Single Writer Multiple Reader (SWMR)¶
Starting with version 2.5.0, h5py includes support for the HDF5 SWMR features.
What is SWMR?¶
The SWMR features allow simple concurrent reading of a HDF5 file while it is being written from another process. Prior to this feature addition it was not possible to do this as the file data and meta-data would not be synchronised and attempts to read a file which was open for writing would fail or result in garbage data.
A file which is being written to in SWMR mode is guaranteed to always be in a valid (non-corrupt) state for reading. This has the added benefit of leaving a file in a valid state even if the writing application crashes before closing the file properly.
This feature has been implemented to work with independent writer and reader processes. No synchronisation is required between processes and it is up to the user to implement either a file polling mechanism, inotify or any other IPC mechanism to notify when data has been written.
The SWMR functionality requires use of the latest HDF5 file format: v110. In
practice this implies using at least HDF5 1.10 (this can be checked via
h5py.version.info) and setting the libver bounding to “latest” when opening or
creating the file.
New v110 format files are not compatible with v18 format. So, files written in SWMR mode with libver=’latest’ cannot be opened with older versions of the HDF5 library (basically any version older than the SWMR feature).
The HDF Group has documented the SWMR features in details on the website: Single-Writer/Multiple-Reader (SWMR) Documentation. This is highly recommended reading for anyone intending to use the SWMR feature even through h5py. For production systems in particular pay attention to the file system requirements regarding POSIX I/O semantics.
Using the SWMR feature from h5py¶
The following basic steps are typically required by writer and reader processes:
- Writer process creates the target file and all groups, datasets and attributes.
- Writer process switches file into SWMR mode.
- Reader process can open the file with swmr=True.
- Writer writes and/or appends data to existing datasets (new groups and datasets cannot be created when in SWMR mode).
- Writer regularly flushes the target dataset to make it visible to reader processes.
- Reader refreshes target dataset before reading new meta-data and/or main data.
- Writer eventually completes and close the file as normal.
- Reader can finish and close file as normal whenever it is convenient.
The following snippet demonstrate a SWMR writer appending to a single dataset:
f = h5py.File("swmr.h5", 'w', libver='latest') arr = np.array([1,2,3,4]) dset = f.create_dataset("data", chunks=(2,), maxshape=(None,), data=arr) f.swmr_mode = True # Now it is safe for the reader to open the swmr.h5 file for i in range(5): new_shape = ((i+1) * len(arr), ) dset.resize( new_shape ) dset[i*len(arr):] = arr dset.flush() # Notify the reader process that new data has been written
The following snippet demonstrate how to monitor a dataset as a SWMR reader:
f = h5py.File("swmr.h5", 'r', libver='latest', swmr=True) dset = f["data"] while True: dset.id.refresh() shape = dset.shape print( shape )
In addition to the above example snippets, a few more complete examples can be found in the examples folder. These examples are described in the following sections.
Dataset monitor with inotify¶
The inotify example demonstrates how to use SWMR in a reading application which monitors live progress as a dataset is being written by another process. This example uses the the linux inotify (pyinotify python bindings) to receive a signal each time the target file has been updated.
""" Demonstrate the use of h5py in SWMR mode to monitor the growth of a dataset on notification of file modifications. This demo uses pyinotify as a wrapper of Linux inotify. https://pypi.python.org/pypi/pyinotify Usage: swmr_inotify_example.py [FILENAME [DATASETNAME]] FILENAME: name of file to monitor. Default: swmr.h5 DATASETNAME: name of dataset to monitor in DATAFILE. Default: data This script will open the file in SWMR mode and monitor the shape of the dataset on every write event (from inotify). If another application is concurrently writing data to the file, the writer must have have switched the file into SWMR mode before this script can open the file. """ import asyncore import pyinotify import sys import h5py import logging #assert h5py.version.hdf5_version_tuple >= (1,9,178), "SWMR requires HDF5 version >= 1.9.178" class EventHandler(pyinotify.ProcessEvent): def monitor_dataset(self, filename, datasetname): logging.info("Opening file %s", filename) self.f = h5py.File(filename, 'r', libver='latest', swmr=True) logging.debug("Looking up dataset %s"%datasetname) self.dset = self.f[datasetname] self.get_dset_shape() def get_dset_shape(self): logging.debug("Refreshing dataset") self.dset.refresh() logging.debug("Getting shape") shape = self.dset.shape logging.info("Read data shape: %s"%str(shape)) return shape def read_dataset(self, latest): logging.info("Reading out dataset [%d]"%latest) self.dset[latest:] def process_IN_MODIFY(self, event): logging.debug("File modified!") shape = self.get_dset_shape() self.read_dataset(shape) def process_IN_CLOSE_WRITE(self, event): logging.info("File writer closed file") self.get_dset_shape() logging.debug("Good bye!") sys.exit(0) if __name__ == "__main__": logging.basicConfig(format='%(asctime)s %(levelname)s\t%(message)s',level=logging.INFO) file_name = "swmr.h5" if len(sys.argv) > 1: file_name = sys.argv dataset_name = "data" if len(sys.argv) > 2: dataset_name = sys.argv wm = pyinotify.WatchManager() # Watch Manager mask = pyinotify.IN_MODIFY | pyinotify.IN_CLOSE_WRITE evh = EventHandler() evh.monitor_dataset( file_name, dataset_name ) notifier = pyinotify.AsyncNotifier(wm, evh) wdd = wm.add_watch(file_name, mask, rec=False) # Sit in this loop() until the file writer closes the file # or the user hits ctrl-c asyncore.loop()
Multiprocess concurrent write and read¶
The SWMR multiprocess example starts two concurrent child processes: a writer and a reader. The writer process first creates the target file and dataset. Then it switches the file into SWMR mode and the reader process is notified (with a multiprocessing.Event) that it is safe to open the file for reading.
