Jungfraujoch/docs/JFJOCH_WRITER.md

# jfjoch_writer

`jfjoch_writer` is a NeXus-compliant HDF5 file writer.

## Acknowledgements
Thanks to Zdenek Matej (MAX IV) and Felix Engelmann (MAX IV) for testing and multiple improvement
suggestions.

## Running directory
The writer needs to run in the base directory for writing files - `file_prefix` is always relative to the writer's running directory.
The writer detects and protects against basic security issues, like `file_prefix` starting with a slash, or starting with `../`, or containing `/../`.

## Usage
Writer needs to be started as a background service, with the following command:
```
jfjoch_writer {options} <address to connect via ZeroMQ to DCU>

Options:
-T        | --tcp                      Use raw TCP/IP instead of ZeroMQ
-j<int>   | --nproc=<int>              Number of forks (only with -T)
-d<path>  | --root_dir=<path>          Root directory for file writing (-R is a deprecated alias)
-r<int>   | --zmq_repub_port=<int>     ZeroMQ port for PUSH socket to republish images
-f<int>   | --zmq_file_port=<int>      ZeroMQ port for PUB socket for notifications on finalized files
-w<int>   | --rcv_watermark=<int>      Receiving ZeroMQ socket watermark (default = 100)
-W<int>   | --repub_watermark=<int>    Republish ZeroMQ socket watermark (default = 1000)
-v        | --verbose                  Verbose output
-h                                     This message
``` 
for example:
```
jfjoch_writer -d /data tcp://dcu-address:5400 
```

## Status and cancellation
When a data collection is finalized, each writer reports its outcome back to `jfjoch_broker` over
the writer notification socket — a ZeroMQ address the broker passes in the START message
(`writer_notification_socket` in the broker configuration) — as a JSON message with the socket
number, run name and number, processed image count, throughput, and on failure an error string.
That is how the broker learns that a writer could not write. On the TCP/IP image stream, failures
additionally come back in-band as negative acknowledgements
(see [Data streams](IMAGE_STREAM.md#tcpip-image-stream)).

To stop a writer, send it `SIGINT`, `SIGQUIT`, `SIGTERM` or `SIGHUP`: it closes the HDF5 files it is
writing and exits. This is only for the case where the broker was terminated or disconnected — it is
not the normal way to end a data collection, which the broker finishes on its own.

## Republish
Republish creates a PUSH socket on the writer, where all the messages are republished for further use by a data analysis pipeline.
Republish is non-blocking, so if there is no receiver on other end or the sending queue is full - images won't be republished. 
In case of START/END messages republishing will attempt sending for 100 ms, but if send times out it won't be retried.

Republish functionality is optional, if republish port number is omitted this functionality is not enabled. 

## Overwriting files
When `jfjoch_writer` creates a HDF5 file, it first adds suffix `.<random>.tmp`. 
Random value depends on current time-stamp and likely will be different from each file of the particular series. 
After file is all saved and closed, it is renamed to remove the suffix.
By default, renaming won't happen if this would overwrite existing file. 
However, this behavior can be changed by setting `overwrite` parameter to true in the file writer configuration.

### When the overwrite conflict is reported
An existing output file is a fatal condition (unless `overwrite` is true). *When* it is detected
depends on whether the transport between the broker and the writer has a back-channel to report the
failure before acquisition starts:

* **Direct HDF5 pusher and TCP writer (back-channel available).** The conflict is detected at
  **start**: the writer that owns the master file checks whether it already exists and refuses to
  start. The direct pusher raises the error in-process; the TCP writer returns a START-failure
  acknowledgement. Either way the broker learns immediately and aborts the data collection *before*
  the detector is armed — no images are taken and nothing is written. Only the master file is
  checked up front: in a multi-writer setup the per-image data files are staggered across writers,
  and checking them at start would make each writer inspect files it never writes (and race the
  writers that do). Data-file conflicts are instead caught by their owning writer at the final
  rename, which for the TCP path surfaces as a write-failure acknowledgement to the broker.
* **ZeroMQ writer (no back-channel).** The ZeroMQ image stream is fire-and-forget: the writer has no
  way to tell the broker to stop, and the broker would keep streaming images regardless. The writer
  therefore does **not** fail at start. It writes the whole series to the `.<random>.tmp` files as
  usual and only fails at the final rename, leaving the `.tmp` files on disk. This is deliberate: the
  acquired images are preserved (in `.tmp` form) rather than being dropped by a writer that aborted
  mid-stream. Rename the `.tmp` files by hand, or re-run with `overwrite` set, to recover them.

