The classes associated with receiving are in the spead2.recv package. A stream represents a logical stream, in that packets with the same heap ID are assumed to belong to the same heap. A stream can have multiple physical transports.

Streams yield heaps, which are the basic units of data transfer and contain both item descriptors and item values. While it is possible to directly inspect heaps, this is not recommended or supported. Instead, heaps are normally passed to spead2.ItemGroup.update().

class spead2.recv.Heap

Heap identifier (read-only)


SPEAD flavour used to encode the heap (see SPEAD flavours)


Returns true if the packet contains a stream start control item.


Returns true if the packet contains a stream stop control item.


Malformed packets (such as an unsupported SPEAD version, or inconsistent heap lengths) are dropped, with a log message. However, errors in interpreting a fully assembled heap (such as invalid/unsupported formats, data of the wrong size and so on) are reported as ValueError exceptions. Robust code should thus be prepared to catch exceptions from heap processing.


Once a stream is constructed, the configuration cannot be changed. The configuration is captured in two classes, StreamConfig and RingStreamConfig. The split is a reflection of the C++ API and not particularly relevant in Python. The configuration options can either be passed to the constructors (as keyword arguments) or set as properties after construction.

class spead2.recv.StreamConfig(**kwargs)
  • max_heaps (int) – The number of partial heaps that can be live at one time, per substream. This affects how intermingled heaps can be (due to out-of-order packet delivery) before heaps get dropped. See Packet ordering for details.

  • substreams (int) – Set the number of parallel streams. The remainder when the heap cnt is divided by this value is used to identify the substream. See Packet ordering for details.

  • bug_compat (int) – Bug compatibility flags (see SPEAD flavours)

  • memcpy (int) – Set the method used to copy data from the network to the heap. The default is MEMCPY_STD. This can be changed to MEMCPY_NONTEMPORAL, which writes to the destination with a non-temporal cache hint (if SSE2 is enabled at compile time). This can improve performance with large heaps if the data is not going to be used immediately, by reducing cache pollution. Be careful when benchmarking: receiving heaps will generally appear faster, but it can slow down subsequent processing of the heap because it will not be cached.

  • memory_allocator (spead2.MemoryAllocator) – Set the memory allocator for a stream. See Memory allocators for details.

  • stop_on_stop_item (bool) – By default, a heap containing a stream control stop item will terminate the stream (and that heap is discarded). In some cases it is useful to keep the stream object alive and ready to receive a following stream. Setting this attribute to False will disable this special treatment. Such heaps can then be detected with is_end_of_stream().

  • allow_unsized_heaps (bool) – By default, spead2 caters for heaps without a HEAP_LEN item, and will dynamically extend the memory allocation as data arrives. However, this can be expensive, and ideally senders should include this item. Setting this attribute to False will cause packets without this item to be rejected.

  • allow_out_of_order (bool) – Whether to allow packets within a heap to be received out-of-order. See Packet ordering for details.

  • stream_id (int) – An arbitrary integer to associate with the stream. This is used to identify chunks generated by spead2.recv.ChunkRingStream.

  • explicit_start (bool) – If set to true, the stream will not receive any data until spead2.recv.Stream.start() is called. See Explicit start for details.


ValueError – if max_heaps is zero.

add_stat(name, mode=StreamStatConfig.COUNTER)

Register a custom statistic. The return value is the index for the statistic.


ValueError – if name already exists


The index that will be returned by the next call to add_stat().


Get the index of statistic name.


IndexError – if name is not a known statistic name


Read-only list of StreamStatConfig describing all the statistics for the stream (including core ones). Positions in this list correspond to indices returned by get_stat_index().

class spead2.recv.RingStreamConfig(**kwargs)
  • heaps (int) – The capacity of the ring buffer between the network threads and the consumer. Increasing this may reduce lock contention at the cost of more memory usage.

  • contiguous_only (bool) – If set to False, incomplete heaps will be included in the stream as instances of IncompleteHeap. By default they are discarded. See Incomplete Heaps for details.

  • incomplete_keep_payload_ranges (bool) – If set to True, it is possible to retrieve information about which parts of the payload arrived in incomplete heaps, using IncompleteHeap.payload_ranges().


