Where Files Are Written¶

In BEC itself, the actual write location is determined from the server-side service configuration, in particular file_writer.base_path.

On PSI deployments, the server reads this from deployment_configs/server.yaml, for example:

redis:
  host: x99sa-bec-001.psi.ch
  port: 6379
file_writer:
  base_path: /sls/x99sa/data/$account/raw
log_writer:
  base_path: /var/log/bec

This server-side config is written automatically during deployment. The file writer then uses file_writer.base_path as the fixed starting point for all scan files.

Account-dependent base paths¶

• If the configured path contains $account, BEC substitutes the currently active account into the path.
• If the configured path does not contain $account, but an account is active, BEC appends the account name as an extra subdirectory.
• If no account is active, BEC uses the base path as-is.

With a configuration such as:

file_writer:
  base_path: /sls/x99sa/data/$account/raw

and an active account p12345, the effective write base path becomes:

/sls/x99sa/data/p12345/raw

Default Directory Structure¶

Below that base path, BEC creates scan-number based directories. In the default layout, scan 1234 is placed under:

S01000-01999/S01234

and the file base name starts with:

S01234

So a typical master file path looks like:

/sls/x99sa/data/p12345/raw/S01000-01999/S01234/S01234_master.h5

The scan directory can also contain additional files written during the same scan, for example detector data files from large detectors such as:

S01234_eiger.h5

The master file is usually the main entry point and can link to these additional files.

Modifying the Structure¶

Scans can modify parts of the file layout through additional scan arguments.

The file_suffix scan argument¶

Scans can provide a file_suffix argument, for example:

scans.line_scan(..., file_suffix="sampleA")

If BEC uses the default scan-bundle structure, it adds this suffix to the scan directory name, for example:

S01000-01999/S01234_sampleA

The same suffix is also appended to filenames that BEC generates for the scan. For example, the master file becomes:

/sls/x99sa/data/p12345/raw/S01000-01999/S01234_sampleA/S01234_master_sampleA.h5

Additional files created during the same scan can use the same suffix as well. For example, a detector file may be written as:

/sls/x99sa/data/p12345/raw/S01000-01999/S01234_sampleA/S01234_eiger_sampleA.h5

The file_directory scan argument¶

Scans can also provide a file_directory argument. This does not replace the configured base path. Instead, it defines the relative directory below the fixed server-side base path.

For example, if a scan is started with:

scans.line_scan(..., file_directory="test_run")

then BEC writes below:

/sls/x99sa/data/p12345/raw/test_run

and the file base name still follows the normal scan-number naming, for example:

/sls/x99sa/data/p12345/raw/test_run/S01234_master.h5

If file_directory is not provided, BEC falls back to the default scan-bundle directory structure such as S01000-01999/S01234.

Why users cannot escape the base path¶

Users cannot escape this base path from the client side.

• The server already fixes the write root through file_writer.base_path.
• User-provided file_directory values are validated and normalized before the final path is constructed.
• BEC checks that the resolved file path stays inside the configured base path.

This prevents one user from writing into another user's directory.