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Cylc Scheduler Internals - Part 2

kinow @ Jul 27, 2018 00:25:37

This is part 2, in a series of posts about Cylc internals. The part 1 had the beginning of the workflow. And here we will have the continuation, from the moment the method configure() is called.

NB: this is a post to remember things, not really expecting to give someone enough information to be able to hack the Cylc Scheduler (though you can and would have fun!).

The configure method is responsible for configuring the Suite Server Program. Which means it will interact with the configuration singletons to retrieve the necessary configuration for the program.

It also interacts with other objects that store state, and also creates basic data structures and objects required for a suite program (e.g. Queues).

This is also where the contact file is created, as well as the HTTP server.

You can download the source file for the diagram used in this post, and edit it with

Multithreaded code and Pandas

kinow @ Jul 22, 2018 13:22:14

Woman looking

Pandas provides high-performance data structures in Python. I think in Java there are similar data structures in projects like Apache Commons Collections, Google Guava, and also Trove.

In the Java libraries thread-safety is always a must-have feature. Probably as it is quite common for a Java program to have more than one thread, especially if the code runs in some sort of web container.

I recently learned that Pandas, on the other hand, does not guarantee any thread-safety. I found that while reading an issue about race condition in the IndexEngine, and after preparing a pull request for that.

from concurrent.futures import ThreadPoolExecutor
import pandas as pd

x = pd.date_range('2001', '2020')
with ThreadPoolExecutor(2) as p:
    assert all( x: x.is_unique, [x]*2))

When you create an index like that, it will delegate most of the hard work to the IndexEngine. Inside the IndexEngine, the values passed for the index are stored, and then an empty Hashtable is created (as well as several flags for the state of the object, such as unique, which defines whether the index has unique elements or not).

Once a user calls a method like is_unique, then the flags are updated, the Hashtable mapping is populated, and while doing so, if not all elements are unique, the flag for unique is set to false, or true otherwise. But if the user does not need that operation, we will avoid populating the mapping until we really need it.

I believe it is done that way for performance. However, at the cost that the state is shared among calls, which makes it harder to use this API in a multithreaded environment - though still possible by moving the synchronization to your caller code.

Some Apache software also do the same, asking users to synchronize, serialize, or handle certain corner cases on their side. There is a huge cost associated with maintaining an Open Source project that promises thread-safety.

But maybe Dask provides an alternative to use Pandas with multiple threads. But I have not used that yet.

♥ Open Source

NB: even though Pandas is not thread-safe, it does not mean you should not use it. Just use with care when using multiple threads

Cylc Scheduler Internals - Part 1

kinow @ Jul 14, 2018 22:42:47

This is the first post in a series of three (or maybe four later) based on diagrams I collected while debugging the Cylc scheduler. The scheduler is called by the cylc start utility.

NB: this is a post to remember things, not really expecting to give someone enough information to be able to hack the Cylc Scheduler (though you can and would have fun!).

Instead of going at length on what happens (and there is quite a bit happening when you run cylc start my.suite), I will use the following diagram, followed by a few paragraphs to highlight certain parts. The code used was based on Cylc 7.7.1.

When a Cylc command like cylc start is invoked, it actually gets translated into bin/cylc-$command_name. cylc-start is a Shell file, that will simply call cylc-run. cylc-run then imports scheduler_cli… but what you need to know is that in the end scheduler_cli will create an instance of Scheduler with the right constructor arguments, and call its start method.

After that point, you are at the left-most lifeline, on the Scheduler constructor (i.e. the init method of the’s Scheduler class).

If you follow the method calls - which are hopefully easy to understand and follow - you will find that the constructor merely creates a few objects, prepares the suite information, and the suite database.

Then the start method kicks things off, interacting with previously created objects, but also with some singletons for logging and configuration. Oh, that Cylc banner is also printed here (in case you would like to customize it as in SpringBoot).

After that, if you are running the suite as daemon, it will be daemonized, by forking the current process, but with Python.

One important step that happens here, is the initialization of the HTTP Server. This server will be used to communicate with the Suite Server Program. It will be listening to connections with the right endpoints available only after the configure method.

Lastly, we have configure and run methods, which are two very important methods to be discussed in the next part of this series, as they are quite extensive, and deserve their own diagrams.

You can download the source file for the diagram used in this post, and edit it with

ImportError when debugging cylc in Eclipse

kinow @ Jul 10, 2018 00:47:13

Since I started reading cylc’s source code in Eclipse to create some sequence diagrams, I have not been able to debug it properly without hitting errors in some part of the program execution.

The error message was “ImportError: cannot import name _remove_dead_weakref”, which was a bit enigmatic as I never heard about that function, but it seemed to be something internal, or at least not from the project code base. And searching the Internet did not help much.

