How to Correctly Implement gRPC Client Reconnection in Kubernetes?

Correct Way to Implement Reconnection with gRPC Client

When interacting with gRPC servers deployed in a Kubernetes environment, it is essential to ensure client resiliency in case of server pod recycling. While gRPC's clientconn.go manages RPC connection handling, it does not automatically reconnect streams, leaving clients responsible for re-establishing connections independently.

Problem Overview:

The code in question attempts to handle stream reconnection based on changes in RPC connection state. However, upon facing connection issues caused by pod recycling, the client was unable to recover and continue processing requests.

Solution:

The key to addressing this issue lies in understanding that stream reconnection requires two distinct steps:

1. Wait for the RPC connection to re-establish (handled by clientconn.go).
2. Obtain a new stream from the server once the connection is back up.

The recommended code structure, provided by Emin Laletovic, effectively implements this approach:

func (grpcclient *gRPCClient) ProcessRequests() error {
  defer grpcclient.Close()

  go grpcclient.process()
  for {
    select {
      case <-grpcclient.reconnect:
        if !grpcclient.waitUntilReady() {
          return errors.New("failed to establish connection within timeout")
        }
        go grpcclient.process()
      case <-grpcclient.done:
        return nil
    }
  }
}

func (grpcclient *gRPCClient) process() {
  reqclient := GetStream() // always obtain a new stream
  for {
    request, err := reqclient.stream.Recv()
    log.Info("Request received")
    if err == io.EOF {
      grpcclient.done <- true
      return
    }
    if err != nil {
      grpcclient.reconnect <- true
      return
    }
    // Process request logic here
  }
}

func (grpcclient *gRPCClient) waitUntilReady() bool {
  // Set timeout duration for reconnection attempt
  // return true if connection is established, false if timeout occurs
}

Corrections to the Solution:

1. WaitForStateChange Issue:

   • clientconn.go's WaitForStateChange function waits for a state change from the current state, not for a specific state change. To wait for a specific state (e.g., READY), use Connect instead.
   • Tracking current state and using Connect when idle ensures continuous connection attempts.

2. Optimization:

   • Introduce a time.Ticker to periodically check and re-establish the connection (instead of an endless loop).

Updated Solution:

func (grpcclient *gRPCClient) isReconnected(check, timeout time.Duration) bool {
  ctx, cancel := context.context.WithTimeout(context.Background(), timeout)
  defer cancel()
  ticker := time.NewTicker(check)

  for {
    select {
      case <-ticker.C:
        grpcclient.conn.Connect()

        if grpcclient.conn.GetState() == connectivity.Ready {
          return true
        }
      case <-ctx.Done():
        return false
    }
  }
}

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