Hi all,
I am happy to announce that nanomsg v0.1 have been released yesterday.
http://nanomsg.org/download.html
The release is an alpha, so expect bugs to exist and don't use the library
in production environments.
Given that there's no previous version, instead of listing changes to the
project itself, here's a list of changes from ZeroMQ, its predecessor:
Licensing
---------
nanomsg library is MIT-licensed. What it means is that, unlike with ZeroMQ,
you can modify the source code and re-release it under a different license,
as a proprietary product et c. More reasoning about the licensing can be
found here: http://250bpm.com/blog:15
POSIX Compliance
----------------
ZeroMQ API, while modeled on BSD socket API, doesn't match the API fully.
nanomsg aims for full POSIX compliance.
- Sockets are represented as ints, not void pointers.
- Contexts, as known in ZeroMQ, don't exist in nanomsg. This means simpler
API (sockets can be created in a single step) as well as the possibility of
using the library for communication between different modules in a single
process (think of plugins implemented in different languages speaking each
to another). More discussion can be found here: http://250bpm.com/blog:23
- Sending are receiving functions (nn_send, nn_sendmsg, nn_recv and
nn_recvmsg) fully match POSIX syntax and semantics.
Implementation Language
-----------------------
The library is implemented in C instead of C++.
- From user's point of view it means that there's no dependency on C++
runtime (libstdc++ or similar) which may be handy in constrained and
embedded environments.
- Form nanomsg developer's point of view it makes life easier.
- Number of memory allocations is drastically reduced as intrusive
containers are used instead of C++ STL containers.
- The above also means less memory fragmentation, less cache misses et c.
- More discussion on the C vs. C++ topic can be found here:
http://250bpm.com/blog:4 and here: http://250bpm.com/blog:8.
Pluggable Transports and Protocols
----------------------------------
In ZeroMQ there was no formal API for plugging in new transports (think
WebSockets, DCCP, SCTP) and new protocols (counterparts to REQ/REP, PUB/SUB
et c.) As a consequence there were no new transports added since 2008. No
new protocols were implemented either. The formal internal transport API
(see here https://raw.github.com/250bpm/nanomsg/master/src/transport.h) and
protocol API (https://raw.github.com/250bpm/nanomsg/master/src/protocol.h)
are meant to mitigate the problem and serve as a base for creating and
experimenting with new transports and protocols. Please, be aware that the
two APIs are still new and may experience some tweaking in the future to
make them usable in wide variety of scenarios.
- nanomsg implements a new SURVEY protocol. The idea is to send a message
("survey") to multiple peers and wait for responses from all of them. For
more details check the article here: http://250bpm.com/blog:5. Also look
here: http://250bpm.com/blog:20.
- In financial services it is quite common to use "deliver messages from
anyone to everyone else" kind of messaging. To address this use case,
there's a new BUS protocol implemented in nanomsg. Check the details here:
http://250bpm.com/blog:17
Threading Model
---------------
One of the big architectural blunders I've done in ZeroMQ is its threading
model. Each individual object is managed exclusively by a single thread.
That works well for async objects handled by worker threads, however, it
becomes a trouble for objects managed by user threads. The thread may be
used to do unrelated work for arbitrary timespan, e.g. an hour, and during
that time the object being managed by it is completely stuck. Some
unfortunate consequences are inability to implemed request resending in
REQ/REP protocol, PUB/SUB subscriptions not being applied while application
is doing other work and similar. In nanomsg the objects are not tightly
bound to particular threads and thus these problems don't exist.
- REQ socket in ZeroMQ cannot be really used in real world environments as
they get stuck if message is lost due to service failure or similar. Users
have to use XREQ instead and implement the request re-trying themselves.
With nanomsg, the re-try functionality is built into REQ socket.
- In nanomsg, both REQ and REP support cancelling the ongoing processing.
Simply send a new request without waiting for a reply (in the case of REQ
socket) or grab a new request without replying to the previous one (in the
case of REP socket).
- In ZeroMQ, due to its threading model, bind-first-then-connect-second
scenario doesn't work for inproc transport. It is fixed in nanomsg.
- For similar reasons auto-reconnect doesn't work for inproc transport in
ZeroMQ. This problem is fixed in nanomsg as well.
- Finally, nanomsg attempts to make nanomsg sockets thread-safe. While using
a single socket from multiple threads in parallel is still discouraged, the
way in which ZeroMQ sockets failed randomly in such circumstances proved to
be painful and hard to debug.
