Why Does the Driver Throw a Timeout During Server Selection?

Each driver operation requires that you choose a healthy server satisfying the server selection criteria. If you do not select an appropriate server within the server selection timeout, the driver throws a server selection timeout exception. The exception looks similar to the following:

A timeout occurred after 30000ms selecting a server using CompositeServerSelector{ Selectors = MongoDB.Driver.MongoClient+AreSessionsSupportedServerSelector, LatencyLimitingServerSelector{ AllowedLatencyRange = 00:00:00.0150000 }, OperationsCountServerSelector }.
Client view of cluster state is
{
    ClusterId : "1",
    Type : "Unknown",
    State : "Disconnected",
    Servers :
    [{
        ServerId: "{ ClusterId : 1, EndPoint : "Unspecified/localhost:27017" }",
        EndPoint: "Unspecified/localhost:27017",
        ReasonChanged: "Heartbeat",
        State: "Disconnected",
        ServerVersion: ,
        TopologyVersion: ,
        Type: "Unknown",
        HeartbeatException: "<exception details>"
    }]
}.

The error message consists of multiple parts:

1. The server selection timeout (30000 ms).

2. The server selectors considered (CompositeServerSelector containing AreSessionsSupportedServerSelector, LatencyLimitingServerSelector, and OperationsCountServerSelector).

3. The driver’s current view of the cluster topology. The list of servers that the driver is aware of is a key part of this view. Each server description contains an exhaustive description of its current state including information about an endpoint, a server version, a server type, and its current health state. If the server encounters issues in reporting its health, HeartbeatException contains the exception from the last failed heartbeat. Analyzing the HeartbeatException on each cluster node can assist in diagnosing most server selection issues. The following heartbeat exceptions are common:

   • No connection could be made because the target machine actively refused it: The driver cannot see this cluster node. This can be because the cluster node has crashed, a firewall is preventing network traffic from reaching the cluster node or port, or some other network error is preventing traffic from being successfully routed to the cluster node.

   • Attempted to read past the end of the stream: This error happens when the driver cannot connect to the cluster nodes due to a network error, misconfigured firewall, or other network issue. To address this exception, ensure that all cluster nodes are reachable. This error commonly occurs when the client machine’s IP address is not configured in the Atlas IPs Access List, which can be found under the Network Access tab for your Atlas Project.

   • The remote certificate is invalid according to the validation procedure: This error typically indicates a TLS/SSL-related problem such as an expired/invalid certificate or an untrusted root CA. You can use tools like openssl s_client to debug TLS/SSL-related certificate problems.