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#63 in Concurrency
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This Rust crate provides a clocked message dispatch service. Clocked dispatch is, at its heart, simply a set of channels. The difference is that each channel is aware of how many messages the system as a whole has processed. This can be useful to have different threads know how up-to-date another thread is.
For example, say a message
m is sent to some receiver
r' who does not receive
m will still be made aware that one
message has been dispatched when it does a subsequent read.
To help illustrate why this is useful, consider two channel pairs, one
r1, and one between
r2. Assume that
r2 occasionally need to communicate. However, they want to wait
until the other has seen any updates preceding those that they have
received. Say that
r1 has seen 7 updates, but
r2 has not seen any.
r2 sufficiently up-to-date? Well, it could be -- all the channel
sends in the system could have been for
Clocked dispatch simplifies by introducing a shared dispatcher that assigns monotonically increasing sequence numbers to all messages. It looks something like this:
s1 s2 +---+----+ | + dispatcher | +---+----+ r1 r2
s1 wishes send a message
r1, it instead sends
m to the
dispatcher. The dispatcher assigns a sequence number to
m, forwards it
r1, and then sends a clock update to
r2. This clock update
r2, in the scenario described above, to see that 7 updates have
passed it by, telling it that it has seen all the updated that
The dispatcher introduces another subtle problem. Specifically, if
is slow to accept updates, it could block the dispatcher when it tries
to send clock updates. To avoid this, the library implements a custom
channel that conveys these sequence numbers without blocking the sender.
Blocking will still happen if a message is being sent, but timestamp
updates will not block the dispatcher. This means that
not block even though
r2 is not currently reading from its input
Clocked dispatch instances can be composed by using
forward instead of
send for messages that have already been assigned a sequence number.
The library ensures that such messages are delivered in-order, by
delaying the delivery of messages until it can guarantee that no earlier
messages will later arrive. See the crate documentation for details
about this mode of operation.
- Avoid one slow path in the graph blocking sends through independent paths. This can be done by not sharing a single channel between all senders to a dispatcher. Instead, the dispatcher should have a separate channel for each sender, and select between those whose recipient has space in their buffer. Unless we switch to an external channel library, this blocks on rust#27800.
Licensed under either of
- Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
- MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)
at your option.
Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.