Skip to main content

Debugging

If you come across bugs, here are some simple debugging suggestions.

Rerun commands with --debug​

The Dagger CLI tries to keep its output concise by default. If you're running into issues and want to debug with more detailed output, you can run any dagger subcommand with the --debug flag to have it reveal all available information.

Enable Python debug logs​

The Python SDK prints debugging information at various steps of the execution flow of a module, which can be very useful in understanding what's being received from and sent to the API.

This is done using standard Python logging, so it's highly configurable (for example, saving to a file or sending to an external system like Sentry). But for a simple lowering of the default logging level to logging.DEBUG, there's a convenience function for that:

import logging

from dagger import function, object_type
from dagger.log import configure_logging

configure_logging(logging.DEBUG)


@object_type
class MyModule:
    @function
    def echo(self, msg: str) -> str:
        return msg
note

With the TUI, you need to use a progress output that doesn't collapse on success like --progress=plain or --debug, otherwise it won't show in the terminal.

Using the command dagger call --debug echo --msg="hello", should print something like:

✔ MyModule.echo(msg: "hello"): String! 0.5s
  ✔ exec /runtime 0.5s
  ┃ [DEBUG] dagger.client._session: Configuring shared connection to GraphQL server
  ┃ [DEBUG] dagger.client._session: Establishing client session to GraphQL server
  ┃ [DEBUG] dagger.mod._module: invoke => {'parent_name': 'MyModule', 'parent_json': '{}', 'name': 'echo', 'input_args': "{'msg': 'hello'}"}
  ┃ [DEBUG] dagger.mod._module: resolver => MyModule.echo
  ┃ [DEBUG] dagger.mod._resolver: func => <Signature (msg: str) -> str>
  ┃ [DEBUG] dagger.mod._resolver: input args => {'msg': 'hello'}
  ┃ [DEBUG] dagger.mod._resolver: structured args => {'msg': 'hello'}
  ┃ [DEBUG] dagger.mod._module: result => 'hello'
  ┃ [DEBUG] dagger.mod._module: output => '"hello"'
  ┃ [DEBUG] dagger.client._session: Closing client session to GraphQL server
DEBUG: executing query="query{myModule{echo(msg:\"hello\")}}"

The above gives a lot of useful information:

  • The function and parent object that the API wants to execute
  • The parent object's state
  • The function's signature
  • The user inputs before and after deserialization
  • The user inputs after being converted to more complex types (structuring)
  • The function's result before and after serialization

Additionally, with --debug the GraphQL query that the CLI produced is also shown.

Access the Dagger Engine logs​

The Dagger Engine runs in a dedicated Docker container and emits log messages as it works. Here's how to access these logs:

DAGGER_ENGINE_DOCKER_CONTAINER="$(docker container list --all --filter 'name=^dagger-engine-*' --format '{{.Names}}')"
docker logs $DAGGER_ENGINE_DOCKER_CONTAINER