The writer process then continue to append chunks to the dataset. After each write it notifies the reader that new data has been written. Whether the new data is visible in the file at this point is subject to OS and file system latencies.
The reader first waits for the initial “SWMR mode” notification from the writer, upon which it goes into a loop where it waits for further notifications from the writer. The reader may drop some notifications, but for each one received it will refresh the dataset and read the dimensions. After a time-out it will drop out of the loop and exit.
""" Demonstrate the use of h5py in SWMR mode to write to a dataset (appending) from one process while monitoring the growing dataset from another process. Usage: swmr_multiprocess.py [FILENAME [DATASETNAME]] FILENAME: name of file to monitor. Default: swmrmp.h5 DATASETNAME: name of dataset to monitor in DATAFILE. Default: data This script will start up two processes: a writer and a reader. The writer will open/create the file (FILENAME) in SWMR mode, create a dataset and start appending data to it. After each append the dataset is flushed and an event sent to the reader process. Meanwhile the reader process will wait for events from the writer and when triggered it will refresh the dataset and read the current shape of it. """ import sys import h5py import numpy as np import logging from multiprocessing import Process, Event class SwmrReader(Process): def __init__(self, event, fname, dsetname, timeout = 2.0): super(SwmrReader, self).__init__() self._event = event self._fname = fname self._dsetname = dsetname self._timeout = timeout def run(self): self.log = logging.getLogger('reader') self.log.info("Waiting for initial event") assert self._event.wait( self._timeout ) self._event.clear() self.log.info("Opening file %s", self._fname) f = h5py.File(self._fname, 'r', libver='latest', swmr=True) assert f.swmr_mode dset = f[self._dsetname] try: # monitor and read loop while self._event.wait( self._timeout ): self._event.clear() self.log.debug("Refreshing dataset") dset.refresh() shape = dset.shape self.log.info("Read dset shape: %s"%str(shape)) finally: f.close() class SwmrWriter(Process): def __init__(self, event, fname, dsetname): super(SwmrWriter, self).__init__() self._event = event self._fname = fname self._dsetname = dsetname def run(self): self.log = logging.getLogger('writer') self.log.info("Creating file %s", self._fname) f = h5py.File(self._fname, 'w', libver='latest') try: arr = np.array([1,2,3,4]) dset = f.create_dataset(self._dsetname, chunks=(2,), maxshape=(None,), data=arr) assert not f.swmr_mode self.log.info("SWMR mode") f.swmr_mode = True assert f.swmr_mode self.log.debug("Sending initial event") self._event.set() # Write loop for i in range(5): new_shape = ((i+1) * len(arr), ) self.log.info("Resizing dset shape: %s"%str(new_shape)) dset.resize( new_shape ) self.log.debug("Writing data") dset[i*len(arr):] = arr #dset.write_direct( arr, np.s_[:], np.s_[i*len(arr):] ) self.log.debug("Flushing data") dset.flush() self.log.info("Sending event") self._event.set() finally: f.close() if __name__ == "__main__": logging.basicConfig(format='%(levelname)10s %(asctime)s %(name)10s %(message)s',level=logging.INFO) fname = 'swmrmp.h5' dsetname = 'data' if len(sys.argv) > 1: fname = sys.argv if len(sys.argv) > 2: dsetname = sys.argv event = Event() reader = SwmrReader(event, fname, dsetname) writer = SwmrWriter(event, fname, dsetname) logging.info("Starting reader") reader.start() logging.info("Starting reader") writer.start() logging.info("Waiting for writer to finish") writer.join() logging.info("Waiting for reader to finish") reader.join()
The example output below (from a virtual Ubuntu machine) illustrate some latency between the writer and reader:
python examples/swmr_multiprocess.py INFO 2015-02-26 18:05:03,195 root Starting reader INFO 2015-02-26 18:05:03,196 root Starting reader INFO 2015-02-26 18:05:03,197 reader Waiting for initial event INFO 2015-02-26 18:05:03,197 root Waiting for writer to finish INFO 2015-02-26 18:05:03,198 writer Creating file swmrmp.h5 INFO 2015-02-26 18:05:03,203 writer SWMR mode INFO 2015-02-26 18:05:03,205 reader Opening file swmrmp.h5 INFO 2015-02-26 18:05:03,210 writer Resizing dset shape: (4,) INFO 2015-02-26 18:05:03,212 writer Sending event INFO 2015-02-26 18:05:03,213 reader Read dset shape: (4,) INFO 2015-02-26 18:05:03,214 writer Resizing dset shape: (8,) INFO 2015-02-26 18:05:03,214 writer Sending event INFO 2015-02-26 18:05:03,215 writer Resizing dset shape: (12,) INFO 2015-02-26 18:05:03,215 writer Sending event INFO 2015-02-26 18:05:03,215 writer Resizing dset shape: (16,) INFO 2015-02-26 18:05:03,215 reader Read dset shape: (12,) INFO 2015-02-26 18:05:03,216 writer Sending event INFO 2015-02-26 18:05:03,216 writer Resizing dset shape: (20,) INFO 2015-02-26 18:05:03,216 reader Read dset shape: (16,) INFO 2015-02-26 18:05:03,217 writer Sending event INFO 2015-02-26 18:05:03,217 reader Read dset shape: (20,) INFO 2015-02-26 18:05:03,218 reader Read dset shape: (20,) INFO 2015-02-26 18:05:03,219 root Waiting for reader to finish