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From Rick McGuire <>
Subject Re: [AsyncHttpClient] data collection & instrumentation
Date Wed, 23 Jan 2008 14:50:56 GMT
Sangjin Lee wrote:
> The modified patch is there on JIRA.  Some follow-up discussions...
> I think the current implementation works well, but one thing that's 
> difficult to do is to collecting timing data.  For example, some of 
> the most important instrumentation data are things like average 
> response time (from request start to request complete) and average 
> connect time (from connect start to connect complete).
> Currently the context object that's available to monitoring listeners 
> is the request object, along with the timestamp of the event itself. 
>  To be able to compute a response time for a given request, one would 
> need to take the timestamp from the request start event, associate it 
> with the request, and store it on the listener.  When the request 
> complete event fires, then one would need to look up the stored data 
> using the request object as a key to retrieve the timestamp for the 
> request start event, compute the delta, and store the delta.
> While all this is possible, it has a number of issues, not the least 
> of which is that one would need to maintain a map of request to start 
> time (as well as request to connect time).  This would bloat memory as 
> well as other implications.
> A substantially easier solution would be to provide the request start 
> time and connect start time as part of the information that's passed 
> to the monitoring listener.  Then listeners could simply compute the 
> diff to get the elapsed time very easily with no need to maintain maps 
> of any kind.  This could be either part of the request object itself, 
> or if desirable, one could consider a separate context or event object 
> that contains this information.  What do you think?
> Thanks,
> Sangjin
> On Jan 22, 2008 1:33 PM, Sangjin Lee < 
> <>> wrote:
>     I took a look at the patch on GERONIMO-3761, and it looks great.
>      Thanks.  I have modified your patch for several things, though,
>     and I'm nearly ready to add it to the JIRA report.  Comments about
>     the changes...
>     - I rewrote the EventQueue class to use an Executor.  Since the
>     Executor implementation provided by the JDK is basically a thread
>     pool associated with a task queue, it provides an identical
>     functionality to what was in EventQueue.  I think that it is good
>     to use the constructs from java.util.concurrent.* whenever it
>     makes sense, and I believe this is one of them.
>     - This change also enables us to remove "synchronized" from
>     notifyMonitoringListener().  The notify method will be called very
>     often and concurrently, and reducing the lock contention will be
>     important.  Using an Executor makes it possible to eliminate
>     synchronization, at least at that level.
>     - I associated a shared thread pool (Executor) for all
>     dispatchers.  I think it is desirable for dispatchers to share
>     this thread pool rather than each instance of dispatchers creating
>     and maintaining its own thread.
>     - Renamed EventQueue to EventDispatcher.
>     - I also moved the monitoring listener list to EventDispatcher.  I
>     also used CopyOnWriteArrayList as the implementation for the list.
>      CopyOnWriteArrayList is an ideal choice for this as it is thread
>     safe and lock-free.  Also, our use case is heavy read-access but
>     very infrequent write-access, which CopyOnWriteArrayList is
>     suitable for.
>     - I moved the connection_failed notification to before the
>     getSession() call.  The getSession() call here always throws an
>     exception (by design), and thus notification needs to be done
>     before calling getSession().
>     - I rewrote the CountingMonitor to use AtomicIntegers.  This
>     should be slightly safer.
>     - I changed the timestamp calls from System.currentTimeMillis() to
>     System.nanoTime()/1000000.  The nanoTime() call is more high-res,
>     as currentTimeMillis() may be tens of milliseconds accurate on
>     some platforms, and thus not suitable for these measurements.
>     I also have some more follow-up questions, which I'll post soon.
>     Thanks,
>     Sangjin
>     On Jan 17, 2008 10:51 AM, Sangjin Lee <
>     <>> wrote:
>         I like your idea of using the event listener as the main way
>         of doing this.  Basically no or multiple listeners would be
>         invoked on a different thread when events occur.
>         The event listener APIs would define those key methods which
>         would contain all the necessary information about the events
>         in an immutable fashion.
>         We could provide a simple adapter that is no op so people can
>         override necessary methods easily.  Also, we could provide one
>         implementation which is a counting listener that does the
>         basic metrics collection.
