Noodle

AKA "someone stop Félix from naming things, please, this is terrible."

Go directly to the usage summary.
Dual-licensed with Apache 2.0 and MIT.
Uses Caretaker Maintainership.

Introduction

Noodle as in pool noodle, because it floats and IPs and... geddit?

The idea is based on MetalLB.

Basically, we emit RFC5944 "gratuitous" ARP announcements.

Instead of the usual ARP request-reply "who has 1.2.3.4?" "I do" cycle, we send "I have 1.2.3.4" and we add 1.2.3.4 to the network interface we do that from, so the network stack handles traffic sent to it properly.

Noodle is the little daemon that does the ARP announcements like that.

But Noodle also listens for ARP on the interface. If it sees ARP on the interface for the same IP it's announcing, but with a different MAC address, it stops and exits!

When it stops, if it's being managed by an active orchestrator or supervisor, it will get restarted. And if it's not, but for some reason wasn't shut down when the orchestrator or supervisor went down, it wont come back up.

In any case, an active Noodle will keep spraying its ARP over the subnet every N seconds, so well behaved devices like routers and VMs will keep their ARP tables updated with the MAC of whatever device Noodle is on.

Essentially it's like MetalLB but we use the network itself as an additional consensus layer, and the main consensus is via whatever orchestrator you use. You can also disable the ARP watcher and/or only send a number of ARPs and then quit, so you can build the orchestration a little more granularly yourself if you want. It's up to you!

BTW this was written for use with Nomad but doesn't in any way depend on Nomad.

Good times

In a graceful move situation

Orchestrator will boot Noodle on the other Node, and then either the orchestrator or the new ARP will kill the old Noodle. ARP tables will be updated subnet-wide and most importantly at the router, and the IP will essentially have "floated" over to the new node in short order.

The orchestrator, when configured properly, can also keep the old service running for a bit before killing it even while starting the new one, so traffic will move over as smoothly as it can (long connections may still get broken).

In a failover situation

The old node will be dead. Traffic goes nowhere. Devices on the subnet and the router still have the old MAC in the ARP tables.

Orchestrator notices a node is down, and reschedules its workload on the alive nodes. Noodle starts, blathers ARP announcements over the network, tables get updated, and traffic starts flowing again.

In a split brain situation

Orchestrator worker node gets separated from the orchestrator's servers, or talks to only one server and it is separated from the others. In either case, that node's orchestrator worker agent will:

appear dead to the rest of the cluster, which will reschedule work as in the above, and
notice that it's dead or doesn't have consensus, and ideally will kill itself.

Proceeds mostly the same as failover, but may proceed as crash:

In an orchestrator crash situation

Orchestrator crashes, leaving the underlying containers or applications running. Rest of the cluster notices the node is dead and reschedules. The new Noodle yells its announce, the old Noodle notices ARP that is not coming from itself, and exits. Traffic moves over to the new node.

Bad times

Partial split brain where the old node still thinks it's meant to be up

Old orchestrator appears dead to the rest of the cluster, which reschedules.

Old Noodle notices ARP, and kills itself.

Old orchestrator restarts its Noodle.

New Noodle notices ARP, and kills itself.

New orchestrator restarts its Noodle.

Round and round it goes, traffic is mightily confused, routers might take a hit.

In a Noodle crash situation

Noodle crashes in the middle of its loops and doesn't clean up the IP on its interface. Linux responds to ARP requests with solicited "it's me!" while other Noodle is screaming out "it's me it's me it's me" announces.

Traffic and routers get confused.

This is always a bug and should be reported.

Operator accidentally scales the Noodle service for one floating IP to 2 or more instances

Proceeds like partial split brain except both nodes are technically legitimate.

Alternatives

Native Cloud Provider Floating IP: absolutely use that if available.
Tunnels: requires router access to set up and tunnel endpoint at the service, can be connected to twice concurrently and then what. Adds a moving piece. But is much quieter on the network, cleaner traffic-wise, maybe clearer to netops.
BGP: requires router access to set up, and complicated daemon on every node, plus management of said daemons' config. Does offer true IP-level load-balancing instead of just directing traffic to one host. BGP can be a pain and also may interfere with netops.
DNS: externalises the problem, very slow to update even with TTL=0 because of client behaviour. Some traffic just never dies.
Single VM acts as load-balancer and is never rebooted: not really an option, this is what we're trying to get away from.
Transferring the entire NIC at the VM hypervisor level between machines instead of doing the ARP dance: certainly an option, relies on having hypervisor API access, needs some kind of centralised control to avoid split brain and conflicts. Implies downtime during the switching.
Just changing the hypervisor MAC of the interfaces: same as previous, plus adds even more downtime as MACs can't conflict across VMs and it needs to find a free "transfer" mac or shut the old NIC down.
Changing the ARP table of the router directly with the router's administrative API: doesn't yell so much into the void, but requires access to router, and some kind of synchronisation to avoid conflicting commands.
VRRP, Tree Spanning, OSPF, bonding, etc: cool tech, may work, requires netops. May not handle intra-subnet traffic, only via router.

Obtain

Only works on Linux.

Currently only ARP (supporting IPv4) is implemented.

From binary release

The release tab on GitHub.

Builds are available for:

x86-64, both gnu and musl
AArch64, both gnu and musl
Arm7 HF, both gnu and musl

It's trivial to add more, so please ask.

With cargo binstall

⚠ Not available yet, depends on netlink#149

cargo binstall passcod-noodle

From source