reconnects automatically or surfaces repeated errors first.
I am also unsure which documented telemetry, if any, would let an operator distinguish "the pool of server connections is exhausted or stale" from unrelated causes such as server overload or network packet loss. Metric names and availability differ across Nomad versions, so any answer should state the version it applies to.
Does the Nomad client perform keepalive or health checking on its multiplexed server connections, and is any of that configurable? Which documented metrics or log entries would confirm stale-connection churn versus server-side slowness? Is there a supported way to bound connection lifetime or force periodic reconnection?
31775 reputation · 25 May 2020, 06:14 UTC
To confirm whether intermittent RPC failures are caused by stale TCP connections (often due to silent drops by firewalls or NAT gateways), you must correlate client-side log events with OS-level TCP state. Because Nomad uses a multiplexed RPC layer (yamux), a single TCP connection carries multiple streams; a stale connection will typically cause all concurrent RPC calls on that pipe to fail simultaneously before the client attempts a reconnect.
The following signals help distinguish stale connections from server-side overload or general packet loss:
nomadcontext deadline exceeded or connection reset by peer. If these are immediately followed by reconnecting to server messages, it indicates the client discovered the transport was dead only upon attempting a request, rather than proactively.nomad.client.rpc.errors if available in your version. A spike in RPC errors without a corresponding spike in server-side CPU, memory, or RPC latency suggests the failure is in the transport layer, not the application logic.net.ipv4.tcp_keepalive_time with your network infrastructure's idle timeout. If the firewall drops sessions at 300 seconds but the OS keepalive is 7200 seconds, the connection will go stale silently.DEBUG. This reveals the exact sequence of connection attempts and the specific point of failure during the RPC handshake.ss -ntp | grep nomad. Look for connections in ESTABLISHED state that have not transmitted data for longer than your firewall's idle timeout.tcpdump on the RPC port. If the client sends a packet and receives no response, or a TCP RST arrives from an intermediate gateway, the connection was dropped by the network rather than the server.This analysis assumes a standard Nomad deployment (v1.0+) using the default yamux multiplexing. Because multiplexing masks individual stream failures, you may not see a connection-lost error until the underlying TCP socket is fully severed or a timeout is reached. Metric names and log formats vary between Nomad versions, so confirm them against your release's documentation.
Missing Diagnostic Detail: To refine this recommendation, please provide the specific Nomad version and the idle timeout setting of your load balancer or firewall.
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