ha

High Availability at the network interface level

npm install ha
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ha - High Availability using NodeJS

'ha' is a very limited high-availability application focused on one use-case:

Two or more Linux servers on one LAN, providing services on one shared (or virtual) IP address.

Updated for NodeJS v0.10.X.

How it Works

Each server is configured with a real ethernet address (e.g. eth0 192.168.1.20) and the shared address (e.g. eth0:1 192.168.1.100). (I've used a virtual ethernet device, but this could easily be a real device on a multi-homed server. There is no requirement that the shared address is in the same subnet as the unique addresses.)

All the servers have unique real addresses (.20, .21, etc.), but all use the same virtual address (.100). The inteface with the virtual address must be configured as 'down' at boot time.

All of the servers multicast a packet each second. Each server receives these packets and if it hears one from a 'more worthy' server, and is ACTIVE, it becomes STANDBY.

If a server doesn't hear from a more 'worthy' server in 3.5 seconds, it becomes ACTIVE.

All servers start in the STANDBY state.

When a server goes from STANDBY to ACTIVE it brings up the interface with the shared address and uses gratuitous arp to tell the LAN about it as quickly as possible.

Shortcomings

When a more 'worthy' node starts multicasting, the less worthy node which is currently hosting the shared address brings it down immediately. The more worthy node takes 3.5 seconds before it comes up. This could be fixed with a more complex protocol and state machines.

This can be partially alleviated in by shortening the multicast interval and tolerance.

This HA solution is not bi- or multi-stable. The most 'worthy' node will host the shared address at all times.

Example /etc/network/interfaces file (Debian/Ubuntu)

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
auto eth0
iface eth0 inet static
 address 192.168.1.20
 netmask 255.255.255.0
 network 192.168.1.0
 broadcast 192.168.1.255
 gateway 192.168.1.254

# The shared cluster address
iface eth0:1 inet static
 address 192.168.1.100
 netmask 255.255.255.0
 network 192.168.1.0
 broadcast 192.168.1.255
 gateway 192.168.1.254

Why NodeJS, not the mature Linux HA?

Because Linux HA was NIH ;)

Using NodeJS makes is extremely easy for us to add HA to our existing NodeJS services and lets us customise HA policy very easily.

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