Tasks
A task describes the how, what, and when to do for a Snap job. A task is described in a task manifest, which can be either JSON or YAML1.
Skip to the TL;DR example here.
The manifest can be divided into two parts: Header and Workflow.
The Header
---
version: 1
schedule:
type: "simple"
interval: "1s"
max-failures: 10
Version
The header contains a version, used to differentiate between versions of the task manifest parser. Right now, there is only one version: 1.
Schedule
The schedule describes the schedule type and interval for running the task. The type of a schedule could be a simple "run forever" schedule, which is what we see above as "simple" or something more complex. Snap is designed in a way where custom schedulers can easily be dropped in. If a custom schedule is used, it may require more key/value pairs in the schedule section of the manifest. At the time of this writing, Snap has three schedules:
- simple schedule which is described above,
- window schedule which adds a start and stop time,
- cron schedule which supports cron-like entries in interval field, like in this example (workflow will fire every hour on the half hour):
"version": 1,
"schedule": {
"type": "cron",
"interval" : "0 30 * * * *"
},
"max-failures": 10,
More on cron expressions can be found here: https://godoc.org/github.com/robfig/cron
Max-Failures
By default, snap will disable a task if there is 10 consecutive errors from any plugins within the workflow. The configuration can be changed by specifying the number of failures value in the task header. If the max-failures value is -1, snap will not disable a task with consecutive failure. Instead, snap will sleep for 1 second for every 10 consective failures and retry again.
For more on tasks, visit SNAPCTL.md.
The Workflow
---
collect:
metrics:
/intel/mock/foo: {}
/intel/mock/bar: {}
/intel/mock/*/baz: {}
config:
/intel/mock:
user: "root"
password: "secret"
tags:
/intel/mock:
experiment: "experiment 11"
/intel/mock/bar:
os: "linux"
process:
-
plugin_name: "passthru"
publish:
-
plugin_name: "file"
config:
file: "/tmp/published"
The workflow is a DAG which describes the how and what of a task. It is always rooted by a collect, and then contains any number of processes and publishes.
Remote Targets
Process and Publish nodes in the workflow can also target remote snap nodes via the 'target' key. The purpose of this is to allow offloading of resource intensive workflow steps from the node where data collection is occuring. Modifying the example above we have:
---
collect:
metrics:
/intel/mock/foo: {}
/intel/mock/bar: {}
/intel/mock/*/baz: {}
config:
/intel/mock:
user: "root"
password: "secret"
tags:
/intel/mock:
experiment: "experiment 11"
/intel/mock/bar:
os: "linux"
process:
-
plugin_name: "passthru"
target: "127.0.0.1:8082"
publish:
-
plugin_name: "file"
target: "127.0.0.1:8082"
config:
file: "/tmp/published"
If a target is specified for a step in the workflow, that step will be executed on the remote instance specified by the ip:port target. Each node in the workflow is evaluated independently so a workflow can have any, all, or none of it's steps being done remotely (if target key is omitted, that step defaults to local). The ip and port target are the ip and port that has a running control-grpc server. These can be specified to snapd via the control-listen-addr and control-listen-port flags. The default is the same ip as the snap rest-api and port 8082.
An example json task that uses remote targets can be found under examples. More information about the architecture behind this can be found here.
collect
The collect section describes which metrics to collect. Metrics can be enumerated explicitly via:
- a concrete namespace
- a wildcard, *
- a tuple, (m1|m2|m3)
The tuple begins and ends with brackets and items inside are separeted by vertical bar. It works like logical or, so it gives an error only if none of these metrics can be collected.
| Metrics declared in task manifest | Collected metrics |
|---|---|
| /intel/mock/* | /intel/mock/foo /intel/mock/bar /intel/mock/*/baz |
| /intel/mock/(foo\ | bar) |
| /intel/mock/*/baz | /intel/mock/*/baz |
The namespaces are keys to another nested object which may contain a specific version of a plugin, e.g.:
---
/foo/bar/baz:
version: 4
If a version is not given, Snap will select the latest for you.
The config section describes configuration data for metrics. Since metric namespaces form a tree, config can be described at a branch, and all leaves of that branch will receive the given config. For example, say a task is going to collect /intel/perf/foo, /intel/perf/bar, and /intel/perf/baz, all of which require a username and password to collect. That config could be described like so:
---
metrics:
/intel/perf/foo: {}
/intel/perf/bar: {}
/intel/perf/baz: {}
config:
/intel/perf:
username: jerr
password: j3rr
Applying the config at /intel/perf means that all leaves of /intel/perf (/intel/perf/foo, /intel/perf/bar, and /intel/perf/baz in this case) will receive the config.
The tag section describes additional meta data for metrics. Similary to config, tags can also be described at a branch, and all leaves of that branch will receive the given tag(s). For example, say a task is going to collect /intel/perf/foo, /intel/perf/bar, and /intel/perf/baz, all metrics should be tagged with experiment number, additonally one metric /intel/perf/bar should be tagged with OS name. That tags could be described like so:
---
metrics:
/intel/perf/foo: {}
/intel/perf/bar: {}
/intel/perf/baz: {}
tags:
/intel/perf:
experiment: "experiment 11"
/intel/perf/bar:
os: "linux"
Applying the tags at /intel/perf means that all leaves of /intel/perf (/intel/perf/foo, /intel/perf/bar, and /intel/perf/baz in this case) will receive the tag experiment: experiment 11.
Applying the tags at /intel/perf/bar means that only /intel/perf/bar will receive the tag os: linux.
A collect node can also contain any number of process or publish nodes. These nodes describe what to do next.
process
A process node describes which plugin to use to process data coming from either a collection or another process node. The config section describes config data which may be needed for the chosen plugin.
A process node may have any number of process or publish nodes.
publish
A publish node describes which plugin to use to process data coming from either a collection or a process node. The config section describes config data which may be needed for the chosen plugin.
A publish node is a pendant vertex (a leaf). It may contain no collect, process, or publish nodes.
TL;DR
Below is a complete example task.
YAML
---
version: 1
schedule:
type: "simple"
interval: "1s"
workflow:
collect:
metrics:
/intel/mock/foo: {}
/intel/mock/bar: {}
/intel/mock/*/baz: {}
config:
/intel/mock:
user: "root"
password: "secret"
tags:
/intel/perf:
experiment: "experiment 11"
/intel/perf/bar:
os: "linux"
process:
-
plugin_name: "passthru"
process: null
publish:
-
plugin_name: "file"
config:
file: "/tmp/published"
JSON
{
"version": 1,
"schedule": {
"type": "simple",
"interval": "1s"
},
"workflow": {
"collect": {
"metrics": {
"/intel/mock/foo": {},
"/intel/mock/bar": {},
"/intel/mock/*/baz": {}
},
"config": {
"/intel/mock": {
"user": "root",
"password": "secret"
}
},
"tags": {
"/intel/mock": {
"experiment": "experiment 11"
},
"/intel/mock/bar": {
"os": "linux"
}
},
"process": [
{
"plugin_name": "passthru",
"process": null,
"publish": [
{
"plugin_name": "file",
"config": {
"file": "/tmp/published"
}
}
]
}
]
}
}
}
footnotes
- YAML is only supported via the snapctl CLI. Only JSON is accepted via the REST API.
- The wildcard must be supported by the target plugin.