New%20Ways%20to%20Fetch%20Work

1
New Ways to Fetch Work

• The new hook infrastructure in Condor 7.1.

2
Whats the problem?

• Users wanted to take advantage of Condors
  resource management daemon (condor_startd) to run
  jobs, but they had their own scheduling system.
• Specialized scheduling needs
• Jobs live in their own database or other storage
  than a Condor job queue

3
Fetch vs. push

• Instead of trying to get these jobs into a
  condor_schedd, or try to push them to the
  condor_startd, just get the condor_startd to
  fetch (pull) the work
• Lower latency than the overhead of matchmaking
  and the schedd
• Fetching only requires an outbound network
  connection which makes life easier if you
  glide-in behind a firewall

4
Whats the dumb solution?

• Put code directly into the condor_startd that can
  talk directly to the other scheduling system(s)
• Wed have to support other protocols
• Wed have to link even more libraries and
  dependencies into our code
• Very inflexible

5
Another dumb solution

• Make it a web service!
• Mostly the same problems
• What protocol?
• What format to describe the jobs?
• Add a dependency on libCurl?
• What if I dont want a webserver to be handling
  my jobs?
• Security? Authentication? Privacy?

6
Our solution (hopefully not dumb)

• Make a system of hooks that you can plug into
• A hook is a point during the life-cycle of a job
  where the Condor daemons will invoke an external
  program
• The hook invocation points have to be hard-coded
  into Condor, but then anyone can implement their
  own hooks to do what they want

7
Why isnt that dumb?

• All the logic, code, libraries, etc, to fetch
  jobs from any given system lives completely
  outside of the Condor source and binaries
• New hooks can be installed without a new version
  of Condor
• No new library dependencies for us
• Hooks are written by people who know what theyre
  doing

8
How does Condor communicate with hooks?

• Passing around ASCII ClassAds via standard input
  and standard output
• Some hooks get control data via a command-line
  argument (argv)
• Hooks can be written in any language (scripts,
  binaries, whatever you want) so long as you can
  read/write STDIN/OUT
• Decades of UNIX wisdom cant be wrong!

9
What hooks are available?

• Hooks for fetching work (condor_startd)
• FETCH_JOB
• REPLY_FETCH
• EVICT_CLAIM
• Hooks for running jobs (condor_starter)
• PREPARE_JOB
• UPDATE_JOB_INFO
• JOB_EXIT

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HOOK_FETCH_JOB

• Invoked by the startd whenever it wants to try to
  fetch new work
• FetchWorkDelay expression
• Hook gets a current copy of the slot ClassAd
• Hook prints the job ClassAd to STDOUT
• If STDOUT is empty, theres no work

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HOOK_REPLY_FETCH

• Invoked by the startd once it decides what to do
  with the job ClassAd returned by HOOK_FETCH_WORK
• Gives your external system a chance to know what
  happened
• argv1 accept or reject
• Gets a copy of slot and job ClassAds
• Condor ignores all output
• Optional hook

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HOOK_EVICT_CLAIM

• Invoked if the startd has to evict a claim thats
  running fetched work
• Informational only you cant stop or delay this
  train once its left the station
• STDIN Both slot and job ClassAds
• STDOUT gt /dev/null

13
HOOK_PREPARE_JOB

• Invoked by the condor_starter when it first
  starts up (only if defined)
• Opportunity to prepare the job execution
  environment
• Transfer input files, executables, etc.
• INPUT both slot and job ClassAds
• OUTPUT ignored, but starter wont continue until
  this hook exits
• Not specific to fetched work

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HOOK_UPDATE_JOB_INFO

• Periodically invoked by the starter to let you
  know whats happening with the job
• INPUT both ClassAds
• Job ClassAd is updated with additional attributes
  computed by the starter
• ImageSize, JobState, RemoteUserCpu, etc.
• OUTPUT ignored

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HOOK_JOB_EXIT

• Invoked by the starter whenever the job exits for
  any reason
• Argv1 indicates what happened
• exit Died a natural death
• evict Booted off prematurely by the startd
  (PREEMPT TRUE, condor_off, etc)
• remove Removed by condor_rm
• hold Held by condor_hold

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HOOK_JOB_EXIT

• HUH!?! condor_rm? What are you talking about?
• The starter hooks can be defined even for regular
  Condor jobs, local universe, etc.
• INPUT copy of the job ClassAd with extra
  attributes about what happened
• ExitCode, JobDuration, etc.
• OUTPUT Ignored
• Except for dumb exceptions the schedd doesnt
  distinguish rm vs. hold when telling the starter
  to go away (yet). Argh!

17
Defining hooks

• Each slot can have its own hook keyword
• Prefix for config file parameters
• Can use different sets of hooks to talk to
  different external systems on each slot
• Global keyword used when the per-slot keyword is
  not defined
• Keyword is inserted by the startd into its copy
  of the job ClassAd and given to the starter

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Defining hooks example

• Most slots fetch work from the database system
• STARTD_JOB_HOOK_KEYWORD DB
• Slot4 fetches and runs work from a web service
• SLOT4_JOB_HOOK_KEYWORD WEB
• The database system needs to both provide work
  and
• know the reply for each attempted claim
• DB_DIR /usr/local/condor/fetch/db
• DATABASE_HOOK_FETCH_WORK (DB_DIR)/fetch_work.ph
  p
• DATABASE_HOOK_REPLY_FETCH (DB_DIR)/reply_fetch.
  php
• The web system only needs to fetch work
• WEB_DIR /usr/local/condor/fetch/web
• WEB_HOOK_FETCH_WORK (WEB_DIR)/fetch_work.php

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Semantics of fetched jobs

• Condor_startd treats them just like any other
  kind of job
• All the standard resource policy expressions
  apply (START, SUSPEND, PREEMPT, RANK, etc).
• Fetched jobs can coexist in the same pool with
  jobs pushed by Condor, COD, etc.
• Fetched work ! Backfill

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Semantics continued

• If the startd is unclaimed and fetches a job, a
  claim is created
• If that job completes, the claim is reused and
  the startd fetches again
• Keep fetching until either
• The claim is evicted by Condor
• The fetch hook returns no more work

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Limitations for fetched jobs

• No schedd/shadow means no standard universe for
  checkpointing, migration, and remote system calls
• Could use stand-alone checkpointing
• Application-specific checkpointing
• Other features that are unavailable
• User policy expressions (e.g. periodic hold)
• No DAGMan (youre on your own)

22
Limitations of the hooks

• If the starter cant run your fetched job because
  your ClassAd is bogus, no hook is invoked to tell
  you about it
• We need a HOOK_STARTER_FAILURE
• No hook when the starter is about to evict you
  (so you can checkpoint)
• Can implement this yourself with a wrapper script
  and the SoftKillSig attribute

23
More information

• New section in the Condor 7.1 manual
• Chapter 4 Miscellaneous Concepts
• 4.4 Job Hooks
• http//www.cs.wisc.edu/condor/manual/v7.1/4_4Job_H
  ooks.html
• Any questions?