daggy/README.md

Daggy: Ya like dags?

Description
==

Daggy is a work orchestration framework for running workflows modeled as directed, acyclic graphs (DAGs). These are
quite useful when modeling data ingestion / processing pipelines.

Below is an example workflow where data is pulled from three sources (A, B, C), some work is done on them, and a report
is generated.

Each step depends on the success of its upstream dependencies, e.g. `Derive_Data_AB` can't run until `Transform_A` and
`Transform_B` have completed successfully.

```mermaid
graph LR
  Pull_A-->Transform_A;
  Pull_B-->Transform_B;
  Pull_C-->Transform_C;
  
  Transform_A-->Derive_Data_AB;
  Transform_B-->Derive_Data_AB;
  Derive_Data_AB-->Derive_Data_ABC;
  Transform_C-->Derive_Data_ABC;
  
  Derive_Data_ABC-->Report;
```

Individual tasks (vertices) are run via a task executor. Daggy supports multiple executors, from local executor (via
fork), to distributed work managers like [slurm](https://slurm.schedmd.com/overview.html)
or [kubernetes](https://kubernetes.io/) (both planned).

State is maintained via state loggers. Currently daggy supports an in-memory state manager (OStreamLogger), and a
filesystem logger (FileSystemLogger). Future plans include supporting [redis](https://redis.io)
and [postgres](https://postgresql.org).

Building
==

**Requirements:**

- git
- cmake >= 3.14
- gcc >= 8

```
git clone https://gitlab.com/iroddis/daggy
cd daggy
mkdir build
cd build
cmake ..
make
```

DAG Run Definition
===

daggy works as a standalone library, but generally runs as a service with a REST interface. This documentation is
specifically for submitting DAGs to the REST server for execution (a DAG run).

DAGs are defined in JSON as a set of `tasks`, along with optional `taskParameters` and `executionParameters` (future).

Basic Definition
--

A DAG Run definition consists of a dictionary that defines a set of tasks. Each task has the following attributes:

| Attribute  |  Required  |  Description                                           |
|------------|------------|--------------------------------------------------------|
| name       |  Yes       |  Name of this task. Must be unique.                    |
| command    |  Yes       |  The command to execute                                |
| maxRetries |  No        | If a task fails, how many times to retry (default: 0)  |
| retry      |  No        | How many seconds to wait between retries.              |
| children   |  No        | List of names of tasks that depend on this task        |
| parents    |  No        | List of names of tasks that this task depends on       |

Defining both `parents` and `children` are not required; one or the other is sufficient. Both are supported to allow you
to define your task dependencies in the way that is most natural to how you think.

Below is an example DAG Run submission:

```json
{
  "tasks": [
    {
      "name": "task_one",
      "command": [
        "/usr/bin/touch",
        "/tmp/somefile"
      ],
      "maxRetries": 3,
      "retryIntervalSeconds": 30
    },
    {
      "name": "task_two",
      "command": [
        "/usr/bin/touch",
        "/tmp/someotherfile"
      ],
      "maxRetries": 3,
      "retryIntervalSeconds": 30,
      "parents": [
        "task_one"
      ]
    }
  ]
}
```

Task Parameters
--

Task commands can be parameterized by passing in an optional `taskParameters` member. Each parameter consists of a name
and either a string value, or an array of string values. Task commands will be regenerated based on the values of the
parameters.

For instance:

```json
{
  "taskParameters": {
    "DIRECTORY": "/var/tmp",
    "FILE": "somefile"
  },
  "tasks": [
    {
      "name": "task_one",
      "command": [
        "/usr/bin/touch",
        "{{DIRECTORY}}/{{FILE}}"
      ],
      "maxRetries": 3,
      "retryIntervalSeconds": 30
    }
  ]
}
``` 

`task_one`'s command, when run, will touch `/var/tmp/somefile`, since the values of `DIRECTORY` and `FILE` will be
populated from the `taskParameters` values.

In the case where a parameter has an array of values, any tasks referencing that value will be duplicated with the
cartesian product of all relevant values.

Example:

```json
{
  "taskParameters": {
    "DIRECTORY": "/var/tmp",
    "FILE": "somefile",
    "DATE": [
      "2021-01-01",
      "2021-02-01",
      "2021-03-01"
    ]
  },
  "tasks": [
    {
      "name": "populate_inputs",
      "command": [
        "/usr/bin/touch",
        "{{DIRECTORY}}/{{FILE}}"
      ]
    },
    {
      "name": "calc_date",
      "command": [
        "/path/to/calculator",
        "{{DIRECTORY}}/{{FILE}}",
        "{{DATE}}"
      ]
    },
    {
      "name": "generate_report",
      "command": [
        "/path/to/generator"
      ]
    }
  ]
}
``` 

Conceptually, this DAG looks like this:

```mermaid
graph LR
    populate_inputs-->calc_date
    calc_date-->generate_report
```

Once the parameters have been populated, the new DAG will look like this:

```mermaid
graph LR
    populate_inputs-->calc_date_1
    populate_inputs-->calc_date_2
    populate_inputs-->calc_date_3
    calc_date_1-->generate_report
    calc_date_2-->generate_report
    calc_date_3-->generate_report
```

- `calc_date_1` will have the command `/path/to/calculator /var/tmp/somefile 2021-01-01`
- `calc_date_2` will have the command `/path/to/calculator /var/tmp/somefile 2021-02-01`
- `calc_date_3` will have the command `/path/to/calculator /var/tmp/somefile 2021-03-01`

Execution Parameters
--
(future work)

The REST server can be configured with multiple pools of executors. For instance, it might be helpful to run certain
jobs on slurm with a specific set of restrictions, or allow for local execution as well as execution on a slurm cluster.

`executionParameters` is a member passed in that alters how the DAG is executed.

| Attribute | Description |
|-----------|-------------|
| pool | Names the executor the DAG should run on |
| poolParameters | Any parameters the executor accepts that might modify how a task is run |