读书笔记: ZeroMQ

https://book.douban.com/people/fleure/annotation/11622703/

Missing Message Problem Solver

<原文开始> - If you start the SUB socket (i.e., establish a connection to a PUB socket) after the PUB socket has started sending out data, you will lose whatever it published before the connection was made. If this is a problem, set up your architecture so the SUB socket starts first, then the PUB socket starts publishing. - Even if you synchronize a SUB and PUB socket, you may still lose messages. It's due to the fact that internal queues aren't created until a connection is actually created. If you can switch the bind/connect direction so the SUB socket binds, and the PUB socket connects, you may find it works more as you'd expect. </原文结束> 如果 SUB socket 的建立在 PUB 发送数据之后，会丢消息。应对：通过某种同步手段先建立 SUB socket 再允许 PUB socket 发送消息； 即使先建立 SUB socket 再建立 PUB socket，依然会丢消息。应对：让 SUB 作服务端来 bind，PUB 作客户端来 connect。 <原文开始>If you're using PUSH sockets, you'll find that the first PULL socket to connect will grab an unfair share of messages. </原文结束> <原文开始>If you're using inproc, make sure both sockets are in the same context.</原文结束> ## High-Water Marks <原文开始>ZeroMQ uses the concept of HWM (high-water mark) to define the capacity of its internal pipes. Each connection out of a socket or into a socket has its own pipe, and HWM for sending, and/or receiving, depending on the socket type. Some sockets (PUB, PUSH) only have send buffers. Some (SUB, PULL, REQ, REP) only have receive buffers. Some (DEALER, ROUTER, PAIR) have both send and receive buffers. </原文结束> <原文开始>In ZeroMQ v2.x, the HWM was infinite by default. This was easy but also typically fatal for high-volume publishers. In ZeroMQ v3.x, it's set to 1,000 by default, which is more sensible</原文结束> <原文开始>When your socket reaches its HWM, it will either block or drop data depending on the socket type. PUB and ROUTER sockets will drop data if they reach their HWM, while other socket types will block. </原文结束> ## Zero-copy <原文开始>You should think about using zero-copy in the specific case where you are sending large blocks of memory (thousands of bytes), at a high frequency. For short messages, or for lower message rates, using zero-copy will make your code messier and more complex with no measurable benefit. Like all optimizations, use this when you know it helps, and measure before and after. </原文结束> ## Multithreading with ZeroMQ <原文开始> If there's one lesson we've learned from 30+ years of concurrent programming, it is: just don't share state.</原文结束> <原文开始>You should follow some rules to write happy multithreaded code with ZeroMQ: - Isolate data privately within its thread and never share data in multiple threads. The only exception to this are ZeroMQ contexts, which are threadsafe. - Stay away from the classic concurrency mechanisms like as mutexes, critical sections, semaphores, etc. These are an anti-pattern in ZeroMQ applications. - Create one ZeroMQ context at the start of your process, and pass that to all threads that you want to connect via inproc sockets. - Use attached threads to create structure within your application, and connect these to their parent threads using PAIR sockets over inproc. The pattern is: bind parent socket, then create child thread which connects its socket. - Use detached threads to simulate independent tasks, with their own contexts. Connect these over tcp. Later you can move these to stand-alone processes without changing the code significantly. - All interaction between threads happens as ZeroMQ messages, which you can define more or less formally. - Don't share ZeroMQ sockets between threads. ZeroMQ sockets are not threadsafe. Technically it's possible to migrate a socket from one thread to another but it demands skill. The only place where it's remotely sane to share sockets between threads are in language bindings that need to do magic like garbage collection on sockets. </原文结束> inproc 的 socket 看起来比较像 golang 的 channel；里面提到 "attached thread" / "PAIR sockets over inproc." 目测用起来会比较像协程或者管道。 <原文开始>ZeroMQ uses native OS threads rather than virtual "green" threads. The advantage is that you don't need to learn any new threading API, and that ZeroMQ threads map cleanly to your operating system. You can use standard tools like Intel's ThreadChecker to see what your application is doing.</原文结束> gevent 这类“绿色线程”的协程库太 overrate 了，不过 python 生态圈也没有什么其他选择。 ## Handing Errors and ETERM <原文开始>ZeroMQ's error handling philosophy is a mix of fail-fast and resilience. Processes, we believe, should be as vulnerable as possible to internal errors, and as robust as possible against external attacks and errors. To give an analogy, a living cell will self-destruct if it detects a single internal error, yet it will resist attack from the outside by all means possible.</原文结束> <原文开始>If it is unclear whether a fault is internal or external, that is a design flaw to be fixed. </原文结束> ## I/O Threads <原文开始>We said that ØMQ does I/O in a background thread. One I/O thread (for all sockets) is sufficient for all but the most extreme applications. When you create a new context, it starts with one I/O thread. The general rule of thumb is to allow one I/O thread per gigabyte of data in or out per second. To raise the number of I/O threads, use the zmq_ctx_set() call before creating any sockets </原文结束>