## Finalized files information
Creates PUB socket to inform about finalized data files. For each closed file, the socket will send a JSON message, with the following structure:

```
{
  "filename": <string>: HDF5 data file name (relative to writer root directory),
  "nimages": <int> number of images in the file (counting from 1!),
  "file_number": <int> number of file within the acquisition,
  "sample_name": <string> name of sample,
  "run_name": <string> name of run,
  "run_number": <int> number of run,
  "experiment_group": <string> number of p-group / proposal (optional),
  "user_data": <any json> user_data,
  "beam_x_pxl": <float> beam center (X) in pixels,
  "beam_y_pxl": <float> beam center (Y) in pixels,
  "detector_distance_m": <float> detector distance in m,
  "detector_height_pxl": <int> detector size (Y) in pixels,
  "detector_width_pxl": <int> detector size (X) in pixels,
  "incident_energy_eV": <float> photon energy of the X-ray beam,
  "pixel_size_m": <float> pixel size in meter (assuming pixel X == Y),
  "saturation": <int> this count and higher mean saturation,
  "space_group_number": <int> space group number (optional),
  "underload": <int> lowest valid count; anything below it is invalid,
  "unit_cell": <optional> unit cell dimensions in Angstrom/degree {
    "a": <float>, "b": <float>, "c": <float>,
    "alpha": <float>, "beta": <float>, "gamma": <float>
  },
}
```
`user_data` is defined as `header_appendix` in the `/start` operation in the `jfjoch_broker`.
Other metadata are also carried over from `/start` operation.

If the `header_appendix` is a string with valid JSON meaning, it will be embedded as JSON, otherwise it will be escaped as string.
For example `header_appendix` of `{"param1": "test1", "param2": ["test1", "test2"]}`, then the example message will look as follows:
```json
{
  "filename": "dataset_name_data_000001.h5",
  "nimages": 1000,
  "file_number": 0,
  "sample_name": "my_sample",
  "run_name": "my_run",
  "run_number": 25,
  "experiment_group": "p00001",
  "beam_x_pxl": 1200,
  "beam_y_pxl": 1500,
  "detector_distance_m": 0.155,
  "detector_height_pxl": 2164,
  "detector_width_pxl": 2068,
  "incident_energy_eV": 12400.0,
  "pixel_size_m": 7.5e-05,
  "saturation": 32766,
  "space_group_number": 96,
  "underload": -32767,
  "unit_cell": {
    "a": 78.0,
    "alpha": 90.0,
    "b": 78.0,
    "beta": 90.0,
    "c": 39.0,
    "gamma": 90.0
  },
  "user_data": {
    "param1": "test1", 
    "param2": ["test1", "test2"]
  }
}
```

Notifications for finalized files are optional, if notification port number is omitted this functionality is not enabled.

## HDF5 file structure

Jungfraujoch writes NXmx-compliant HDF5, with substantial derived metadata (spot finding, indexing,
integration, azimuthal integration, per-image statistics and timing) stored *beyond* the NXmx
standard. The complete file layout — master vs data files, the three format variants
(`NXmxLegacy`, `NXmxVDS`, `NXmxIntegrated`), every NXmx field that is populated and every
Jungfraujoch extension — is documented in [HDF5 / NeXus data format](HDF5.md).

If data collection was configured with a `header_appendix` containing a key `hdf5` whose value is a
JSON object of numbers and strings, those entries are written to `/entry/user`.

## Other formats (CBF and TIFF)
Earlier versions could also write Crystallographic Binary File (CBF, miniCBF) and TIFF images. These
writers have been removed: Jungfraujoch now writes only NXmx HDF5. The `CBF` and `TIFF` values are
retained in the file-format enum for wire back-compatibility, but a request to write either format
is rejected.

## No file option(s)
There are two options to disable writing of files by the writer:
* Setting `file_prefix` to empty string - this will disable sending files on ZeroMQ image socket.
* Setting file format to `NoFile` - files are streamed over ZeroMQ socket, but `jfjoch_writer` will not write anything. 
This can be useful for debugging purposes, or if you only rely on republishing functionality of the `jfjoch_writer`