ValueError – if ring_heaps is zero.

Blocking receive

To do blocking receive, create a spead2.recv.Stream, and add transports to it with add_buffer_reader(), add_udp_reader(), add_tcp_reader() or add_udp_pcap_file_reader(). Then either iterate over it, or repeatedly call get().

class spead2.recv.Stream(thread_pool, stream_config=StreamConfig(), ring_config=RingStreamConfig())

Stream configuration passed to the constructor (read-only)


Ringbuffer configuration passed to the constructor (read-only)


Feed data from an object implementing the buffer protocol.

add_udp_reader(port, max_size=DEFAULT_UDP_MAX_SIZE, buffer_size=DEFAULT_UDP_BUFFER_SIZE, bind_hostname='', socket=None)

Feed data from a UDP port.

  • port (int) – UDP port number

  • max_size (int) – Largest packet size that will be accepted.

  • buffer_size (int) – Kernel socket buffer size. If this is 0, the OS default is used. If a buffer this large cannot be allocated, a warning will be logged, but there will not be an error.

  • bind_hostname (str) – If specified, the socket will be bound to the first IP address found by resolving the given hostname. If this is a multicast group, then it will also subscribe to this multicast group.

add_udp_reader(multicast_group, port, max_size=DEFAULT_UDP_MAX_SIZE, buffer_size=DEFAULT_UDP_BUFFER_SIZE, interface_address)

Feed data from a UDP port (IPv4 only). This is intended for use with multicast, but it will also accept a unicast address as long as it is the same as the interface address.

  • multicast_group (str) – Hostname/IP address of the multicast group to subscribe to

  • port (int) – UDP port number

  • max_size (int) – Largest packet size that will be accepted.

  • buffer_size (int) – Kernel socket buffer size. If this is 0, the OS default is used. If a buffer this large cannot be allocated, a warning will be logged, but there will not be an error.

  • interface_address (str) – Hostname/IP address of the interface which will be subscribed, or the empty string to let the OS decide.

add_udp_reader(multicast_group, port, max_size=DEFAULT_UDP_MAX_SIZE, buffer_size=DEFAULT_UDP_BUFFER_SIZE, interface_index)

Feed data from a UDP port with multicast (IPv6 only).

  • multicast_group (str) – Hostname/IP address of the multicast group to subscribe to

  • port (int) – UDP port number

  • max_size (int) – Largest packet size that will be accepted.

  • buffer_size (int) – Kernel socket buffer size. If this is 0, the OS default is used. If a buffer this large cannot be allocated, a warning will be logged, but there will not be an error.

  • interface_index (str) – Index of the interface which will be subscribed, or 0 to let the OS decide.

add_tcp_reader(port, max_size=DEFAULT_TCP_MAX_SIZE, buffer_size=DEFAULT_TCP_BUFFER_SIZE, bind_hostname='')

Receive data over TCP/IP. This will listen for a single incoming connection, after which no new connections will be accepted. When the connection is closed, the stream is stopped.

  • port (int) – TCP port number

  • max_size (int) – Largest packet size that will be accepted.

  • buffer_size (int) – Kernel socket buffer size. If this is 0, the OS default is used. If a buffer this large cannot be allocated, a warning will be logged, but there will not be an error.

  • bind_hostname (str) – If specified, the socket will be bound to the first IP address found by resolving the given hostname.

add_tcp_reader(acceptor, max_size=DEFAULT_TCP_MAX_SIZE)

Receive data over TCP/IP. This is similar to the previous overload, but takes a user-provided socket, which must already be listening for connections. It duplicates the acceptor socket, so the original can be closed immediately.

  • acceptor (socket.socket) – Listening socket

  • max_size (int) – Largest packet size that will be accepted.

add_udp_pcap_file_reader(filename, filter='')

Feed data from a pcap file (for example, captured with tcpdump or mcdump). An optional filter selects a subset of the packets from the capture file. This is only available if libpcap development files were found at compile time.

  • filename (str) – Filename of the capture file

  • filter (str) – Filter to apply to packets from the capture file


Feed data from an in-process queue. Refer to In-process transport for details.


