# Strange DRb and fork issues **URL:** \ **Category:** Ruby\ **Created:** [April 29, 2007, 1:17pm UTC](https://www.ruby-forum.com/t/strange-drb-and-fork-issues/93215 "2007-04-29T13:17:45Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![Farrel\_L](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/9d8465/32.png) [@Farrel\_L](https://www.ruby-forum.com/u/Farrel_L)\ **Post date:** [April 29, 2007, 1:17pm UTC](https://www.ruby-forum.com/t/strange-drb-and-fork-issues/93215/1 "2007-04-29T13:17:45Z")
Hi all, I’m busy experimenting with DRb and I’ve hit some strange behaviour which I can’t get seem to find the cause of. I think it might be something to do with DRb and forks but I’m unsure. The process I’m trying to achieve is to have a Server process running as a DRb service. It can spawn client processes (the create\_client method) which also run using a DRb service. Upon spawning the clients create DRb proxy objects to the server and register using the register\_client method passing through their client process pid and url. The server then creates DRb proxy objects with the information it receives and stores it in a hash with the client pid as the key. We can retrieve an array of client pid using the client\_pids method on the server. I have encountered the following problem. When I start the server process and then start the client process independent from the server process everything seems to work fine. I can retrieve from the server the array of client pids and everything exits gracefully. However when I create the client from within the server process using the create\_client method (like it hopefully would normally do) the server registers the client pid and stores it in the hash but on subsequent calls to get the array of pids it comes back empty(!) and a ruby process hangs around afterwards. I’ve attached some files showing this behaviour. Both tests in theory should be doing the same thing; registering a client with a server and then retrieving the array of client pids. farrel@nicodemus ~/Desktop $ ruby server.rb -n test\_register\_client Loaded suite server Started Registered client druby://localhost:41362, 1 clients registered . Finished in 0.310428 seconds. 1 tests, 1 assertions, 0 failures, 0 errors farrel@nicodemus ~/Desktop $ ps PID TTY TIME CMD 5404 pts/4 00:00:00 bash 5920 pts/4 00:00:00 ps farrel@nicodemus ~/Desktop $ ruby server.rb -n test\_create\_client Loaded suite server Started Registered client druby://localhost:2276, 1 clients registered F Finished in 0.315975 seconds. 1. Failure: test\_create\_client(TestServer) [server.rb:68]: \<[5923]\> expected but was \<[]\>. 1 tests, 1 assertions, 1 failures, 0 errors farrel@nicodemus ~/Desktop $ ps PID TTY TIME CMD 5404 pts/4 00:00:00 bash 5923 pts/4 00:00:00 ruby 5924 pts/4 00:00:00 ps As you can see the pid array comes back empty despite the server printing out the message that it has registered the client and for some reason there’s a ruby process still hanging around. Any DRb experts out there who can help? Thanks Farrel ---
**Author:** ![Farrel\_L](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/9d8465/32.png) [@Farrel\_L](https://www.ruby-forum.com/u/Farrel_L)\ **Post date:** [April 29, 2007, 7:29pm UTC](https://www.ruby-forum.com/t/strange-drb-and-fork-issues/93215/2 "2007-04-29T19:29:19Z")
On Apr 29, 2007, at 4:17 AM, Farrel L. wrote: > Hi all, > > I’m busy experimenting with DRb and I’ve hit some strange behaviour > which I can’t get seem to find the cause of. I think it might be > something to do with DRb and forks but I’m unsure. The process I’m > trying to achieve is to have a Server process running as a DRb > service. It can spawn client processes (the create\_client method) > which also run using a DRb service. > > Any DRb experts out there who can help? > > Thanks > Farrel > \ > \ > \ Hey Farrel- I’ll save you a lot of hassle. Go grab the slave gem off rubyforge. It does exactly what you are trying to do. It allows you to fork child objects and it sets up a drb connection and a heartneat between the child and parent so if the parent dies, the children will also die. [http://codeforpeople.com/lib/ruby/slave/slave-1.2.1/README](http://codeforpeople.com/lib/ruby/slave/slave-1.2.1/README) Cheers- – Ezra Z. – Lead Rails Evangelist – removed\_email\_address@domain.invalid – Engine Y., Serious Rails Hosting – (866) 518-YARD (9273) ---
**Author:** ![Farrel\_L](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/9d8465/32.png) [@Farrel\_L](https://www.ruby-forum.com/u/Farrel_L)\ **Post date:** [April 29, 2007, 9:37pm UTC](https://www.ruby-forum.com/t/strange-drb-and-fork-issues/93215/3 "2007-04-29T21:37:31Z")
On 29/04/07, Ezra Z. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > I’ll save you a lot of hassle. Go grab the slave gem off rubyforge. > It does exactly what you are trying to do. It allows you to fork > child objects and it sets up a drb connection and a heartneat between > the child and parent so if the parent dies, the children will also die. Thanks, I’ll check it out, although I would really like to work out what the problem is with my code. I’ll look through the source of Slave and see if I can find some hints. Farrel ---
