Use Case 17

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Use Case #17: long term resources

As shown in [Use Case 16], the "don't create resource" behavior can generate a race condition: the resource will never created because the producer task finished before the consumer task starts.
The following picture illustrates the race condition:

flom supports long term resources to deal with this type of situations.

Open two terminals and try this experiment:

1. inside the first terminal write this command at prompt, and press "enter": "flom --resource-idle-lifespan=5000 -- ping -c 3 localhost"
2. wait the command terminates to ping and wait approximately 2-3 more seconds
3. inside the second terminal write this command at prompt, and press "enter": "flom --resource-create=no -- ping -c 4 localhost"

Expected result:

1. the first terminal executes "ping -c 3 localhost" command
2. the second terminal executes "ping -c 4 localhost" command

Terminal 1 output:

tiian@ubuntu:~$ flom --resource-idle-lifespan=5000 -- ping -c 3 localhost
PING localhost (127.0.0.1) 56(84) bytes of data.
64 bytes from localhost (127.0.0.1): icmp_seq=1 ttl=64 time=0.081 ms
64 bytes from localhost (127.0.0.1): icmp_seq=2 ttl=64 time=0.069 ms
64 bytes from localhost (127.0.0.1): icmp_seq=3 ttl=64 time=0.055 ms

--- localhost ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 1998ms
rtt min/avg/max/mdev = 0.055/0.068/0.081/0.012 ms

Terminal 2 output:

tiian@ubuntu:~$ flom --resource-create=no -- ping -c 4 localhost
PING localhost (127.0.0.1) 56(84) bytes of data.
64 bytes from localhost (127.0.0.1): icmp_seq=1 ttl=64 time=0.057 ms
64 bytes from localhost (127.0.0.1): icmp_seq=2 ttl=64 time=0.118 ms
64 bytes from localhost (127.0.0.1): icmp_seq=3 ttl=64 time=0.053 ms
64 bytes from localhost (127.0.0.1): icmp_seq=4 ttl=64 time=0.062 ms

--- localhost ping statistics ---
4 packets transmitted, 4 received, 0% packet loss, time 2997ms
rtt min/avg/max/mdev = 0.053/0.072/0.118/0.027 ms

Explanation:

command "ping -c 3 localhost" creates and locks the default resource; the resource will last at least 5000 milliseconds after command completion; command "ping -c 4 localhost" locks the default resource.

Rewind it...

What happens if you wait more than 5000 milliseconds between first command completion and second command execution? Try it:

1. inside the first terminal write this command at prompt, and press "enter": "flom --resource-idle-lifespan=5000 -- ping -c 3 localhost"
2. wait the command terminates to ping and wait approximately 7-8 more seconds
3. inside the second terminal write this command at prompt, and press "enter": "flom --resource-create=no -- ping -c 4 localhost"

Expected result:

The resource created by the first command disappears (automatically cleaned by flom daemon) and the second command waits indefinitely.

Summary

This use case allows you to implement the producer/consumer pattern even if the commands are fired at different times.
The long term resource behavior can be obtained using "-i milliseconds", "--resource-idle-lifespan=milliseconds" command option or "Resource/IdleLifespan" property inside configuration files.

See also

Flom available arguments are documented in man page: use man flom.
Flom [Configuration] explains how you can specify flom behavior without using command line arguments.