Here is the complete console output in Eclipse.

pydev debugger: starting (pid: 15124)
timeout 10 ps -opid,args 13640  # return 1

            | |            The Cylc Suite Engine [7.7.1-37-g09c8a]    
._____._. ._| |_____.           Copyright (C) 2008-2018 NIWA          
| .___| | | | | .___|  _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
| !___| !_! | | !___.  This program comes with ABSOLUTELY NO WARRANTY;
!_____!___. |_!_____!  see `cylc warranty`.  It is free software, you 
      .___! |           are welcome to redistribute it under certain  
      !_____!                conditions; see `cylc conditions`.       
2018-07-10T01:00:47+12 INFO - Suite starting: server=localhost:44444 pid=15124
2018-07-10T01:00:47+12 INFO - Cylc version: 7.7.1-37-g09c8a
2018-07-10T01:00:47+12 INFO - Run mode: live
2018-07-10T01:00:47+12 INFO - Initial point: 1
2018-07-10T01:00:47+12 INFO - Final point: None
2018-07-10T01:00:47+12 INFO - Cold Start 1
2018-07-10T01:00:47+12 DEBUG - [hello.1] -released to the task pool
2018-07-10T01:00:47+12 DEBUG - BEGIN TASK PROCESSING
2018-07-10T01:00:47+12 DEBUG - [hello.1] -waiting => queued
2018-07-10T01:00:47+12 DEBUG - 1 task(s) de-queued
2018-07-10T01:00:47+12 INFO - [hello.1] -submit-num=1, owner@host=localhost
2018-07-10T01:00:47+12 DEBUG - [hello.1] -queued => ready
2018-07-10T01:00:47+12 DEBUG - END TASK PROCESSING (took 0.023609161377 seconds)
2018-07-10T01:00:48+12 DEBUG - ['cylc', 'jobs-submit', '--debug', '--', '/home/kinow/Development/python/workspace/example-suite/log/job', '1/hello/01']
2018-07-10T01:00:48+12 ERROR - [jobs-submit cmd] cylc jobs-submit --debug -- /home/kinow/Development/python/workspace/example-suite/log/job 1/hello/01
    [jobs-submit ret_code] 1
    [jobs-submit err]
    Traceback (most recent call last):
      File "/home/kinow/Development/python/workspace/cylc/bin/cylc-jobs-submit", line 52, in <module>
        from cylc.batch_sys_manager import BatchSysManager
      File "/home/kinow/Development/python/workspace/cylc/lib/cylc/", line 114, in <module>
        from cylc.task_message import (
      File "/home/kinow/Development/python/workspace/cylc/lib/cylc/", line 26, in <module>
        from logging import getLevelName, WARNING, ERROR, CRITICAL
      File "/home/kinow/Development/python/anaconda2/lib/python2.7/logging/", line 26, in <module>
        import sys, os, time, cStringIO, traceback, warnings, weakref, collections
      File "/home/kinow/Development/python/anaconda2/lib/python2.7/", line 14, in <module>
        from _weakref import (
    ImportError: cannot import name _remove_dead_weakref
2018-07-10T01:00:48+12 ERROR - [jobs-submit cmd] cylc jobs-submit --debug -- /home/kinow/Development/python/workspace/example-suite/log/job 1/hello/01
    [jobs-submit ret_code] 1
    [jobs-submit out] 2018-07-10T01:00:48+12|1/hello/01|1
2018-07-10T01:00:48+12 INFO - [hello.1] -(current:ready) submission failed at 2018-07-10T01:00:48+12
2018-07-10T01:00:48+12 ERROR - [hello.1] -submission failed
2018-07-10T01:00:48+12 DEBUG - [hello.1] -ready => submit-failed
2018-07-10T01:00:48+12 DEBUG - BEGIN TASK PROCESSING
2018-07-10T01:00:48+12 DEBUG - 0 task(s) de-queued
2018-07-10T01:00:48+12 DEBUG - END TASK PROCESSING (took 0.00175499916077 seconds)
2018-07-10T01:00:49+12 WARNING - suite stalled

As the current diagram I am working on has quite a few if‘s and else‘s, I decided to investigate why this error was occurring. Then, after some elimination I found that it was due to the missing Anaconda 2 entry in my $PATH environment variable.

I had this variable configured in a custom script I load whenever I decide to use Anaconda 2. And reproducing the same behaviour in Eclipse was easy.

A screen shot of Eclipse with source code
Locating the bug

Et voilà! Eclipse was happily debugging again!

A screen shot of Eclipse with source code
Locating the bug

So if you have a similar problem, try comparing your environment variables and check if you have some entries missing, and try adding them in Eclipse Debug configuration.

Happy cycling!

A simple cylc suite

kinow @ Jul 08, 2018 18:59:13

I have been writing more suites for cylc lately, and found an example that has proved to be useful for debugging certain parts of the code.

It is an extremely simple suite, similar to what is in cylc’s documentation. It sleeps for N seconds, and prints a message.

What makes it extra simpler, is that it cycles through integers, and has a limit of 1 maximum active points.

It is essentially the same as running the command in your shell session. With the difference that it will run through all cylc’s internal, only once, and allow you to debug and diagnostic parts nor related to cycling and graphs (as for these parts you would probably need a more elaborate example).

    cycling mode = integer
    initial cycle point = 1
    max active cycle points = 1
            graph = "hello"
        script = "sleep 10; echo PING"

I also combine this suite with the following global.rc.

    terminal = vim 
    gui = gvim -f

    base port = 44444
    method = http
    maximum number of ports = 1

With “base port” set to 44444, and the maximum number of ports to 1, I will be able to run only one task. But that way I can configure Wireshark and other tools to default to 44444/HTTP, for ease of debugging.

Then initialize the suite with something like: cylc start --non-daemon --debug /home/kinow/Development/python/workspace/example-suite/

Happy cycling!