>         What do you think?
Only if it can be done without having to maintain the same sort of 
request-to-start time map that you don't wish to do with the listener.  
The process of adding data collection should cause memory bloat there 
either, particularly if monitoring is not being used (the more likely 
case).  It seems more reasonable that this type of processing should be 
pushed into the monitoring implementation rather than have the async 
client try to keep track of everything.  This way, the overhead is only 
introduced while metrics are being gathered.  A very simple and 
relatively low cost means might be to add a couple of time stamp fields 
to the request object, but only for the most significant events.  
Perhaps request start and connection start, but nothing else.  Another 
possible approach would be to have a mechanism that would allow the 
monitor to attach an annotation object to the request that could be used 
to implement a lifecycle memory if needed.  The cost of doing this is 
relatively minor when this type of information is not needed, but it's 
flexible enough to be tailored to any type of data collection.

>         Thanks,
>         Sangjin
>         On Jan 17, 2008 2:58 AM, Rick McGuire <
>         <>> wrote:
>             Thunderbird is playing very strange games with me this
>             morning, somehow
>             deleting the original post.   Anyway, here are my comments
>             on this.
>             > I'd like to propose changes to enable some basic stat
>             collection
>             > and/or instrumentation to have visibility into
>             performance of AHC.
>             >  For a given *AsyncHttpClient*, one might want to know
>             metrics like
>             >
>             > - total request count
>             > - total success count
>             > - total exception count
>             > - total timeout count
>             > - connection attempt count
>             > - connection failure count
>             > - connect time average
>             > - connection close count
>             > - average response time (as measured from the invocation
>             time to
>             > having the response ready)
>             > - and others?
>             Collection of metric information would, I think, be a good
>             thing.
>             However, I think we should separate the consolidation of
>             the information
>             from the collection.  That is, the client should just have
>             different
>             types of events for data collection, and the event
>             listener would be
>             responsible for presenting the information appropriately.
>             For example, to create the list above, I'd see the
>             following set of
>             events needed:
>             - request made
>             - request completed
>             - request failed
>             - request timeout
>             - connection attempt started
>             - connection failed
>             - connection closed
>             All events would be timestamped, which would allow metrics
>             like "average
>             request time" to be calculated.  This set of events would
>             mean the
>             client would not need to maintain any metric accumulators,
>             and if the
>             event information is done correctly, would even allow more
>             fine grained
>             monitoring (e.g., average connection time for requests to
>             domain
>             " <>").
>             >
>             > Collecting these metrics should have little effect on
>             the overall
>             > performance.  There would be an API to access these stats.
>             >
>             > I was initially thinking of an IoFilter to consolidate
>             these hooks,
>             > but I realize some of these metrics are not readily
>             available to an
>             > IoFilter (e.g. connect-related numbers).  It might be
>             unavoidable to
>             > spread the instrumentation in a couple of places (IoHandler,
>             > ConnectFutureListener, etc.).
>             >
>             > Taking this one step further, one might think of
>             callbacks or
>             > listeners for various key events such as connect
>             complete, request
>             > sent, etc., so callers can provide instrumenting/logging
>             code via
>             > event notification.  However, I think this should be
>             used judiciously
>             > as such injected code may cause havoc.
>             I think listeners would be the way to go.  This would
>             allow multiple
>             monitoring types to be attached to the pipe to gather data
>             as needed.
>             Perhaps the approached used with the javamail API might be
>             of use here.
>             The javamail Store APIs have a number of listener events
>             that are
>             broadcast (new mail arrived, message delete, folder
>             created, etc.).
>             Because there are similar concerns of havoc, the events
>             get posted to a
>             queue, and are dispatched on to a separate thread.  The
>             queue is only
>             created (and the associated thread) are only created when
>             there are
>             listeners available to handle the events.  This allows the
>             events to
>             very low overhead when there are no interested parties and
>             prevents the
>             listeners from interfering with normal javamail operations
>             by being
>             processed on a different thread.
>             >
>             > Thoughts?  Suggestions?

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