According to the documentation for zumero_sync:
If a large amount of information needs to be pulled from the server,
this function may need to be called more than once.
In my Android app that uses Zumero that's no problem; I just keep calling zumero_sync until the return value doesn't start with "0;".
However, now I'm trying to write an admin script that also syncs with my server dbfiles. I'd like to use the sqlite3 shell, and have the script pass the SQL to execute via command line arguments. I need to call zumero_sync in a loop (which SQLite doesn't support) to make sure the db is fully synced. If I had to, I could invoke sqlite3 in a loop (reading its output, looking for "0;"), or even write a C++ app to call the SQLite/Zumero functions natively. But it certainly would be easier if a single zumero_sync was enough.
I guess my real question is: could zumero_sync be changed so it completes the sync before returning? If there are cases where the existing behavior is more useful, maybe there could be a parameter for specifying which mode to use?
I see two basic questions here:
(1) Why does zumero_sync() work the way it does?
(2) Can it work differently?
I'll answer (2) first, since it's easier: Yes, it could work differently. Rather, we could (and probably will, soon, you brought this up) implement an additional function, named something like zumero_sync_complete(), which performs [the guts of] zumero_sync() in a loop and returns after the sync is complete.
We didn't implement zumero_sync_complete() because it doesn't add much value. It's a simple loop, so you can darn well write it yourself. :-)
Er, except in scripting environments which don't support loops. Like the sqlite3 shell.
Answer to (1):
The Zumero sync protocol is designed to give the server the flexibility to return partial results if it wants to do so. And for the sake of reducing load on the server (and increasing its scalability) it often does want to do exactly that.
Given that, one reason to expose this to the client is to increase the client's flexibility as well. As long we're making multiple roundtrips, we might as well give the client an opportunity to do something (like, maybe, update a progress bar) in between them.
Another thing a client might want to do in between loop iterations is handle an error.
Or, in the case of a multithreaded client, it might want to deal with changes that happened on the client while the sync is going on.
Which raises the question of how locking should be managed? Do we hold the sqlite write lock during the entire loop? Or only when absolutely necessary?
Bottom line: A robust app would probably want to implement the loop itself so that it can make its own decisions and retain full control over things.
But, as you observe, the sqlite3 shell doesn't have loops. And it's not an app. And it doesn't have threads. Or progress bars. So it's a use case where a simpler-and-less-powerful form of zumero_sync() would make sense.
Related
I am confused by a number of aspects of QTcpSocket::write.
The documentation suggests that it can write fewer bytes than the length of the buffer being sent. This implies that multiple calls are potentially needed. What is the recommended way to deal with this (bearing in mind following points)?
My initial attempt at calling write did not appear actually to send any data. I found that calling waitForBytesWritten solved this. If I need multiple write calls as per previous point, how do I use waitForBytesWritten in conjunction with these? Do I associate a waitForBytesWritten with each write or do I loop over write and then use waitForBytesWritten.
The documentation suggests that waitForBytesWritten can fail randomly on Windows so ideally I do not want to rely on it at all. It suggests using the bytesWritten signal instead but I have found very little information on how one is supposed to use this properly. In particular, if I have to deal with my concern in the first point, do I not get into the recursive call situation warned about in the documentation of bytesWritten?
I would like to know Why we need to compile the program of progress 4GL? Really what is happening behind there? Why we are getting .r file after compiled the program? When we check the syntax if its correct then we will get one message box 'Syntax is correct' how its finding the errors and showing the messages.Any explanations welcome and appreciated.
Benefits of compiled r-code include:
Syntax checking
Faster execution (r-code executes faster)
Security (r-code is not "human readable" and tampering with it will likely be noticed)
Licensing (r-code runtime licenses are much less expensive)
For "how its finding the errors and showing the messages" -- at a high level it is like any compiler. It evaluates the provided source against a syntax tree and lets you know when you violate the rules. Compiler design and construction is a fairly advanced topic that probably isn't going to fit into a simple SO question -- but if you had something more specific that could stand on its own as a question someone might be able to help.
