mirror of https://github.com/hashicorp/consul
101 lines
3.7 KiB
Markdown
101 lines
3.7 KiB
Markdown
---
|
|
layout: docs
|
|
page_title: Network Coordinates
|
|
sidebar_title: Network Coordinates
|
|
description: A Decentralized Network Coordinate System, with several improvements based on several follow-on papers.
|
|
---
|
|
|
|
# Network Coordinates
|
|
|
|
Consul uses a [network tomography](https://en.wikipedia.org/wiki/Network_tomography)
|
|
system to compute network coordinates for nodes in the cluster. These coordinates
|
|
allow the network round trip time to be estimated between any two nodes using a
|
|
very simple calculation. This allows for many useful applications, such as finding
|
|
the service node nearest a requesting node, or failing over to services in the next
|
|
closest datacenter.
|
|
|
|
All of this is provided through the use of the [Serf library](https://www.serf.io/).
|
|
Serf's network tomography is based on ["Vivaldi: A Decentralized Network Coordinate System"](http://www.cs.ucsb.edu/~ravenben/classes/276/papers/vivaldi-sigcomm04.pdf),
|
|
with some enhancements based on other research. There are more details about
|
|
[Serf's network coordinates here](https://www.serf.io/docs/internals/coordinates.html).
|
|
|
|
## Network Coordinates in Consul
|
|
|
|
Network coordinates manifest in several ways inside Consul:
|
|
|
|
- The [`consul rtt`](/commands/rtt) command can be used to query for the
|
|
network round trip time between any two nodes.
|
|
|
|
- The [Catalog endpoints](/api/catalog) and
|
|
[Health endpoints](/api/health) can sort the results of queries based
|
|
on the network round trip time from a given node using a "?near=" parameter.
|
|
|
|
- [Prepared queries](/api/query) can automatically fail over services
|
|
to other Consul datacenters based on network round trip times. See the
|
|
[Geo Failover](https://learn.hashicorp.com/tutorials/consul/automate-geo-failover) for some examples.
|
|
|
|
- The [Coordinate endpoint](/api/coordinate) exposes raw network
|
|
coordinates for use in other applications.
|
|
|
|
Consul uses Serf to manage two different gossip pools, one for the LAN with members
|
|
of a given datacenter, and one for the WAN which is made up of just the Consul servers
|
|
in all datacenters. It's important to note that **network coordinates are not compatible
|
|
between these two pools**. LAN coordinates only make sense in calculations with other
|
|
LAN coordinates, and WAN coordinates only make sense with other WAN coordinates.
|
|
|
|
## Working with Coordinates
|
|
|
|
Computing the estimated network round trip time between any two nodes is simple
|
|
once you have their coordinates. Here's a sample coordinate, as returned from the
|
|
[Coordinate endpoint](/api/coordinate).
|
|
|
|
```
|
|
"Coord": {
|
|
"Adjustment": 0.1,
|
|
"Error": 1.5,
|
|
"Height": 0.02,
|
|
"Vec": [0.34,0.68,0.003,0.01,0.05,0.1,0.34,0.06]
|
|
}
|
|
```
|
|
|
|
All values are floating point numbers in units of seconds, except for the error
|
|
term which isn't used for distance calculations.
|
|
|
|
Here's a complete example in Go showing how to compute the distance between two
|
|
coordinates:
|
|
|
|
```
|
|
import (
|
|
"math"
|
|
"time"
|
|
|
|
"github.com/hashicorp/serf/coordinate"
|
|
)
|
|
|
|
func dist(a *coordinate.Coordinate, b *coordinate.Coordinate) time.Duration {
|
|
// Coordinates will always have the same dimensionality, so this is
|
|
// just a sanity check.
|
|
if len(a.Vec) != len(b.Vec) {
|
|
panic("dimensions aren't compatible")
|
|
}
|
|
|
|
// Calculate the Euclidean distance plus the heights.
|
|
sumsq := 0.0
|
|
for i := 0; i < len(a.Vec); i++ {
|
|
diff := a.Vec[i] - b.Vec[i]
|
|
sumsq += diff * diff
|
|
}
|
|
rtt := math.Sqrt(sumsq) + a.Height + b.Height
|
|
|
|
// Apply the adjustment components, guarding against negatives.
|
|
adjusted := rtt + a.Adjustment + b.Adjustment
|
|
if adjusted > 0.0 {
|
|
rtt = adjusted
|
|
}
|
|
|
|
// Go's times are natively nanoseconds, so we convert from seconds.
|
|
const secondsToNanoseconds = 1.0e9
|
|
return time.Duration(rtt * secondsToNanoseconds)
|
|
}
|
|
```
|