| package coordinate |
| |
| import ( |
| "fmt" |
| "math" |
| "sort" |
| "sync" |
| "time" |
| ) |
| |
| // Client manages the estimated network coordinate for a given node, and adjusts |
| // it as the node observes round trip times and estimated coordinates from other |
| // nodes. The core algorithm is based on Vivaldi, see the documentation for Config |
| // for more details. |
| type Client struct { |
| // coord is the current estimate of the client's network coordinate. |
| coord *Coordinate |
| |
| // origin is a coordinate sitting at the origin. |
| origin *Coordinate |
| |
| // config contains the tuning parameters that govern the performance of |
| // the algorithm. |
| config *Config |
| |
| // adjustmentIndex is the current index into the adjustmentSamples slice. |
| adjustmentIndex uint |
| |
| // adjustment is used to store samples for the adjustment calculation. |
| adjustmentSamples []float64 |
| |
| // latencyFilterSamples is used to store the last several RTT samples, |
| // keyed by node name. We will use the config's LatencyFilterSamples |
| // value to determine how many samples we keep, per node. |
| latencyFilterSamples map[string][]float64 |
| |
| // mutex enables safe concurrent access to the client. |
| mutex sync.RWMutex |
| } |
| |
| // NewClient creates a new Client and verifies the configuration is valid. |
| func NewClient(config *Config) (*Client, error) { |
| if !(config.Dimensionality > 0) { |
| return nil, fmt.Errorf("dimensionality must be >0") |
| } |
| |
| return &Client{ |
| coord: NewCoordinate(config), |
| origin: NewCoordinate(config), |
| config: config, |
| adjustmentIndex: 0, |
| adjustmentSamples: make([]float64, config.AdjustmentWindowSize), |
| latencyFilterSamples: make(map[string][]float64), |
| }, nil |
| } |
| |
| // GetCoordinate returns a copy of the coordinate for this client. |
| func (c *Client) GetCoordinate() *Coordinate { |
| c.mutex.RLock() |
| defer c.mutex.RUnlock() |
| |
| return c.coord.Clone() |
| } |
| |
| // SetCoordinate forces the client's coordinate to a known state. |
| func (c *Client) SetCoordinate(coord *Coordinate) { |
| c.mutex.Lock() |
| defer c.mutex.Unlock() |
| |
| c.coord = coord.Clone() |
| } |
| |
| // ForgetNode removes any client state for the given node. |
| func (c *Client) ForgetNode(node string) { |
| c.mutex.Lock() |
| defer c.mutex.Unlock() |
| |
| delete(c.latencyFilterSamples, node) |
| } |
| |
| // latencyFilter applies a simple moving median filter with a new sample for |
| // a node. This assumes that the mutex has been locked already. |
| func (c *Client) latencyFilter(node string, rttSeconds float64) float64 { |
| samples, ok := c.latencyFilterSamples[node] |
| if !ok { |
| samples = make([]float64, 0, c.config.LatencyFilterSize) |
| } |
| |
| // Add the new sample and trim the list, if needed. |
| samples = append(samples, rttSeconds) |
| if len(samples) > int(c.config.LatencyFilterSize) { |
| samples = samples[1:] |
| } |
| c.latencyFilterSamples[node] = samples |
| |
| // Sort a copy of the samples and return the median. |
| sorted := make([]float64, len(samples)) |
| copy(sorted, samples) |
| sort.Float64s(sorted) |
| return sorted[len(sorted)/2] |
| } |
| |
| // updateVivialdi updates the Vivaldi portion of the client's coordinate. This |
| // assumes that the mutex has been locked already. |
| func (c *Client) updateVivaldi(other *Coordinate, rttSeconds float64) { |
| const zeroThreshold = 1.0e-6 |
| |
| dist := c.coord.DistanceTo(other).Seconds() |
| if rttSeconds < zeroThreshold { |
| rttSeconds = zeroThreshold |
| } |
| wrongness := math.Abs(dist-rttSeconds) / rttSeconds |
| |
| totalError := c.coord.Error + other.Error |
| if totalError < zeroThreshold { |
| totalError = zeroThreshold |
| } |
| weight := c.coord.Error / totalError |
| |
| c.coord.Error = c.config.VivaldiCE*weight*wrongness + c.coord.Error*(1.0-c.config.VivaldiCE*weight) |
| if c.coord.Error > c.config.VivaldiErrorMax { |
| c.coord.Error = c.config.VivaldiErrorMax |
| } |
| |
| delta := c.config.VivaldiCC * weight |
| force := delta * (rttSeconds - dist) |
| c.coord = c.coord.ApplyForce(c.config, force, other) |
| } |
| |
| // updateAdjustment updates the adjustment portion of the client's coordinate, if |
| // the feature is enabled. This assumes that the mutex has been locked already. |
| func (c *Client) updateAdjustment(other *Coordinate, rttSeconds float64) { |
| if c.config.AdjustmentWindowSize == 0 { |
| return |
| } |
| |
| // Note that the existing adjustment factors don't figure in to this |
| // calculation so we use the raw distance here. |
| dist := c.coord.rawDistanceTo(other) |
| c.adjustmentSamples[c.adjustmentIndex] = rttSeconds - dist |
| c.adjustmentIndex = (c.adjustmentIndex + 1) % c.config.AdjustmentWindowSize |
| |
| sum := 0.0 |
| for _, sample := range c.adjustmentSamples { |
| sum += sample |
| } |
| c.coord.Adjustment = sum / (2.0 * float64(c.config.AdjustmentWindowSize)) |
| } |
| |
| // updateGravity applies a small amount of gravity to pull coordinates towards |
| // the center of the coordinate system to combat drift. This assumes that the |
| // mutex is locked already. |
| func (c *Client) updateGravity() { |
| dist := c.origin.DistanceTo(c.coord).Seconds() |
| force := -1.0 * math.Pow(dist/c.config.GravityRho, 2.0) |
| c.coord = c.coord.ApplyForce(c.config, force, c.origin) |
| } |
| |
| // Update takes other, a coordinate for another node, and rtt, a round trip |
| // time observation for a ping to that node, and updates the estimated position of |
| // the client's coordinate. Returns the updated coordinate. |
| func (c *Client) Update(node string, other *Coordinate, rtt time.Duration) *Coordinate { |
| c.mutex.Lock() |
| defer c.mutex.Unlock() |
| |
| rttSeconds := c.latencyFilter(node, rtt.Seconds()) |
| c.updateVivaldi(other, rttSeconds) |
| c.updateAdjustment(other, rttSeconds) |
| c.updateGravity() |
| return c.coord.Clone() |
| } |
| |
| // DistanceTo returns the estimated RTT from the client's coordinate to other, the |
| // coordinate for another node. |
| func (c *Client) DistanceTo(other *Coordinate) time.Duration { |
| c.mutex.RLock() |
| defer c.mutex.RUnlock() |
| |
| return c.coord.DistanceTo(other) |
| } |