The short answer is that when you compile, you're translating your program to a language the machine understands. You're asking two different questions here, so let me give you a simple answer to the first: you don't NEED to compile if you're the only one using the program, for example. But in order to have your program optimized (since it's already at the machine language level) and guarantee no one is messing with your logic, we compile the code and usually don't allow regular users to access the source code.
The second question, how does the syntax checker work, I believe it would be better for you to Google and choose some articles to read about compilers. They're complex, but in a nutshell what they do is take what Progress expects as full, operational commands, and compare to what you do. For example, if you do a
Find first customer where customer.active = yes no-error.
Progress will check if customer is a table, if customer.active is a field in that table, if it's the logical type, since you are filtering if it is yes, and if your whole conditions can be translated to one single true or false Boolean value. It goes on to check if you specified a lock (and default to shared if you haven't, like in my example, which is a no-no, by the way), what happens if there are multiple records (since I said first, then get just the first one) and finally what happens if it fails. If you check the find statement, there are more options to customize it, and the compiler will simply compare your use of the statement to what Progress can have for it. And collect all errors if it can't. That's why sometimes compilers will give you generic messages. Since they don't know what you're trying to do, all they can do is tell you what's basically wrong with what you wrote.
Hope this helps you understand.
So just started playing with Meteor and trying to get my head around the security model. It seems there's two ways to modify data.
The Meteor.call way which seems pretty standard - pretty much just a call to the server with its own set of business rules implemented.
Then there is the Collection.allow method which seems much more different to anything I've done before. So it seems that if you put an collection.allow, you're saying that the client can make any write operation to that collection as long as it can get past the validations in its allow function.
That makes me feel uneasy cause it's feels like a lot of freedom and my allow function would need to be pretty long to make sure it's locked down securely enough.
For instance, mongodb has no schema, so you'd have to basically have a rule that defines which fields would be accepted and the format those fields must be in.
Wouldn't you also have to put in the business logic for every type of update that might be made to your system.
So say, I had a SoccerTeam collection. There may be several situations I may need to make a change, like if I'm adding or removing a player, changing the team name, team status has changed etc.
It seems to me that you'd have to put everything into this one massive function. It just sounds like a radical idea, but it seems Meteor.call methods would just be a lot simpler.
Am I thinking about this in the wrong manner (or for the wrong use case?) Does anyone have any example of how they can structure an allow or deny function with a list of what I may need to check in my allow function to make my collection secure?
You are following the same line of reasoning I used in deciding how to handle data mutations when building Edthena. Out of the box, meteor provides you with the tools to make a simple tradeoff:
Do I trust the client and get a more responsive UI (latency compensation)? Or do I require strict control over data validation, but force the client to wait for an update?
I went with the latter, and exclusively used method calls for a few reasons:
I sleep better a night knowing there exists exactly one way to update each of my collections.
I found that some of my updates required side effects that only made sense to execute on the server (e.g. making denormalized updates to other collections).
At present, there isn't a clear benefit to latency compensation for our app. We found the delay for most writes was inconsequential to the user experience.
allow and deny rules are weak tools. They are essentially only good for validating ownership and other simple checks.
At the time when we first released to production (August 2013) this seemed like a radical conclusion. The meteor docs, the API, and the demos highlight the use of client-side writes, so I wasn't entirely sure I had made the right decision. A couple of months later I had my first opportunity to sit down with several of the meteor core devs - this is a summary of their reaction to my design choices:
This seems like a rational approach. Latency compensation is really useful in some contexts like mobile apps, and games, but may not be worth it for all web apps. It also makes for cool demos.
So there you have it. As of this writing, my advice for production apps would be to use client-side updates where you really need the speed, but you shouldn't feel like you are doing something wrong by making heavy use of methods.
