self cancelling watcher

[mckenziec]

• etcd3-py version: latest
• Python version: 2.7.5
• Operating System: CENTOS 7.5

Description

Maybe this is a Python thing I can work out, but I'm trying to have a watcher that'll self cancel within an event callback. Like watch once, or watch until some condition is satisfied, then cancel itself.

I've designed a kind of request/approval protocol on top of etcd and I'd like the watch to end when my approval watch fires. Any suggestions?

Excellent, stable library BTW. Thanks!

What I Did

            def watch_apr(ev):
                k = ev.key
                v = ev.value
                # cancel here somehow

            apr_watcher.onEvent(EventType.PUT, watch_apr)
            apr_watcher.runDaemon()
            while apr_watcher <> None:
                time.sleep(1)

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[Revolution1]

you can use watcher.watch_once
https://etcd3-py.readthedocs.io/en/latest/etcd3.stateful.html#etcd3.stateful.watch.Watcher.watch_once

[mckenziec]

I knew that! Thanks Revolution1.

[mckenziec]

Actually I played with watch_once() and it blocks the main thread (fine) but doesn't release after the callback executes. Maybe I'm using it wrong:

            apr_watcher = cli.Watcher(key=apath, progress_notify=True, prev_kv=False, prefix=True)

            def watch_apr(ev):
                if ev.type == EventType.PUT:
                    k = ev.key
                    v = ev.value
                    # do stuff...
                    print "watch complete!"
            print "watching once..."
            apr_watcher.watch_once(watch_apr)
            print "watching twice?"

During execution I'll see:

watching once...
watch complete!

And it won't continue. Any suggestions? Thanks.

[mckenziec]

Ah got it. I have to return the event in my once run callback:

            apr_watcher = cli.Watcher(key=apath, progress_notify=True, prev_kv=False, prefix=True)

            def watch_apr(ev):
                if ev.type == EventType.PUT:
                    k = ev.key
                    v = ev.value
                    # do stuff...
                    print "watch complete!"
                    return ev

            print "watching once..."
            apr_watcher.watch_once(watch_apr)
            print "watching twice?"

[Revolution1]

uh... no
watch_once accepts a filter, not callback
A filter only tells the dispatcher whether to keep this event or just drop it.
A filter can be either a EventType, a Regex pattern string or a function that returns True if event match what you need.

So you are using the filter as a callback.
If you don't return a True value, the dispather treats the event as a uneeded one, and keep on waiting for another valid event.

the correct way to use is:

w = Watcher(...)
e = w.watch_once() # default None accept all kinds of event
# or
e = w.watch_once('.*123') # regex filter the event of which key ends with '123'

The you can use the returned Event e

[mckenziec]

Ok, that makes sense. I was confused by the onEvent() taking a filter_or_cb and also cb, and that the constructor for Watcher includes the key'ing, but of course I can further filter for something like EventType. Great, I can remove the callback, which was a pain to debug as it runs in another thread. Anyways, thanks for the follow up!

[Revolution1]

The purpose of filter_or_cb is:

In these use case:

1. watch only one key or prefix

w = Watcher(key="key or prefix you want to watch")
w.onEvent(lambda event: do sth...) # the first arg is a callback

2. watch only one multiple key or prefix

w = Watcher(key="wider key or prefix that covers your need")
w.onEvent("filter of key or prefix 1", lambda event: do sth...) # the first arg is a filter
w.onEvent("filter of key or prefix 2", lambda event: do sth...) # the first arg is a filter

thus, only one watcher is created, only one stream open

In conclusion, the first argument or onEvent can be either a filter or callback, so it's named filter_or_cb