From: Alex Rousskov <rousskov@measurement-factory.com>
Date: Fri, 16 Jul 2010 16:20:13 +0000 (-0600)
Subject: Added initial extended documentation for AsyncJob API.
X-Git-Tag: SQUID_3_2_0_1~55
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=3889d00c545dcb05060d081d73ecd2fac6f12fd5;p=thirdparty%2Fsquid.git

Added initial extended documentation for AsyncJob API.

Needs dox syntax check and polishing.
---

diff --git a/src/base/AsyncJobs.dox b/src/base/AsyncJobs.dox
new file mode 100644
index 0000000000..a16dea352b
--- /dev/null
+++ b/src/base/AsyncJobs.dox
@@ -0,0 +1,107 @@
+/**
+\defgroup AsyncJobs Asynchronous Jobs
+\ingroup Components
+ 
+\section Terminology Terminology
+
+- \b Job: an AsyncJob object.
+- \b Creator: the code creating the job. Usually the Initiator.
+- \b Start: the act of calling AsyncStart with a job pointer.
+- \b Initiator: the code starting the job. Usually the Creator.
+
+\section Life Typical life cycle
+
+-# Creator creates and initializes a job.
+-# Initiator starts the job. If Initiator expects
+to communicate with the started job, then it stores the job pointer
+returned by AsyncStart.
+-# The job's start() method is called. The method usually schedules
+some I/O or registers to receive some other callbacks.
+-# The job runs and does what it is supposed to do. This usually involves
+scheduling I/O, setting timeouts, receiving Comm or Store callbacks, and
+then notifying Initiator of the final result.
+-# The job reaches its goal or encounters an error condition.
+-# The swanSong() method is called.
+-# The job object is destroyed.
+
+
+If you want to do something before starting the job, do it in the constructor
+or some custom method that the job creator will call _before_ calling
+AsyncStart():
+
+    std::auto_ptr<MyJob> job(new MyJob(...)); // sync/blocking
+    job->prepare(...); // sync/blocking
+    job->prepareSomethingElse(...); // sync/blocking
+    AsyncStart(job.release()); // non-blocking
+
+If you do not need complex preparations, it is better to do this instead:
+
+    AsyncStart(new MyJob(...));
+
+Keep in mind that you have no async debugging, cleanup, and protections until
+you call AsyncStart with a job pointer.
+
+
+\section Rules Basic rules
+
+- To start a job, use AsyncStart. Do not call start() directly. There
+are examples in the code.
+
+- Never call swanSong() directly. If you are outside an AsyncCall
+handler, and want to kill the job, then call deleteThis(). If you are
+inside an AsyncCall handler, you have several options for job termination:
+
+  -# Call mustStop(reason) for errors that require further processing in
+the same method(s) chain, below/after the mustStop() call. Efficient.
+
+  -# Throw (via Must or directly) for errors that do not require further
+processing in the same method(s) chain, below/after the mustStop() call.
+Inefficient but simple and allows exiting from deeply nested method calls.
+
+  -# Otherwise, just finish the call. Your doneAll() should return true
+and the job will terminate successfully.
+
+swanSong() will be called automatically in all of these cases when the
+job is being terminated. It is a general cleanup method, like a
+destructor. The only difference is that a destructor must not throw.
+
+
+- Do not assume swanSong() is called in some perfectly nice job state.
+The job code or the code it calls may throw at any time after start()
+was called. The entry may be gone, the Abort may have been called, the
+fd may have been closed, etc.
+
+
+- Never call deleteThis() in contexts other than those documented above.
+It is a hack for the old-style code. You can avoid it and other
+old-style special precautions altogether if you convert sync calls into
+async ones. This is especially easy for old-style calls that have only
+one parameter ("data") or two simple parameters.
+
+
+- In swanSong, always call swanSong() of the parent, after you are done
+cleaning up your job. It does not matter whether the [current] parent
+swanSong() does nothing.
+
+
+- If a job does not have a doneAll() method implemented, it is probably
+buggy. Any job must know what it wants to accomplish. Please note that
+doneAll() is for defining the successful termination goal/condition.
+Errors are handled by mustStop() or throw, as discussed above.
+
+Similarly, if your doneAll() implementation looks like "return isDone;",
+you are doing it wrong. Compute the condition directly rather than
+expecting various job methods to maintain some isDone variable correctly.
+
+
+- If a job does Comm I/O, it probably needs a Comm closing handler.
+
+- If a job stores a StoreEntry, it probably needs an entry Abort handler.
+
+- Ask yourself what the user will see/experience when the job throws,
+which could happen as early as in the start() method (technically, it
+can happen even earlier, during job creation and initialization). Are you
+OK with that?
+
+
+*/