Schedule durable tasks across multiple processes and machines.
What is it?
The zc.async package provides an easy-to-use Python tool that schedules work persistently and reliably across multiple processes and machines.
- Web apps: maybe your web application lets users request the creation of a large PDF, or some other expensive task.
- Postponed work: maybe you have a job that needs to be done at a certain time, not right now.
- Parallel processing: maybe you have a long-running problem that can be made to complete faster by splitting it up into discrete parts, each performed in parallel, across multiple machines.
- Serial processing: maybe you want to decompose and serialize a job.
High-level features include the following:
- easy to use;
- flexible configuration, changeable dynamically in production;
- supports high availability;
- good debugging tools;
- well-tested; and
- friendly to testing.
While developed as part of the Zope project, zc.async can be used stand-alone.
How does it work?
The system uses the Zope Object Database (ZODB), a transactional, pickle-based Python object database, for communication and coordination among participating processes.
zc.async participants can each run in their own process, or share a process (run in threads) with other code.
The Twisted framework supplies some code (failures and reactor implementations, primarily) and some concepts to the package.
Where can I read more?
Quickstarts and in-depth documentation are available in the package and in the new and exciting on-line documentation.
- Rearrange ftesting.setUp to avoid provoking a DemoStorage bug present in ZODB <= 3.9.3.
- Resolved the following testing issues: signal handlers weren’t cleaned up properly in some tests, Twisted was leaking file descriptors during ftesting tearDown (http://twistedmatrix.com/trac/ticket/3063), and the twisted.txt regression test was not repeatable due to Twisted’s recalcitrance when it comes to stopping and subsequently starting a reactor instance.
- Fix two undefined variables that could trigger exceptions in corner cases.
- Made zc.async.subscribers.ThreadedDispatcherInstaller keep track of signal handlers it installs in a module global “signal_handlers.”
- Made zc.async.ftesting.tearDown restore the signal handlers that were replaced by ThreadedDispatcherInstaller.
- Fix a bug in zc.async.ftesting.setUp and zc.async.testing.print_logs which would result in the default argument for log_file becoming “fixated” with an incorrect value across tests.
- Make the ftesting.txt test exercise the ‘zc.async’ logger in addition to ‘zc.async.event’.
- zc.async.utils.dt_to_long coerces return value to long (test pass on 64-bit Python).
- Tests pass on Python 2.6
- The callable of a zc.async.job.Job (or one of its subclasses) can be a method on the Job itself.
- Fix a bug where zc.async.testing._datetime.now did not accept the same keyword arguments as datetime.datetime, added tests.
- Fix a bug where zc.async.testing._datetime.astimezone did not accept the same keyword arguments as datetime.datetime, added tests.
- Add a performance optimization with isinstance before providedBy.
- Add support for filters to AgentInstaller.
- Fix a bug which caused a condition to be always true in monitor.Encoder.
- Documentation improvements. Converted documentation into Sphinx system.
- Made “other” commit errors for the RetryCommonForever retry policy have an incremental backoff. By default, this starts at 0 seconds, and increments by a second to a maximum of 60 seconds.
- Work around a memory leak in zope.i18nmessageid (https://bugs.launchpad.net/zope3/+bug/257657). The change should be backward-compatible. It also will produce slightly smaller pickles for Jobs, but that was really not a particular goal.
- Added zc.async.partial.Partial for backward compatibility purposes.
- Fix support for Twisted installed reactor.
- Fix retry behavior for parallel and serial jobs
- Tweaked the uuid.txt to mention zdaemon/supervisor rather than Zope 3.
- Fixed some bugs in egg creation.
- Changed quotas to not use a container that has conflict resolution, since these values should be a strict maximum.
- We only want to claim a job if we are activated. Make the agent check the activated and dead attributes of the parent dispatcher before claiming.
- When activating, also clean out jobs from the dispatcher’s agents, just as with deactivating. This should protect from unusual race conditions in which the dispatcher got a job after being deactivated.
- Change dispatcher to ping before claiming jobs.
- when a ping reactivates a dispatcher, use new method reactivate rather than activate. This fires a new DispatcherReactivated event.
- It’s still theoretically possible (for instance, with a badly-behaved long commit that causes a sibling to believe that the process is dead) that an async worker process would be working on a job that it shouldn’t be. For instance, the job has been taken away, and is another process’ responsibility now. Now, whenever a process is about to start any work (especially a retry), it should double-check that the job is registered as being performed by itself. If not, the process should abort the transaction, make an error log, and give up on the job. Write conflict errors on the job should protect us from the edge cases in this story.
- The dispatcher’s getActiveJobs method now actually tells you information about what’s going on in the threads at this instant, rather than what’s going on in the database. The poll’s active jobs keys continues to report what was true in the database as of the last poll. This change also affects the async jobs monitor command.
- The dispatcher method getJobInfo (and the monitor command async job) now returns the name of the queue for the job, the name of the agent for the job, and whether the job has been, or was reassigned.
- zc.async events inherit from ‘zc.component.interfaces.IObjectEvent’ instead of a zc.async specific IObjectEvent (thanks to Satchit Haridas).
