Advanced Usage: Stdout Target
Relevant source files
The following files were used as context for generating this wiki page:
The stdout target in the Meltano cron extension provides an alternative to automatic crontab management. While the extension's default behavior is to modify the system's crontab directly, the stdout target allows users to generate cron-formatted entries and pipe them to the standard output. This is particularly useful for environments where the user does not have permission to edit the crontab or when utilizing external scheduling tools that consume cron syntax.
When using this target, the extension shifts from being a management tool (responsible for installing and uninstalling entries) to a generation tool. It computes the necessary shell scripts and cron timing strings based on Meltano's schedule configuration but leaves the final application of these entries to the user or an external orchestration process.
Sources: README.md:16-18, README.md:65-67, cron_ext/stores/stdout.py:11-13
Architecture and Data Flow
The stdout target is implemented through the StdoutEntryStore class, which inherits from the EntryStore abstract base class. Unlike the CrontabEntryStore, which interacts with the system via subprocess calls to crontab -l and crontab -, the stdout variant is considered "unmanaged."
Store Comparison
The following table highlights the differences between the default crontab target and the stdout target:
| Feature | Crontab Target | Stdout Target |
|---|---|---|
| Class | CrontabEntryStore | StdoutEntryStore |
| Is Managed | True | False |
| Primary Action | Modifies system crontab | Prints to console |
| Uninstallation | Supported | Not Supported |
| Persistence | Permanent (in crontab) | Ephemeral (standard output) |
Sources: cron_ext/stores/base.py:13-15, cron_ext/stores/crontab.py:18-18, cron_ext/stores/stdout.py:13-13, cron_ext/extension.py:167-168
Logic Flow for Stdout Generation
The flow for the stdout target begins with the CLI invocation. The Cron extension class initializes the store based on the user-provided target argument.
The diagram shows how the extension processes Meltano schedules into script files before piping the resulting cron strings to the terminal.
Sources: cron_ext/main.py:101-105, cron_ext/extension.py:48-52, cron_ext/extension.py:141-144
Implementation Details
The StdoutEntryStore Class
The StdoutEntryStore is designed with minimal logic because it does not maintain a state of "currently installed" entries.
entries(getter): Always returns an empty tuple. Because the store is unmanaged, it cannot "read" what was previously output to a terminal.entries(setter): Takes an iterable of cron strings and prints them using the Pythonprintfunction with a newline separator and a forced flush.
Sources: cron_ext/stores/stdout.py:15-28
Limitations on Unmanaged Stores
Because StdoutEntryStore.is_managed is set to False, certain operations are restricted. The most significant restriction is the inability to use the uninstall command.
The sequence diagram illustrates the failure path when attempting to uninstall from an unmanaged store.
Sources: cron_ext/extension.py:167-168, cron_ext/main.py:121-123
Usage and Examples
To use the stdout target, the --target=stdout flag must be passed to the install or list commands.
Installation Command
The install command generates the cron lines. Note that even though the target is stdout, the extension still generates physical shell scripts in the project directory (under .meltano/run/cron-ext/) because the cron entry must point to a file on disk.
meltano invoke cron install --target=stdout
Example Output:
0 0 * * * '/path/to/project/.meltano/run/cron-ext/daily.sh'
Sources: README.md:65-67, cron_ext/extension.py:100-105
List Command
When using the list command with the stdout target, the extension will return no results. This is because the StdoutEntryStore does not have a persistent memory of previous outputs.
# cron_ext/stores/stdout.py:20-21
@property
def entries(self) -> tuple[str, ...]:
return ()
Sources: cron_ext/stores/stdout.py:20-21, cron_ext/main.py:90-97
Summary
The stdout target serves as a non-destructive way to generate cron configurations. By utilizing the StdoutEntryStore, the extension bypasses the standard crontab utility, printing the results directly to the user. This is an "unmanaged" workflow, meaning the extension provides the configuration strings but does not track, update, or remove them once they have been emitted. Users must ensure that the generated .sh scripts in the .meltano directory remain accessible to whatever system eventually runs the cron entries.
Sources: README.md:16-18, cron_ext/stores/stdout.py:11-28, cron_ext/extension.py:167-168