I'm writing a CMS in Go and have a session type (user id, page contents to render, etc). Ideally I'd like that type to be a global variable so I'm not having to propagate it through all the nested functions, however having a global variable like that would obviously mean that each new session would overwrite it's predecessor, which, needlessly to say, would be an epic fail.
Some languages to offer a way of having globals within threads that are preserved within that thread (ie the value of that global is sandboxed within that thread). While I'm aware that Goroutines are not threading, I just wondered if there was a similar method at my disposal or if I'd have to pass a local pointer of my session type down through the varies nested routines.
I'm guessing channels wouldn't do this? From what I can gather (and please correct me if I'm wrong here), but they're basically just a safe way of sharing global variables?
edit: I'd forgotten about this question! Anyhow, an update for anyone who is curious. This question was written back when I was new to Go and the CMS was basically my first project. I was coming from a C background with familiarity with POSIX thread but I quickly realised a better approach was to write the code in a mode functional design with session objects passed down as pointers in function parameters. This gave me both the context-sensitive local scope I was after while also minimizing the amount to data I was copying about. However being a 7 year old project and one that was at the start of my transition to Go, it's fair to say the project could do with a major rewrite anyway as there are a lot of mistakes made. That's a concern for another day though - currently it works and I have enough other projects on the go at.
You'll want to use something like a Context:
http://blog.golang.org/context
Basically, the pattern is to create a Context for each unique thing you want to do. (A web request in your case.) Use context.WithValue to embed multiple variables in the context. Then always pass it as the first parameter to other methods that are doing further work in other goroutines.
Getting the variable you need out of the context is a matter of calling context.Value from within any goroutine. From the above link:
A Context is safe for simultaneous use by multiple goroutines. Code can pass a single Context to any number of goroutines and cancel that Context to signal all of them.
I had an implementation where I was explicitly sending variables as method parameters, and I discovered that embedding these variables using contexts significantly cleaned up my code.
Using a Context also helps because it provides ways to end long-running tasks by using channels, select, and a concept called a "done channel." See this article for a great basic review and implementation:
http://blog.golang.org/pipelines
I'd recommend reading the pipelines article first for a good flavor of how to manage communication among goroutines, then the context article for a better idea of how to level-up and start embedding variables to pass around.
Good luck!
Don't use global variables. Use Go goroutine-local variables.
go-routine Id..
There are already goroutine-local variables: they are called function
arguments, function return values, and local variables.
Russ
If you have more than one user, then wouldn't you need that info for each connection? So I would think that you'd have a struct per connected user. It would be idiomatic Go to pass a pointer to that struct when setting up the worker goroutine, or passing the pointer over a channel.
Related
I'm familiarizing myself with NSPersistentContainer. I wonder if it's better to spawn an instance of the private context with newBackgroundContext every time I need to insert/fetch some entities in the background or create one private context, keep it and use for all background tasks through the lifetime of the app.
The documentation also offers convenience method performBackgroundTask. Just trying to figure out the best practice here.
I generally recommend one of two approaches. (There are other setups that work, but these are two that I have used, and tested and would recommend.)
The Simple Way
You read from the viewContext and you write to the viewContext and only use the main thread. This is the simplest approach and avoid a lot of the multithread issues that are common with core-data. The problem is that the disk access is happening on the main thread and if you are doing a lot of it it could slow down your app.
This approach is suitable for small lightweight application. Any app that has less than a few thousand total entities and no bulk changes at once would be a good candidate for this. A simple todo list, would be a good example.
The Complex Way
The complex way is to only read from the viewContext on the main thread and do all your writing using performBackgroundTask inside a serial queue. Every block inside the performBackgroundTask refetches any managedObjects that it needs (using objectIds) and all managedObjects that it creates are discarded at the end of the block. Each performBackgroundTask is transactional and saveContext is called at end of the block. A fuller description can be found here: NSPersistentContainer concurrency for saving to core data
This is a robust and functional core-data setup that can manage data at any reasonable scale.
The problem is that you much always make sure that the managedObjects are from the context you expect and are accessed on the correct thread. You also need a serial queue to make sure you don't get write conflicts. And you often need to use fetchedResultsController to make sure entities are not deleted while you are holding pointers to them.
I'm relatively new to software development, and I'm on my way to completing my first app for the iPhone.
While learning Swift, I learned that I could add functions outside the class definition, and have it accessible across all views. After a while, I found myself making many global functions for setting app preferences (registering defaults, UIAppearance, etc).
Is this bad practice? The only alternate way I could think of was creating a custom class to encapsulate them, but then the class itself wouldn't serve any purpose and I'd have to think of ways to passing it around views.
Global functions: good (IMHO anyway, though some disagree)
Global state: bad (fairly universally agreed upon)
By which I mean, it’s probably a good practice to break up your code to create lots of small utility functions, to make them general, and to re-use them. So long as they are “pure functions”
For example, suppose you find yourself checking if all the entries in an array have a certain property. You might write a for loop over the array checking them. You might even re-use the standard reduce to do it. Or you could write a re-useable function, all, that takes a closure that checks an element, and runs it against every element in the array. It’s nice and clear when you’re reading code that goes let allAboveGround = all(sprites) { $0.position.y > 0 } rather than a for…in loop that does the same thing. You can also write a separate unit test specifically for your all function, and be confident it works correctly, rather than a much more involved test for a function that includes embedded in it a version of all amongst other business logic.