**Author:** ![Farrel\_L](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/9d8465/32.png) [@Farrel\_L](https://www.ruby-forum.com/u/Farrel_L)\ **Post date:** [April 30, 2007, 1:10am UTC](https://www.ruby-forum.com/t/strange-drb-and-fork-issues/93215/4 "2007-04-30T01:10:49Z")
On 30/04/07, removed\_email\_address@domain.invalid [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > i reccomend trying to using slave.rb if you can - of course, as the author, > i’m biased. it’s really tricky connecting parent/child drb processes: the key > is that you cannot carry the drb objects across the fork, you must setup each > new drb object in the child and arrange for the parent to find it. this, as > ezra mentions, exactly what slave does. if you have examle code we can > suggest how to plug slave into it. Hi Ara, In my code I used fork with a block (taken from server.rb): def create\_client(hostname=nil,port=nil) pid = fork do @klass.start(hostname,port,self.url) end Process.detach(pid) pid end end Could that be causing the strange issue I’m having? I assumed that was a safe way of using fork as the child only runs what’s in the block. My main puzzle is why the server loses it’s reference to the client process after it has registered. I’m trying to learn some of the low level details of DRb, but unfortunately the documentation out there is not very detailed. Farrel ---
**Author:** ![Farrel\_L](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/9d8465/32.png) [@Farrel\_L](https://www.ruby-forum.com/u/Farrel_L)\ **Post date:** [April 30, 2007, 3:22pm UTC](https://www.ruby-forum.com/t/strange-drb-and-fork-issues/93215/5 "2007-04-30T15:22:41Z")
On Mon, 30 Apr 2007, Farrel L. wrote: > end > end that looks reasonable. > Could that be causing the strange issue I’m having? I assumed that was a > safe way of using fork as the child only runs what’s in the block. My main > puzzle is why the server loses it’s reference to the client process after it > has registered. hard to say without seeing the code 😉 > I’m trying to learn some of the low level details of DRb, but unfortunately > the documentation out there is not very detailed. the samples directory covers nearly everything - do you have that? cheers. -a ---
**Author:** ![Farrel\_L](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/9d8465/32.png) [@Farrel\_L](https://www.ruby-forum.com/u/Farrel_L)\ **Post date:** [April 30, 2007, 3:34pm UTC](https://www.ruby-forum.com/t/strange-drb-and-fork-issues/93215/6 "2007-04-30T15:34:43Z")
On 30/04/07, removed\_email\_address@domain.invalid [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > > Process.detach(pid) > > hard to say without seeing the code 😉 Hi Ara, Thanks for the help, I hope you don’t mind me badgering you but from my time on the list I know you are one of the resident DRb experts so I’m grabbing the opportunity to get some expert advice here. I attached the code I’m using in my first email, comprising three files (base.rb, client.rb and server.rb) with the unit tests illustrating my problems in server.rb . If it got stripped out let me know and I can try and resend it. In the meantime I will check out the DRb samples directorty and see if that helps. Thanks again, Farrel ---
**Author:** ![Farrel\_L](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/9d8465/32.png) [@Farrel\_L](https://www.ruby-forum.com/u/Farrel_L)\ **Post date:** [May 1, 2007, 12:11am UTC](https://www.ruby-forum.com/t/strange-drb-and-fork-issues/93215/7 "2007-05-01T00:11:01Z")
On Mon, 30 Apr 2007, Farrel L. wrote: > know and I can try and resend it. In the meantime I will check out the > DRb samples directorty and see if that helps. > > Thanks again, > Farrel i’ll try to look at it soon… cheers. -a ---
**Author:** ![Farrel\_L](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/9d8465/32.png) [@Farrel\_L](https://www.ruby-forum.com/u/Farrel_L)\ **Post date:** [April 30, 2007, 12:06am UTC](https://www.ruby-forum.com/t/strange-drb-and-fork-issues/93215/8 "2007-04-30T00:06:44Z")
On Mon, 30 Apr 2007, Farrel L. wrote: > On 29/04/07, Ezra Z. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > > > I’ll save you a lot of hassle. Go grab the slave gem off rubyforge. > > It does exactly what you are trying to do. It allows you to fork > > child objects and it sets up a drb connection and a heartneat between > > the child and parent so if the parent dies, the children will also die. > > Thanks, I’ll check it out, although I would really like to work out what the > problem is with my code. I’ll look through the source of Slave and see if I > can find some hints. i reccomend trying to using slave.rb if you can - of course, as the author, i’m biased. it’s really tricky connecting parent/child drb processes: the key is that you cannot carry the drb objects across the fork, you must setup each new drb object in the child and arrange for the parent to find it. this, as ezra mentions, exactly what slave does. if you have examle code we can suggest how to plug slave into it. regards. -a