Returns the next heap, blocking if necessary. If the stream has been stopped, either by calling stop() or by receiving a stream control packet, it raises spead2.Stopped. However, heap that were already queued when the stream was stopped are returned first.

A stream can also be iterated over to yield all heaps.


Like get(), but if there is no heap available it raises spead2.Empty.


Start receiving data. This only needs to be called if the explicit_start parameter to spead2.recv.StreamConfig is set to True, although it is harmless to call it even if not. If explicit_start is true, then after calling start(), it is no longer possible to add more readers.


Shut down the stream and close all associated sockets. It is not possible to restart a stream once it has been stopped; instead, create a new stream.


The read end of a pipe to which a byte is written when a heap is received. Do not read from this pipe. It is used for integration with asynchronous I/O frameworks (see below).


Statistics about the stream.


The internal ringbuffer of the stream (see Statistics).

Asynchronous receive

Asynchronous I/O is supported through Python’s asyncio module. It can be combined with other asynchronous I/O frameworks like twisted and Tornado.

class spead2.recv.asyncio.Stream(*args, **kwargs)

See spead2.recv.Stream (the base class) for other constructor arguments.


Coroutine that yields the next heap, or raises spead2.Stopped once the stream has been stopped and there is no more data. It is safe to have multiple in-flight calls, which will be satisfied in the order they were made.

The stream is also asynchronously iterable, i.e., can be used in an async for loop to iterate over the heaps.

Packet ordering

SPEAD is typically carried over UDP, and by its nature, UDP allows packets to be reordered. Packets may also arrive interleaved if they are produced by multiple senders. We consider two sorts of packet ordering issues:

  1. Re-ordering within a heap. By default, spead2 assumes that all the packets that form a heap will arrive in order, and discards any packet that does not have the expected payload offset. In most networks this is a safe assumption provided that all the packets originate from the same sender (IP address and port number) and have the same destination.

    If this assumption is not appropriate, it can be changed with the allow_out_of_order attribute of spead2.recv.StreamConfig. This has minimal impact when packets do in fact arrive in order, but reassembling arbitrarily ordered packets can be expensive. Allowing for out-of-order arrival also makes handling lost packets more expensive (because one must cater for them arriving later), which can lead to a feedback loop as this more expensive processing can lead to further packet loss.

  2. Interleaving of packets from different heaps. This is always supported, but to a bounded degree so that lost packets don’t lead to heaps being kept around indefinitely in the hope that the packet may arrive. The max_heaps attribute of spead2.recv.StreamConfig determines the amount of overlap allowed: once a packet in heap \(n\) is observed, it is assumed that heap \(n - \text{max\_heaps}\) is complete. When there are many producers it will likely be necessary to increase this value. Larger values increase the memory usage for partial heaps, and have a small performance impact.

    It’s possible to get more predictable results when the producers interleave their heap cnts (for example, by using spead2.send.Stream.set_cnt_sequence()) such that the remainder when dividing the heap cnt by the number of producers identifies the producer. In this case, set the substreams attribute of spead2.recv.StreamConfig to the number of producers. Note that max_heaps applies separately to each producer, and can usually be very low (1 or 2) if the producer sends one heap at a time.

Memory allocators

To allow for performance tuning, it is possible to use an alternative memory allocator for heap payloads. A few allocator classes are provided; new classes must currently be written in C++. The default (which is also the base class for all allocators) is spead2.MemoryAllocator, which has no constructor arguments or methods. An alternative is spead2.MmapAllocator.

class spead2.MmapAllocator(flags=0, prefer_huge=False)

An allocator using mmap(2). This may be slightly faster for large allocations, and allows setting custom mmap flags. This is mainly intended for use with the C++ API, but is exposed to Python as well.

  • flags (int) – Extra flags to pass to mmap(2). Finding the numeric values for OS-specific flags is left as a problem for the user.

  • prefer_huge (bool) – If true, allocations will try to use huge pages (if supported by the OS), and fall back to normal pages if that fails.

The most important custom allocator is spead2.MemoryPool. It allocates from a pool, rather than directly from the system. This can lead to significant performance improvements when the allocations are large enough that the C library allocator does not recycle the memory itself, but instead requests memory from the kernel.