As for the future, I'd imagine that post-1.0 we'll start to see things like built-in schema enforcement on both the client and server which will go a long way towards resolving my concerns. I see Collection2 as a significant first step in that direction, but I haven't tried it yet in any meaningful way.
stubs
A logical follow-up question is "Why not use stubs?". I spent some time investigating this but reached the conclusion that method stubbing wasn't useful to our project for the following reasons:
I like to keep my server code on the server. Stubbing requires that I either ship all of my model code to the client or selectively repeat parts of it again. In a large app, I don't see that as practical.
I found the the overhead required to separate out what may or may not run on the client to be a maintenance challenge.
In order for the stub to do anything other than reject a database mutation, you'd need to have an allow rule in place - otherwise you'd end up with a lot of UI flicker (the client allows the write but the server immediately invalidates it). But having an allow rule defeats the whole point, because a user could still write to the db from the console.
The usual allow methods I have are these:
MyCollection.allow({
insert: false
update: false
remove: false
})
And then, I have methods which take care of all insertions. These methods perform the type checks and permission assessment. I have found that to be a much more maintainable method: completely decoupling the data layer from the code which runs on the client.
For instance, mongodb has no schema, so you'd have to basically have a rule that defines which fields would be accepted and the format those fields must be in.
Take a look at Collection2. They support schema checking at run-time before inserting documents into the Collection.
another quick question, I want to make simple console based game, nothing too fancy, just to have some weekend project to get more familiar with C. Basically I want to make tetris, but I end up with one problem:
How to let the game engine go, and in the same time wait for input? Obviously cin or scanf is useless for me.
You're looking for a library such as ncurses.
Many Rogue-like games are written using ncurses or similar.
There's two ways to do it:
The first is to run two threads; one waits for input and updates state accordingly while the other runs the game.
The other (more common in game development) way is to write the game as one big loop that executes many times a second, updating game state, redrawing the screen, and checking for input.
But instead of blocking when you get key input, you check for the presence of pending keypresses, and if nothing has happened, you just continue through your loop. If you have multiple input sources (keyboard, network, etc.) they all get put there in the loop, checking one after another.
Yes, it's called polling. No, it's not efficient. But high-end games are usually all about pulling the maximum performance and framerates out of the computer, not running cool.
For added efficiency, you can optionally block with a timeout -- saying "wait for a keypress, but no longer than 300 milliseconds" so you can continue on with your loop.
select() comes to mind, but there are other ways of waiting or checking for input as well.
You could work out how to change stdin to non-blocking, which would enable you to write something like tetris, but the game might be more directly expressed in an event-driven paradigm. Maybe it's a good excuse to learn windows programming.
Anyway, if you want to go the console route, if you are using the microsoft compiler, then you should have kbhit() available (via conio.h) which can tell you whether a call to fgetc on stdin would block.
Actually should mention that the MinGW gcc compiler 3.4.5 also supports kbhit().
Looking into asynchronous address resolution in winsock it seems that the only two options are either to use the blocking gethostbyname on a seperate thread, or use WSAAsyncGetHostByName. The latter is designed for some reason to work with window messages, instead of overlapped operations and completion ports/routines.
Is there any version of gethostbyname that works asynchronously with overlapped operations in a similiar manner to the rest of the winsock API?
Unfortunately there isn't at present, although GetAddrInfoEx() has placeholders for all the right things for async operation via all of the 'usual' routes (including IOCP) so I expect there will be eventually... Unfortunately, at this time, the docs say that all of these must be set to NULL and are marked as 'reserved'. :(
I'm just about to write one (have been for a while)... It's unfortunate that WSAAsyncGetHostByName doesn't even allow concurrent name resolution, so it's pretty useless as a base for what I want; but, then again, since it doesn't handle IPv6 that also makes it pretty useless to me. I expect I'll start from scratch; possibly using something like this (beerware) as a base.
Sorry, there is no overlapped version of gethostbyname().