Backends¶

Local JSONL history is always on. Remote backends are optional mirrors: they receive the same metrics, and none of them can break the run if they fail.

et.init(project="demo", backends=["wandb"])            # local + wandb
et.init(project="demo", backends=["wandb", "trackio"]) # local + both
et.init(project="demo", backends=[])                   # local only

"jsonl" is accepted for backward compatibility and ignored — local history is not optional.

What gets forwarded¶

Call	Forwarded as
committed history row	`backend.log(metrics, step=<resolved>, commit=True)`
`et.init(...)`	`backend.init(project, name, config, entity, tags, notes, resume, ...)`
`et.define_metric(name, **kw)`	`backend.define_metric` if it has one
`et.log_artifact(...)`	`backend.log_artifact` if it has one
`et.finish(exit_code)`	`backend.finish()`

Local history assembles the row first. Summary, alerts and remote backends then receive that same committed row, including span and plugin metrics. Two log() calls for one step therefore produce one backend call, and a step dropped by the local step policy is never forwarded. A backend that does not accept commit (trackio) simply does not receive it.

Failure handling¶

A backend that cannot be imported is skipped with the exact install command.
A backend whose init() fails is dropped; the run continues without it.
A failing log(), define_metric(), log_artifact() or finish() is logged and ignored.

In every case the local history is complete.

Per-backend options¶

et.init(
    project="demo",
    backends=["wandb"],
    backend_kwargs={"wandb": {"group": "ablation", "job_type": "train"}},
)

Custom backends¶

Anything with init, log and finish works:

class MyBackend:
    def init(self, **kwargs): ...
    def log(self, data, step=None, commit=None): ...
    def finish(self, **kwargs): ...

et.init(project="demo", backends=[MyBackend()])

It is registered under its lowercased class name.

wandb compatibility¶

Migrating an existing script is usually one line:

# import wandb as et
import expr_tracker as et

wandb	expr_tracker	Notes
`init(project, name, config, tags, notes, entity, dir, resume)`	same	plus `alert`, `alert_rules`, history tuning
`log(data, step, commit)`	same	identical semantics
`finish(exit_code, quiet)`	same	arguments kept for drop-in use
`alert(title, text, level)`	`alert(..., channels=...)`	a superset
`log_artifact` / `use_artifact` / `Artifact`	same	local implementation
`define_metric(name, ...)`	same	forwarded; a no-op locally
`wandb.run`	`et.get_run()`	the current `Run`
`run.summary`	`et.summary()`	persisted as `summary.json`
`run.step` / `run.dir` / `run.url`	same	`url` needs the wandb backend
`wandb.Image` / `Table` / `Video`	not implemented	forwarded to wandb; stored locally as `repr`
`wandb.watch` / `sweep` / `save`	not implemented	out of scope

Note

Recent wandb API keys are 86 characters (wandb_v1_...) and need a current wandb release; older versions reject them and the backend is skipped with a warning.