code: func NewHostCertificateCache(keygen sshca.Authority, authClient auth.ClientI) (*certificateCache, error) {
  	cache, err := ttlmap.New(defaults.HostCertCacheSize)
  	if err != nil {
  		return nil, trace.Wrap(err)
  	}
  
  	return &certificateCache{
  		keygen:     keygen,
  		cache:      cache,
  		authClient: authClient,
  	}, nil
  }
docs: // NewHostCertificateCache creates a shared host certificate cache that is
  // used by the forwarding server.