HDF5-HL Packet Tables

The Situation: A Stream of Data An instrument takes measurements at regular intervals Data arrives in "packets"--one value at a time Data is arriving in real-time Or multiple instruments are being used Packets consist of one or more measurements Packets vary in size

The Solution: Packet Tables A high-level API for HDF5 Designed to support streams of data High-performance for real-time data Supports both fixed-length packets and variable-length packets Available in C and C++ Packet Tables are always 1-D lists of packets. They're not higher-performance than normal HDF5 calls, but they are much faster than other High-Level APIs.

Packet Tables vs. H5TB Tables The "Packet Table" and "Table" interfaces both create tables in HDF5. H5TB Tables are flexible. H5TB Tables support insertions. Packet Tables are high-performance and support variable-length entries. A table is one or the other, but not both! Packet Tables are lower-level; they have to be opened and closed. H5TB Tables calls are atomic. H5TB tables store metadata about field names, allow tables to be combined, etc. Packet Tables support appends, but not insertions (this feature could be added if there is demand for it, but it would much slower than appending).

Using Packet Tables A Packet Table contains either fixed-length or variable-length packets. Use H5PTcreate_fl or H5PTcreate_vl Once set, a Packet Table's type never changes Packet Tables need to be opened and closed like HDF5 datasets. Use H5PTopen and H5PTclose

Using Packet Tables Write packets from the data stream Use H5PTappend Read packets back in order Set the starting point with H5PTset_index Use H5PTget_next to move through the data …Or, out of order Use H5PTread_packets If you set the index to point to packet 1 and call H5PTget_next, you'll get packet 1. Next time you call H5PTget_next, you'll get packet 2, and so on. You can also get more than one packet at a time. H5PTread_packets gives you random read access without bothering with indices, etc.

Fixed-length vs. Variable-length Time Data a. Fixed length packets. b. Variable length packets. This is what we mean when we talk about "Fixed length" and "Variable-length" packets. This is also a good picture of what Packet Tables look like in general.

Fixed-Length vs. Variable Length Both types of Packet Table use the same API calls Fixed-length tables use HDF5 datatypes Variable-Length Packet Tables use hvl_t structs HDF5's natural support for variable-size data During reads, a buffer is allocated and must be freed -- use H5PTfree_vlen_readbuff Essentially, variable-length packet tables are fixed-length packet tables that take a different kind of data. All the functions are the same, but fixed-length tables expect buffers full of some HDF5 datatype, and variable-length tables expect buffers full of hvl_t's.

Packet Tables in Action An overview of Packet Tables http://hdf.ncsa.uiuc.edu/HDF5/hdf5_hl/doc/RM_hdf5pt_intro.html See the Packet Table use cases: http://hdf.ncsa.uiuc.edu/HDF5/hdf5_hl/doc/RM_hdf5pt_usecases.html Simple examples of Packet Tables in use