- Added new monitoring and introspection tools: the asyncdb zc.monitor command (and, for Python, the code in monitordb.py). This code provides easy spellings to examine the database’s view of what is happening in zc.async. Because it is the database, it also has a much longer historical view than the async tools. The best way to learn about these tools is to read the extensive documentation provided within zc.monitor by using asyncdb help and asyncdb help <TOOL NAME>.
- Added new preferred way of filtering agent choices: the new filter attribute. Using filters, rather than “choosers,” allows several asyncdb tools to filter pending jobs based on what an agent is willing to do. It also is a smaller contract, and so a filter requires less code than a chooser in the common case. On the other hand, using a filter alone doesn’t allow the agent to try to prefer certain tasks.
- Deprecated agent.chooseFirst. It is no longer necesary, since an agent without a chooser and with a filter of None has the same behavior. It is retained for legacy databases.
- Moved deprecated legacy code to new legacy module.
- Tried to be significantly reduce the chance of spurious timing errors in the tests, at the expense of causing the tests to take longer to run.
- monitoring support depends on the new zc.monitor package, which is not Zope specific. This means non-Zope 3 apps can take advantage of the monitoring support. To use, use the [monitor] target; this only adds simplejson, zc.ngi, and zc.monitor to the basic dependencies.
- Make ftesting try to join worker threads, in addition to polling thread, to try to eliminate intermittent test-runner warnings in ftests that a thread is left behind. If the threads do not end, inform the user what jobs are not letting go. (thanks to Patrick Strawderman)
- Fix a bug where zc.async.testing._datetime.now did not accept the same keyword arguments as datetime.datetime, added tests.
- The new serial and parallel helpers did not allow the postprocess argument to be a partial closure, and were being naughty. Fixed.
- Added tests and demos for advanced features of serial and parallel.
- More tweaks to the new Quickstart S5 document.
Mentioned in ftesting.txt that Zope 3 users should uses zope.app.testing 3.4.2 or newer. Also added a summary section at the beginning of that file.
Added logging of critical messages to __stdout__ for ftesting.setUp. This can help discovering problems in callback transactions. This uses a new helper function , print_logs, in zc.async.testing, which is primarily intended to be used for quick and dirty debugging
Changed testing.wait_for_result and testing.wait_for_annotation to ignore ReadConflictErrors, so they can be used more reliably in tests that use MappingStorage, and other storages without MVCC.
Support <type ‘builtin_function_or_method’> for adaptation to Job.
Add warning about long commits to tips and tricks.
After complaining about a polling dispatcher that is deactivated not really being dead in the logs, reactivate.
No longer use intermediate job to implement the success/failure addCallbacks behavior. Introduce an ICallbackProxy that can be used for this kind of behavior instead. This change was driven by two desires.
- Don’t log the intermediate result. It makes logs harder to read with unnecessary duplications of pertinent data hidden within unimportant differences in the log entries.
- Don’t unnecessarily remember errors in success/failure callbacks. This can cause unnecessary failures in unusual situations.
The callback proxy accepts callbacks, which are added to the selected job (success or failure) when the job is selected.
This change introduces some hopefully trivial incompatibilities, which basically come down to the callback being a proxy, not a real job. Use the convenience properties success and failure on the proxy to look at the respective jobs. After the proxy is evaluated, the job attribute will hold the job that was actually run. status and result are conveniences to get the status and result of the selected job.
Add parallel and serial convenience functions to zc.async.job to make it trivial to schedule and process decomposed jobs.
Add start convenience function to zc.async.configure to make it trivial to start up a common-case configuration of a zc.async dispatcher.
No longer use protected attributes of callbacks in resumeCallbacks.
The “local” code is now moved out from the dispatcher module to threadlocal. This is to recognize that the local code is now modified outside of the dispatcher module, as described in the next bullet.
Jobs, when called, are responsible for setting the “local” job value. This means that zc.async.local.getJob() always returns the currently running job, whether it is a top-level job (as before) or a callback (now).
Start on S5 QuickStart presentation (see QUICKSTART_1_VIRTUALENV.txt in package).
- added “Tips and Tricks” and incorporated into the PyPI page.
- added setUp and tearDown hooks to Job class so that code can run before and after the main job’s code. The output of setUp is passed as an argument to tearDown so that one can pass state to the other, if needed. setUp is run immediately before the actual job call. tearDown runs after the transaction is committed, or after it was aborted if there was a failure. A retry requested by a retry policy causes the methods to be run again. A failure in setUp is considered to be a failure in the job, as far as the retryPolicy is concerned (i.e., the job calls the retry policy’s jobError method). If setUp fails, the job is not called, bit tearDown is. tearDown will fail with a critical log message, but then processing will continue.
- using the new setUp and tearDown hooks, added a Zope 3-specific Job subclass (see zc.async.z3.Job) that remembers the zope.app.component site and interaction participants when instantiated. These can be mutated. Then, when the job is run, the setUp sets up the site and a security interaction with the old participants, and then the tearDown tears it all down after the transaction has committed.
- changed retry policy logs to “WARNING” level, from “INFO” level.