A memory pool has a range of sizes that it will handle from its pool, by allocating the upper bound size. Thus, setting too wide a range will waste memory, while setting too narrow a range will prevent the memory pool from being used at all. A memory pool is best suited for cases where the heaps are all roughly the same size.

A memory pool can optionally use a background task (scheduled onto a thread pool) to replenish the pool when it gets low. This is useful when heaps are being captured and stored indefinitely rather than processed and released.

class spead2.MemoryPool(thread_pool, lower, upper, max_free, initial, low_water, allocator=None)

Constructor. One can omit thread_pool and low_water to skip the background refilling.

  • thread_pool (ThreadPool) – thread pool used for refilling the memory pool

  • lower (int) – Minimum allocation size to handle with the pool

  • upper (int) – Size of allocations to make

  • max_free (int) – Maximum number of allocations held in the pool

  • initial (int) – Number of allocations to put in the free pool initially.

  • low_water (int) – When fewer than this many buffers remain, the background task will be started and allocate new memory until initial buffers are available.

  • allocator (MemoryAllocator) – Underlying memory allocator


Whether to issue a warning if the memory pool becomes empty and needs to allocate new memory on request. It defaults to true.

Incomplete Heaps

By default, an incomplete heap (one for which some but not all of the packets were received) is simply dropped and a warning is printed. Advanced users might need finer control, such as recording metrics about the number of these heaps. To do so, set contiguous_only to False in the RingStreamConfig. The stream will then yield instances of IncompleteHeap.

class spead2.recv.IncompleteHeap

Heap identifier (read-only)


SPEAD flavour used to encode the heap (see SPEAD flavours)


The expected number of bytes of payload (-1 if unknown)


The number of bytes of payload that were actually received


A list of pairs of heap offsets. Each pair is a range of bytes that was received. This is only non-empty if incomplete_keep_payload_ranges was set in the RingStreamConfig; otherwise the information is dropped to save memory.

When using this, you should also set allow_out_of_order to True in the StreamConfig, as otherwise any data after the first lost packet is discarded.


Returns true if the packet contains a stream start control item.


Returns true if the packet contains a stream stop control item.


Refer to Receiver stream statistics for general information about statistics.

class spead2.recv.StreamStats

Collection of statistics. It present both dictionary-like and sequence-like interfaces. Iteration is dictionary-like, iterating over the keys (names of statistics). Indexing with negative indices is not supported.


List of spead2.recv.StreamStatConfig describing the available statistics in further detail. This gives the same list as StreamConfig.stats.

class spead2.recv.StreamStatConfig
class Mode
name: str

Name of the statistic

mode: Mode

Mode for updating long-term statistics from per-batch statistics

combine(a, b)

Combine two samples according to the mode.

Additional statistics are available on the ringbuffer underlying the stream (ringbuffer property), with similar caveats about synchronisation.

class spead2.recv.Stream.Ringbuffer

Number of heaps currently in the ringbuffer.


Maximum number of heaps that can be held in the ringbuffer (corresponds to the heaps attribute of RingStreamConfig).

The spead2.recv.stream_stat_indices module contains constants for indices that can be used to retrieve core statistics by index, without needing to look up the index.


Explicit start

When using multiple readers with a stream or multiple streams, it is sometimes desirable to have them all begin listening to the network at the same time, so that their data can be matched up. Adding readers can be slow (mostly due to the cost of allocating buffers), so when adding multiple readers serially, they will start listening at very different times.

If one sets the explicit_start parameter to spead2.recv.StreamConfig to true, then adding a reader will do the expensive work of allocating buffers, but will not start it listening to the network. That is done by calling spead2.recv.Stream.start(). This will still iterate serially over the readers, so they will not start listening at exactly the same time, but the skew will be much smaller because the operation is much more light-weight.

When this feature is in use, no readers can be added to a stream after calling start() (doing so will raise an exception). This gives the implementation a hint that adding readers cannot happen concurrently with packets arriving. At present the implementation does not take advantage of this assumption, but that is subject to change in future versions of spead2.