High Availability at the network interface level
npm install haha - High Availability using NodeJS
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'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
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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
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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)
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``This file describes the network interfaces available on your system
and how to activate them. For more information, see interfaces(5).
Why NodeJS, not the mature Linux HA?
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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.