- changed many dispatcher errors to “CRITICAL” level from “ERROR” level.
- added “CRITICAL” level logs for “other” commit retries on the RetryCommonForever retry policy.
- added remove method on queue.
- added helpers for setting up and tearing down Zope 3 functional tests (ftesting.py), and a discussion of how to write Zope 3 functional tests with layers (zope.app.testing.functional) in ftesting.txt.
- remove obsolete retry approach for success/failure callbacks (completeStartedJobArguments): it is now handled by retry policies.
- remove odd full-path self-references within the utils module.
- renamed zc.async.utils.try_transaction_five_times to zc.async.utils.try_five_times.
- doc improvements and fixes (thanks to Zvezdan Petkovic and Gintautas Miliauskas).
- the z3 “extra” distutils target now explicitly depends on zope.security, zope.app.security, and zope.app.component. This almost certainly does not increase the practical dependencies of the z3 extras, but it does reflect new direct dependencies of the z3-specific modules in the package.
- made the log for finding an activated agent report the pertinent queue’s oid as an unpacked integer, rather than the packed string blob. Use ZODB.utils.p64 to convert back to an oid that the ZODB will recognize.
- Bugfix: in failing a job, the job thought it was in its old agent, and the fail call failed. This is now tested by the first example in new doctest catastrophes.txt.
- jobs no longer default to a begin_by value of one hour after the begin_after. The default now is no limit.
- Made dispatcher much more robust to transaction errors and ZEO ClientDisconnected errors.
- Jobs now use an IRetryPolicy to decide what to do on failure within a job, within the commit of the result, and if the job is interrupted. This allows support of transactional jobs, transactional jobs that critically must be run to completion, and non-transactional jobs such as communicating with an external service.
- The default retry policy supports retries for ClientDisconnected errors, transaction errors, and interruptions.
- job.txt has been expanded significantly to show error handling and the use of retry policies. New file catastrophes.txt shows handling of other catastrophes, such as interruptions to polling.
- job errors now go in the main zc.async.event log rather than in the zc.async.trace log. Successes continue to go in the trace log.
- callback failures go to the main log as a CRITICAL error, by default.
- handleInterrupt is the new protocol on jobs to inform them that they were active in a dispatcher that is now dead. They either fail or reschedule, depending on the associated IRetryPolicy for the job. If they reschedule, this should either be a datetime or timedelta. The job calls the agent’s reschedule method. If the timedelta is empty or negative, or the datetime is earlier than now, the job is put back in the queue with a new putBack method on the queue. This is intended to be the opposite of claim. Jobs put in the queue with putBack will be pulled out before any others.
- convert to using zope.minmax rather than locally defined Atom.
- Fix (and simplify) last_ping code so as to reduce unnecessarily writing the state of the parent DispatcherAgents collection to the database whenever the atom changed.
- Depends on new release of zc.twist (1.3)
- Switched dispatcher’s in-memory storage of job and poll information to be per job or per poll, respectively, rather than per time period, so as to try and make memory usage more predictable (for instance, whether a dispatcher is whipping through lots of jobs quickly, or doing work more slowly).
- more README tweaks.
- converted all reports from the dispatcher, including the monitor output, to use “unpacked” integer oids. This addresses a problem that simplejson was having in trying to interpret the packed string blobs as unicode, and then making zc.ngi fall over. To get the object, then, you’ll need to use ZODB.utils.p64, like this: connection.get(ZODB.utils.p64(INTEGER_OID)), where INTEGER_OID indicates the integer oid of the object you want to examine.
- added several more tests for the monitor code.
- made the async jobs monitor command be “up to the minute”. Before, it included all of the new and active jobs from the previous poll; now, it also filters out those that have since completed.
- The async job command was broken, as revealed by a new monitor test. Fixed, which also means we need a new version of zope.bforest (1.2) for a new feature there.
Fired events when the IQueues and IQueue objects are installed by the QueueInstaller (thanks to Fred Drake).
Dispatchers make agent threads keep their connections, so each connection’s object cache use is optimized if the agent regularly requests jobs with the same objects.
README improved (thanks to Benji York and Sebastian Ware).
Callbacks are logged at start in the trace log.
All job results (including callbacks) are logged, including verbose tracebacks if the callback generated a failure.
Had the ThreadedDispatcherInstaller subscriber stash the thread on the dispatcher, so you can shut down tests like this:
>>> import zc.async.dispatcher >>> dispatcher = zc.async.dispatcher.get() >>> dispatcher.reactor.callFromThread(dispatcher.reactor.stop) >>> dispatcher.thread.join(3)
Added getQueue to zc.async.local as a convenience (it does what you could already do: zc.async.local.getJob().queue).
Clarified that IQueue.pull is the approved way of removing scheduled jobs from a queue in interfaces and README.
reports in the logs of a job’s success or failure come before callbacks are started.
Added a section showing how the basic_dispatcher_policy.zcml worked, which then pushed the former README_3 examples into README_3b.
Put ZPL everywhere I was supposed to.
Moved a number of helpful testing functions out of footnotes and into zc.async.testing, both so that zc.async tests don’t have to redefine them and client packages can reuse them.
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