State Machines
--------------
Internal interactions inside the nanomsg library are modeled as a set of
state machines. The goal is to avoid the incomprehensible shutdown mechanism
as seen in ZeroMQ and thus make the development of the library easier.
- For more discussion see http://250bpm.com/blog:24 and
http://250bpm.com/blog:25.
IOCP Support
------------
One of the long-standing problems in ZeroMQ was that internally it uses BSD
socket API even on Windows platform where it is a second class citizen.
Using IOCP instead, as appropriate, would require major rewrite of the
codebase and thus, in spite of multiple attemps, was never implemented. IOCP
is supposed to have better performance characteristics and, even more
importantly, it allows to use additional transport mechanisms such as
NamedPipes which are not accessible via BSD socket API. For these reasons
nanomsg uses IOCP internally on Windows platforms.
Level-triggered Polling
-----------------------
One of the aspects of ZeroMQ that proved really confusing for users was the
ability to integrate ZeroMQ sockets into an external event loops by using
ZMQ_FD file descriptor. The main source of confusion was that the descriptor
is edge-triggered, i.e. it signals only when there were no messages before
and a new one arrived. nanomsg uses level triggered file descriptors instead
that simply signal when there's a message available irrespective of whether
it was available in the past.
Routing Priorities
------------------
nanomsg implements priorities for outbound traffic. You may decide that
messages are to be routed to a particular destination in preference, and
fall back to an alternative destination only if the primary one is not
available.
- For more discussion see here: http://250bpm.com/blog:14
TCP Transport Enhancements
--------------------------
There's a minor enhancement to TCP transport. When connecting, you can
optionally specify the local interface to use for the connection, like this:
nn_connect (s, "tcp://eth0;192.168.0.111:5555").
Asynchronous DNS
----------------
DNS queries (e.g. converting hostnames to IP addresses) are done in
asynchronous manner. In ZeroMQ such queries were done synchronously, which
meant that when DNS was unavailable the whole library, including the sockets
that haven't used DNS, just hung.
Zero-Copy
---------
While ZeroMQ offers a "zero-copy" API, it's not true zero-copy. Rather it's
"zero-copy till the message gets to the kernel boundary". From that point on
data are copied as with standard TCP. nanomsg, on the other hand, aims at
supporting true zero-copy mechanisms such as RDMA (CPU by-pass, direct
memory-to-memory copying) and shmem (transfer of data between processes on
the same box by using shared memory). The API entrypoint for zero-copy
messaging are nn_allocmsg and nn_freemsg functions in combination with
NN_MSG option passed to send/recv functions.
Efficient Subscription Matching
-------------------------------
In ZeroMQ, simple tries are used to store and match PUB/SUB subscriptions.
The subscription mechanism was intended for up to 10,000 subscriptions where
simple trie works well. However, there are users who use as much as
150,000,000 subscriptions. In such cases there's a need for a more efficient
data structure. Thus, nanomsg uses memory-efficient version of Patricia trie
instead of simple trie.
- For more details check this article: http://250bpm.com/blog:19.
Unified Buffer Model
--------------------
ZeroMQ has a strange double-buffering behaviour. Both the outgoing and
incoming data are stored in a message queue AND in TCP's tx/rx buffers. What
it means, for example, is that if you want to limit the amount of outgoing
data, you have to set both ZMQ_SNDBUF and ZMQ_SNDHWM socket options. Given
that there's no semantic difference between the two, nanomsg uses only TCP's
(or equivalent's) buffers to store the data.
Scalability Protocols
---------------------
Finally, on philosophical level, nanomsg aims at implementing different
"scalability protocols" rather than being a generic networking.
Specifically:
- Different protocols are fully separated, you cannot connect REQ socket to
SUB socket or similar.
- Each protocol embodies a distributed algorithm with well-defined
prerequsites (e.g. "the service has to be stateless" in case of REQ/REP) and
guarantees (if REQ socket stays alive request will be ultimately processed).
- Partial failure is handled by the protocol, not by the user. In fact, it
is transparent to the user.
- The specifications of the protocols are in /rfc subdirectory.
- The goal is to standardise the protocols via IETF.
- There's no generic UDP-like socket (ZMQ_ROUTER), you should use L4
protocols for that kind of functionality.
Future Development
------------------
The most important features to be implemented in the future:
- TCP port multiplexing would allow to funnel all nanomsg communication
through a single TCP port.
- Monitoring tools. These would allow admins to locate and fix the problems
in nanomsg topologies, such as unavaialable endpoints, protocol mismatches,
node overload et c.
- IPC transport on Windows (via NamedPipes).
Enjoy!
Martin
