Container dispatch » History » Revision 17
Revision 16 (Tom Clegg, 12/23/2015 09:43 PM) → Revision 17/26 (Tom Clegg, 01/06/2016 08:46 PM)
h1. Container dispatch
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h2. Summary
A dispatcher uses available compute resources to execute queued containers.
Dispatch is meant to be a small simple component rather than a pluggable framework: e.g., "slurm dispatch" can be a small standalone program, rather than a plugin for a big generic dispatch program.
h2. Pseudocode
* Notice there is a queued container
* Decide whether the required resources are available to run the container
* Lock the container (this avoids races with other dispatch processes)
* Translate the container's runtime constraints and priority to instructions for the lower-level scheduler, if any
* Invoke the "crunch2 run" executor
* When the priority changes on a container taken by this dispatch process, update the lower-level scheduler accordingly (cancel if priority is zero)
* If the lower-level scheduler indicates the container is finished or abandoned, but the Container record is locked by this dispatcher and has state=Running, fail the container
h2. Examples
slurm batch mode
* Use "sinfo" to determine whether it is possible to run the container
* Submit a batch job to the queue: "echo crunch-run --job {uuid} | sbatch -N1"
* When container priority changes, use scontrol and scancel to propagate changes to slurm
* Use strigger to run a cleanup script when a container exits
standalone worker
* Inspect /proc/meminfo, /proc/cpuinfo, "docker ps", etc. to determine local capacity
* Invoke crunch-run as a child process (or perhaps a detached daemon process)
* Signal crunch-run to stop if container priority changes to zero
h2. Arvados API support
Each dispatch process has an Arvados API token that allows it to see queued containers.
* No two dispatch processes can run at the same time with the same token. One way to achieve this is to make a user record for each dispatch service.
Container APIs relevant to a dispatch program:
* List Queued containers (might be a subset of Queued containers)
* List containers with state=Locked or state=Running associated with current token
* Receive event when container is created or modified and state is Queued (it might become runnable)
* Change state Queued->Locked
* Change state Locked->Queued
* Change state Locked->Running
* Change state Running->Complete
* Receive event when priority changes
* Receive event when state changes to Complete
* Create a unique API token to pass to crunch-run (expires when the container stops)
* Create events/logs
** Decided not to run this container
** Decided to run this container (e.g., no node with those resources)
** Lock failed
** Dispatched to crunch-run
** Cleaned up crashed crunch-run (lower-level scheduler indicates the job finished, but crunch-run didn't leave the container in a final state)
** Cleaned up abandoned container (container belongs to this process, but dispatch and lower-level scheduler don't know about it)
h2. Non-responsibilities
Dispatch doesn't retry failed containers. If something needs to be reattempted, a new container will appear in the queue.
Dispatch doesn't fail a container that it can't run. It doesn't know whether other dispatchers will be able to run it.
h2. Additional notes
(see also #6429 and #6518 and #8028) #6518)
Using websockets to listen for container events (new containers added, priority changes) will benefit from some Go SDK support.