Breaking up your code into smaller functions can also help avoid needing to use var so much. For example, in the above example you would probably need a var to track the result of your looping but the result of the all function can be assigned using let. Favoring immutable variables declared with let can help make your program easier to reason about and debug.
What you shouldn’t do, as #drewag points out in his answer, is write functions that change global variables (or access singletons which amount to the same thing). Any global function you write should operate only on their inputs and produce the exact same results every time regardless of when they are called. Global functions that mutate global state (i.e. make changes to global variables (or change values of variables passed to them as arguments by reference) can be incredibly confusing to debug due to unexpected side-effects they might cause.
There is one downside to writing pure global functions,* which is that you end up “polluting the namespace” – that is, you have all these functions lying around that might have specific relevance to a particular part of your program, but accessible everywhere. To be honest, for a medium-sized application, with well-written generic functions named sensibly, this is probably not an issue. If a function is purely of use to a specific struct or class, maybe make it a static method. If your project really is getting too big, you could perhaps factor out your most general functions into a separate framework, though this is quite a big overhead/learning exercise (and Swift frameworks aren’t entirely fully-baked yet), so if you are just starting out so I’d suggest leaving this for now until you get more confident.
* edit: ok two downsides – member functions are more discoverable (via autocomplete when you hit .)
Updated after discussion with #AirspeedVelocity
Global functions can be ok and they really aren't much different than having type methods or even instance methods on a custom type that is not actually intended to contain state.
The entire thing comes down mostly to personal preference. Here are some pros and cons.
Cons:
They sometimes can cause unintended side effects. That is they can change some global state that you or the caller forgets about causing hard to track down bugs. As long as you are careful about not using global variables and ensure that your function always returns the same result with the same input regardless of the state of the rest of the system, you can mostly ignore this con.
They make code that uses them difficult to test which is important once you start unit testing (which is a definite good policy in most circumstances). It is hard to test because you can't mock out the implementation of a global function easily. For example, to change the value of a global setting. Instead your test will start to depend on your other class that sets this global setting. Being able to inject a setting into your class instead of having to fake out a global function is generally preferable.
They sometimes hint at poor code organization. All of your code should be separable into small, single purpose, logical units. This ensures your code will remain understandable as your code base grows in size and age. The exception to this is truly universal functions that have very high level and reusable concepts. For example, a function that lets you test all of the elements in a sequence. You can also still separate global functions into logical units by separating them into well named files.
Pros:
High level global functions can be very easy to test. However, you cannot ignore the need to still test their logic where they are used because your unit test should not be written with knowledge of how your code is actually implemented.
Easily accessible. It can often be a pain to inject many types into another class (pass objects into an initializer and probably store it as a property). Global functions can often remove this boiler plate code (even if it has the trade off of being less flexible and less testable).
In the end, every code architecture decision is a balance of trade offs each time you go to use it.
I have a Framework.swift that contains a set of common global functions like local(str:String) to get rid of the 2nd parameter from NSLocalize. Also there are a number of alert functions internally using local and with varying number of parameters which makes use of NSAlert as modal dialogs more easy.
So for that purpose global functions are good. They are bad habit when it comes to information hiding where you would expose internal class knowledge to some global functionality.
The golang package "net/http" uses the global variable DefaultServeMux to register handlers. Is this considered a good practice or even an golang idiom? Is it a global variable after all?
The two main reasons not to use global variables are AFAIK 1) that they add to complexity and 2) are problematic in concurrent programs.
Maybe 1) is not considered important in this case because the developer can choose not to use DefaultServerMux? What about 2)? Are global variables always thread/goroutine safe in Go? Still, I'm surprised that it's used in Go's standard library. I've never seen such practice in other languages / standard libraries.
Is it a global variable after all?
Yes. The variable is defined on root level, which makes it global throughout the package.
However, this is not a global variable which stores all the sensible information
of the net/http package. It is merely a convenience setup which uses the content of
the net/http package to provide an quickstart opportunity to the user.
This also means, that is does not add much complexity.
Is this considered a good practice or even an golang idiom?
IMO, it is good practice to aid the user with the usage of a package.
If you're finding that you could save the user some time by providing a
good default configuration, do so.
However, you should be careful when you're about to export variables.
They should be made ready for concurrent access.
The DefaultServeMux (or better, the underlying ServeMux), for example, is using a mutex to be thread safe.
Are global variables always thread/goroutine safe in Go?
No. Without proper synchronization (mutex, channel, ...), everything that is accessed concurrently is problematic and will most certainly blow everything to bits and pieces.
I've never seen such practice in other languages / standard libraries.
Python's logging module, for example, provides a function to retrieve the root logging object, which one can call methods on to customize the logging behaviour. This could be seen as a global object, as it is mutable and defined in the module.
