问题
I've heard that you can lose information if you use the Post
method instead of the SendAsync
method of an ActionBlock<T>
object, when you decide to utilize it's BoundedCapacity
property.
Could someone please explain why that is so?
回答1:
The Post method attempts to post an item synchronously and returns true
or false
, depending on whether the block accepted the item or not. Reasons to not accept an item:
- The block is marked as completed (by calling its
Complete
method). - The block is completed, either successfully or unsuccessfully (its Completion.IsCompleted property returns
true
). - The block has a bounded capacity (option BoundedCapacity != -1), and its buffer is currently full.
The SendAsync method attempts to post an item asynchronously and returns a Task<bool>
. This task will always be completed, unless the block has a bounded capacity, its buffer is currently full, and it's not currently completed or marked as completed. This is the only case that the SendAsync
will behave asynchronously. After awaiting the task, the bool
result of the task indicates whether the block accepted the item or not. Reasons to not accept an item:
- The block was marked as completed either before calling the
SendAsync
, or during the awaiting. - The block was completed either before calling the
SendAsync
, or during the awaiting as a result of an exception, or because its Fault method was invoked.
So the difference between Post
and SendAsync
is the point (3). They behave differently in the case of a bounded-capacity block with a full buffer. In this case the Post
rejects immediately the item, while the SendAsync
will asynchronously accept it when the buffer has free space again.
In most cases the behavior of SendAsync
is the desirable one. Using the Post
instead of the SendAsync
can be seen as a bug that is waiting to happen, when some time later the block is reconfigured as bounded, to solve newly discovered problems related with excessive memory usage.
It is a good idea to not dismiss the return value of both methods, because a return value of false
indicates in most cases a bug. it is quite rare to expect and be ready to handle a false
result. Some ideas:
if (!block.Post(item)) throw new InvalidOperationException();
if (!await block.SendAsync(item)) throw new InvalidOperationException();
var accepted = block.Post(item); Debug.Assert(accepted);
var accepted = await block.SendAsync(item); Debug.Assert(accepted);
回答2:
Yes you can lose information, Post
has higher potential to do that but SendAsync
also can lose information. Let's say you have an ActionBlock that is taking 1000 ms to complete, during this time period 10 messages are posted. BoundedCapacity
is set to 5 for the ActionBlock
. As a result, the last 5 message is not processed, information lost.
Here are some detail about it: TPL Dataflow, whats the functional difference between Post() and SendAsync()?
See second answer.
来源:https://stackoverflow.com/questions/62016856/avoiding-use-of-actionblocktinput-post-when-postdataflowblockoptions-boundedca