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Part 1 - The Sensor and Its Node

This is Part 1 of the Resilience tutorial. You start three runtimes that stand in for three machines, pin the sensor to the one with the probe, and then kill that runtime. Along the way you learn about placement tags - how a cell says where it may run - and restart policies - what the swarm does when a cell's runtime dies.

Step 1 - Start Three Runtimes​

A runtime carries tags: words that describe what the machine is or has. Some it gets for free - every Linux runtime carries linux, and every runtime carries its own id as @<id>. Others you give it when you start it. A cell, in turn, can require tags, and it is only ever placed on a runtime that carries all of them.

In Terminal 1, start the three runtimes. Each gets a name and one tag. --detached puts them in the background so one terminal is enough:

myrmic runtimes start -n node1 --tag soil-probe --detached
myrmic runtimes start -n node2 --tag compute --detached
myrmic runtimes start -n node3 --tag compute --detached

Expected output, once per runtime:

INFO  runtime "node1" pid file: /run/user/1000/myrmic/node1.pid
INFO  Starting runtime "node1"(112a7f3f15aa4f7ead00258a4c8c609a)

Give them a few seconds to find each other, then look at the swarm:

myrmic network status
Discovered 3 node(s)

  #  name   kind   id          tags
  0  node1  linux  [1]12a7f3f  soil-probe, @112a7f3f15aa4f7ead00258a4c8c609a, linux
  1  node2  linux  [c]a157d28  compute, @ca157d28d8cf44689a97832a30327a8c, linux
  2  node3  linux  [4]f741c9b  compute, @4f741c9b645144daa3d0e8776399c7bf, linux

Three nodes, one swarm. node1 is the only one that carries soil-probe; node2 and node3 both carry compute. Keep this table in mind: the id column is how the swarm names a node, and you will translate ids back to names more than once.

Step 2 - Pin the Sensor to Its Node​

Go to your greenhouse workspace and deploy the sensor. Two new flags appear:

cd greenhouse
myrmic deploy moisture-sensor -t soil-probe --policy always
INFO  deploying cell (srn = moisture-sensor, sri = 365d6cfe-e9c2-5914-bfed-3a17d4ecd6da)
INFO  deployed cell (srn = moisture-sensor, sri = 365d6cfe-e9c2-5914-bfed-3a17d4ecd6da)
  • -t soil-probe is the cell's requirement. Only node1 carries that tag, so only node1 qualifies - the probe is wired to that machine and nowhere else.
  • --policy always is the cell's restart policy: if the cell stops for any reason other than you removing it, the swarm starts it again. The default is never.
myrmic cells
  cell             sri                                   kind  runtime     age  policy  class            srn
──── moisture-sensor ───────────────────────────────────────────────────────────────────────────────────────────────────
  moisture-sensor  365d6cfe-e9c2-5914-bfed-3a17d4ecd6da  wasm  [1]12a7f3f  14s  always  moisture-sensor  moisture-sensor

In a terminal myrmic cells is a live view that redraws every couple of seconds until you press Ctrl-C; add --once for a single listing instead - which is also what you get when the output is piped or redirected.

The runtime column says [1]12a7f3f - node1, as it must. Open Terminal 2 and listen to it:

myrmic subscribe moisture
[2026-09-06T18:12:33.728Z] event=moisture sender=365d6cfe-... payload=8 bytes
59.79998

[2026-09-06T18:12:34.730Z] event=moisture sender=365d6cfe-... payload=8 bytes
59.39998

One reading a second, the mock weather slowly drying the soil.

Step 3 - Kill the Node, Bring It Back​

Now pull the plug on the Raspberry Pi. Find node1's process id and kill it the way a power cut would, with no chance to shut down cleanly:

myrmic runtimes list
INFO  3 local runtime(s)
node1	running	pid=99396
node2	running	pid=99413
node3	running	pid=99430
kill -9 99396

Terminal 2 falls silent at once. The sensor ran on node1 and node1 is gone; there is nothing left to publish.

Wait as long as you like - nothing changes. The sensor has a restart policy, but restarting it means placing it on a runtime that carries soil-probe, and there is no such runtime in the swarm right now. myrmic cells will stop listing the sensor after about a minute, once the swarm has given node1 up; myrmic network status shows two nodes. A restart policy cannot conjure up hardware.

Now the electrician arrives. Start node1 again, with the same name and tag:

myrmic runtimes start -n node1 --tag soil-probe --detached

Watch Terminal 2. About 20 seconds after the runtime is up, the readings resume:

[2026-09-06T18:13:19.431Z] event=moisture sender=365d6cfe-... payload=9 bytes
63.799995

[2026-09-06T18:13:20.433Z] event=moisture sender=365d6cfe-... payload=9 bytes
63.399994

Nobody redeployed anything. The swarm remembered that the sensor should be running, saw a runtime with soil-probe reappear, and put the sensor back on it - that is --policy always doing its job. myrmic cells lists it again, with a fresh age.

You may notice the reading jumped - from the high 50s before the kill to the mid 60s after. That is the mock sensor's own doing: its startup code resets the simulation to 65 %. Whether a cell's memory survives a restart is a bigger question than this cell can answer, and it is the subject of Part 2.

What Have You Learned​

  • A runtime carries tags, some automatic (linux, @<id>), some given at start (--tag). A cell requires tags with -t, and is only placed on a runtime that carries all of them.
  • A requirement pins a cell to hardware. The sensor lives on node1 because the probe does; no other node ever qualifies.
  • --policy always makes the swarm restart a cell that stopped for any reason other than being removed. If no qualifying runtime exists, it waits; when one appears, the cell is back within a few seconds.
  • myrmic cells tells you where each cell runs; myrmic network status translates the runtime id to a name; myrmic runtimes list gives you the process id to kill.

Next: Part 2 - The Grow-Bed and Its State, where the cell can move - and its memory may not follow.

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