The globvar is, in this case, as safe and as good choice as the analogue seen in e.g package "log" is.
IOW, claim 1 is as vague as it can get and claim 2 is constrained: sometime/somewhere true, otherwise false == doesn't hold in general even though used just like that.
Let's say I have an event and the corresponding function is called. This function interacts with the outside world and so can sometimes have long delays. If the function waits or hangs then my UI will freeze and this is not desirable. On the other hand, having to break up my function into many parts and re-emitting signals is long and can break up the code alot which would make hard to debug and less readable and slows down the development process. Is there a special feature in event driven programming which would enable me to just write the process in one function call and be able to let the mainThread do its job when its waiting? For example, the compiler could reckognize a keyword then implement a return then re-emit signals connected to new slots automatically? Why do I think this would be a great idea ;) Im working with Qt
Your two options are threading, or breaking your function up somehow.
With threading, it sounds like your ideal solution would be Qt::Concurrent. If all of your processing is already in one function, and the function is pretty self-contained (doesn't reference member variables of the class), this would be easy to do. If not, things might get a little more complicated.
For breaking your function up, you can either do it as you suggested and break it into different functions, with the different parts being called one after another, or you can do it in a more figurative way, but scattering calls to allow other processing inside your function. I believe calling processEvents() would do what you want, but I haven't come across its use in a long time. Of course, you can run into other problems with that unless you understand that it might cause other parts of your class to run once more (in response to other events), so you have to treat it almost as multi-threaded in protecting variables that have an indeterminate state while you are computing.
"Is there a special feature in event driven programming which would enable me to just write the process in one function call and be able to let the mainThread do its job when its waiting?"
That would be a non-blocking process.
But your original query was, "How can I implement a blocking process in a single slot without freezing the GUI?"
Perhaps what you're looking for a way to stop other processing when some - any - process decides it's time to block? There are typically ways to do this, yes, by calling a method on one of the parental objects, which, of course, will depend on the specific objects you are using (eg a frame).
Look to the parent objects and see what methods they have that you'd like to use. You may need to overlay one of them to get your exactly desired results.
If you want to handle a GUI event by beginning a long-running task, and don't want the GUI to wait for the task to finish, you need to do it concurrently, by creating either a thread or a new process to perform the task.
You may be able to avoid creating a thread or process if the task is I/O-bound and occasional callbacks to handle I/O would suffice. I'm not familiar with Qt's main loop, but I know that GTK's supports adding event sources that can integrate into a select() or poll()-style loop, running handlers after either a timeout or when a file descriptor becomes ready. If that's the sort of task you have, you could make your event handler add such an event source to the application's main loop.
I normally work on single threaded applications and have generally never really bothered with dealing with threads. My understanding of how things work - which certainly, may be wrong - is that as long as we're always dealing with single threaded code (i.e. no forks or anything like that) it will always be executed in the same thread.
Is this assumption correct? I have a fuzzy idea that UI libraries/frameworks may spawn off threads of their own to handle GUI stuff (which accounts for the fact that the Windows task manager tells me that my 'single threaded' application is actually running on 10 threads) but I'm guessing that this shouldn't affect me?
How does this apply to COM? For instance, if I were to create an instance of a COM component in my code; and that COM component writes some information to a thread-based location (using System.Threading.Thread.GetData for instance) will my application be able to get hold of that information?
So in summary:
In single threaded code, can I be sure that whatever I store in a thread-based location can be retrievable from anywhere else in the code?
If that single threaded code were to create an instance of a COM component which stores some information in a thread-based location, can that be similarly retrievable from anywhere else?
UI usually has the opposite constraint (sadly): it's single threaded and everything must happen on that thread.
The easiest way to check if you are always in the same thread (for, say, a function) is to have an integer variable set at -1, and have a check function like (say you are in C#):
void AssertSingleThread()
{
if (m_ThreadId < 0) m_ThreadId = Thread.CurrentThread.ManagedThreadId;
Debug.Assert(m_ThreadId == Thread.CurrentThread.ManagedThreadId);
}
That said:
I don't understand the question #1, really. Why store in a thread-based location if your purpose is to have a global scope ?
About the second question, most COM code runs on a single thread and, most often, on the thread where your UI message processing lives - this is because most COM code is designed to be compatible with VB6 which is single-thread.
The reason your program has about 10 threads is because both Windows (if you use some of its features like completion ports, or some kind of timers) and the CLR (for example for the GC or, again, some types of timers) may create threads in your process space (technically any program with enough priviledges, can too).
Think about having the model of having a single dataStore class running in your mainThread that all threads can read and write their instance variables to. This will avoid a lot of problems that might arise accessing threads all over the shop.
Simple idea, until you reach the fun part of threading. Concurrency and synchronization; simply, if you have two threads that want to read and write to the same variable inside dataStore at the same time, you have a problem.
Java handles this by allowing you to declare a variable or method synchronized, allowing only one thread access at a time.
I believe some .NET objects have Lock and Synchronized methods defined on them, but I know no more than this.