Distributed Objects + Grand Central Dispatch

Question

Not a specific question as such, I'm more trying to test the waters. I like distributed objects, and I like grand central dispatch; How about I try to combine the two?

Does that even make sense? Has anyone played around in these waters? Would I be able to use GCD to help synchronize object access across machines? Or would it be better to stick to synchronizing local objects only? What should I look out for? What design patterns are helpful and what should I avoid?

as an example, I use GCD queues to synchronize accesses to a shared resource of some kind. What can I expect to happen if I make this resource public via distributed objects? Questions like: How nicely to blocks play with distributed objects? Can I expect to use the everything as normal across machines? If not, can I wrangle it to do so? What difficulties can I expect?

This is too broad and vague to answer. What do you hope to achieve? — jscs, May 14 '12 at 00:16

score 2 · Accepted Answer · answered May 14 '12 at 01:34

2

I very much doubt this will work well. GCD objects are not Cocoa objects, so you can't reference them remotely. GCD synchronization primitives don't work across process boundaries.

While blocks are objects, they do not support NSCoding, so they can't be transmitted across process boundaries. (If you think about it, they are not much more than function pointers. The pointed-to function must have been compiled into the executable. So, it doesn't make sense that two different programs would share a block.)

Also, Distributed Objects depends on the connection being scheduled in a given run loop. Since you don't manage the threads used by GCD, you are not entitled to add run loop sources except temporarily.

Frankly, I'm not even sure how you envision it even theoretically working. What do you hope to do? How do you anticipate it working?

answered May 14 '12 at 01:34

Ken Thomases

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I didn't really have a specific purpose in mind, I'm just begun to get used to using GCD to coordinate shared resources and I'm wondering if that could apply to distributed objects, and if not then what is the recommended way to do that sort of thing? – dented42 May 14 '12 at 02:51
Although, if one added the restriction that only instances of the same binary would attempt to connect to each other then wouldn't passing function pointers work? – dented42 May 14 '12 at 02:52
No. Each process has its own address space and things are not located at the same address in each. – Ken Thomases May 14 '12 at 03:19

score 0 · Answer 2 · answered Apr 02 '16 at 00:36

Running across machines -- as in a LAN, MAN, or WAN?

In a LAN, distributed objects will probably work okay as long as the server you are connecting to is operational. However, most programmers you meet will probably raise an eyebrow and just ask you, "Why didn't you just use a web server on the LAN and just build your own wrapper class that makes it 'feel' like Distributed Objects?" I mean, for one thing, there are well-established tools for troubleshooting web servers, and it's easier and often cheaper to hire someone to build a web service for you rather than a distributed object server.

On a MAN or WAN, however, this would be slow and a very bad idea for most uses. For that type of communication, you're better off using what everyone else uses -- REST-like APIs with HTTPS/HTTP, sending either XML, JSON, or key/value data back and forth. So, you could make a class wrapper that makes this "feel" sort of like distributed objects. And my gut feeling tells me that you'll need to use tricks to speed this up, such as caching chunks of data locally on the client so that you don't have to keep fetching from the server, or even caching on the server so that it doesn't have to interact with a database as often.

GCD, Distributed Objects, Mach Ports, XPC, POSIX Message Queues, Named Pipes, Shared Memory, and many other IPC mechanisms really only make the most sense on local, application to application communication on the same computer. And they have the added advantage of privilege elevation if you want to utilize that. (Note, I said POSIX Message Queues, which are workstation-specific. You can still use a 'message queue service' on a LAN, MAN, or WAN -- there are many products available for that.)

Distributed Objects + Grand Central Dispatch

2 Answers2