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<h1>Pegasus: A Multi-Node SSH Command Runner</h1>
</div>
Run a list of commands on a set of SSH nodes. With a bit of optional parametrization.
## Demo
[](https://asciinema.org/a/zERwhKrj0AQ3qW0g9RCj8MBsr)
## Features
- Passwordless SSH is all you need.
- **Simple** config for simple use cases, **flexible** config for advanced ones.
- Two modes:
- **Broadcast** mode runs each command on every node.
- **Queue** mode runs each command once on the next free node.
- Modify the **file-based queue** (`queue.yaml`) while Pegasus is running.
- **Parametrize** hosts and commands.
## Getting Started with Examples
To use Pegasus,
1. Install Pegasus, either from GitHub Release or `cargo install pegasus-ssh`.
2. Setup passwordless SSH for your nodes.
3. Populate `hosts.yaml` and `queue.yaml`, and run Pegasus.
Pegasus will remove one entry at a time from the top of `queue.yaml` and move it to `consumed.yaml` as it begins to execute it.
### Queue Mode: Getting a Bag of Jobs Done
Run four Python commands using two nodes.
```yaml
# hosts.yaml
- node-1
- node-2
```
```yaml
# queue.yaml
- . /opt/miniconda3/etc/profile.d/conda.sh; python train.py --bs 8
- . /opt/miniconda3/etc/profile.d/conda.sh; python train.py --bs 16
- . /opt/miniconda3/etc/profile.d/conda.sh; python train.py --bs 32
- . /opt/miniconda3/etc/profile.d/conda.sh; python train.py --bs 64
```
```console
$ pegasus q # stands for Queue
```
### Broadcast Mode: Terraforming Nodes
Run identical commands for multiple nodes.
```yaml
# queue.yaml
- mkdir workspace
- cd workspace && git clone https://github.com/jaywonchung/dotfiles.git
- . workspace/dotfiles/install.sh
```
```console
$ pegasus b # stands for Broadcast
```
### Parallelizing Execution with Node Parameters
Split nodes into sub-nodes that run commands in parallel. Below, *four* SSH connections are kept, and *four* commands run in parallel.
```yaml
# hosts.yaml
- hostname:
- node-1
- node-2
container:
- gpu0
- gpu1
```
When parametrizing nodes, just make sure you specify the `hostname` key.
You can use these parameters in your commands. By the way, the templating engine is Handlebars.
```yaml
# queue.yaml
- docker exec {{ container }} python train.py --bs 8
- docker exec {{ container }} python train.py --bs 16
- docker exec {{ container }} python train.py --bs 32
- docker exec {{ container }} python train.py --bs 64
```
Four sub-nodes and four jobs. So all jobs will start executing at the same time.
### Parametrizing Commands for Conciseness
If you can parametrize nodes, why not commands?
```yaml
# queue.yaml
- command:
- docker exec {{ container }} python train.py --bs {{ bs }}
bs: [8, 16, 32, 64]
```
This results in the exact same jobs with the example above.
When parametrizing commands, just make sure you specify the `command` key.
### Quiz
How many commands will execute in Queue mode?
```yaml
# hosts.yaml
- hostname:
- node-1
- node-2
laziness:
- 1
- hostname:
- node-3
laziness:
- 2
```
```yaml
# queue.yaml
- echo hi from {{ hostname }}
- command:
- for i in $(seq {{ low }} {{ high }}); do echo $i; sleep {{ laziness }}; done
- echo bye from {{ hostname }}
low:
- 1
- 2
high:
- 3
- 4
```
Note that although `echo bye from {{ hostname }}` doesn't really use the `low` or `high` parameters, it will run `2 * 2 = 4` times regardless.
The answer is `1 + 2 * 2 * 2`.
### Lock Mode: Modifying the Queue
`queue.yaml` is **actually** the queue.
Pegasus removes the first entry in `queue.yaml` whenver there's a free host available.
If you delete entries before Pegasus pulls it, they will not execute.
If you add entreis to `queue.yaml`, they will execute.
#### Q. Why do I need this?
Think about when the number of remaining commands is less than the number of free nodes. Without a way to submit more jobs to Pegasus, those free nodes will stay idle until all the commands finish and you start a fresh new instance of Pegasus.
By providing a way to add to the queue while commands are still running, users may achieve higher node utilization. Being able to delete from the queue is just a byproduct; adding to the queue is the key feature.
#### Q. But that's a race condition on `queue.yaml`.
Lock mode will lock `queue.yaml` and launch a command line editor for you.
```console
$ pegasus l --editor nvim # l stands for Lock
```
Editor priority is `--editor` > `$EDITOR` > `vim`.
When you save and exit, the queue lock is released and Pegasus is allowed access to `queue.yaml`.
#### Q. What if Pegasus terminates before I add to `queue.yaml`?
Enable daemon mode, and Pegasus will not terminate even if `queue.yaml` is empty. It will stand waiting for you to populate `queue.yaml` again, and execute them.
```console
$ pegasus q --daemon
```
## Details
### `queue.yaml`
This is the queue file. Entries in `queue.yaml` are consumed from the top, one by one. Also, entries are consumed only when a new host is available to execute new commands. Consumed entries are immediately appended to `consumed.yaml` in "canonical form", where every entry has a `command` key. Thus you might do something like `tail -n 2 consumed.yaml > queue.yaml` to re-execute your previous single-line command.
As mentioned earlier, always use the Lock Mode when you need to modify `queue.yaml`.
### Broadcast Mode
In broadcast mode, hosts are kept in sync with each other. That is, the next command is fetched from `queue.yaml` and executed on all hosts when all the hosts are done executing the previous command.
Consider the following situation:
```
fast-host slow-host
- command1 success success
- command2 success fail!
- command3 success
- command4 running
```
In this case, we would want to prepend a undo command for `command2` (e.g., `rm -rf repo || true`) and restart from that, but `fast-host` is already far ahead, making things complicated. Thus, especially when you're terraforming nodes with Pegasus, keeping hosts in sync should be beneficial.
There is also a `-e` or `--error-aborts` flag in Broadcast Mode, which aborts Pegasus automatically when a host fails on a command.
### Cancelling and killing
It is very difficult to find a generic way to cancel commands that started running via SSH (See [#11](https://github.com/jaywonchung/pegasus/issues/11)).
Therefore, the caveat of Pegasus at the moment is that it works very well when things go well, but it's difficult to cancel and kill when things go not quite as planned.
You need to walk into every node and manually kill commands.
That said, you can still use Broadcast mode to automate that.
