mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-11-14 13:21:28 +00:00
234 lines
9.5 KiB
Text
234 lines
9.5 KiB
Text
Progress Reporting
|
|
------------------
|
|
|
|
This document describes the design and use cases for the progress reporting
|
|
messages.
|
|
|
|
PROGRESS messages are posted on the bus to inform the application about the
|
|
progress of asynchronous operations in the pipeline. This should not be confused
|
|
with asynchronous state changes.
|
|
|
|
We accommodate for the following requirements:
|
|
|
|
- Application is informed when an async operation starts and completes.
|
|
- It should be possible for the application to generically detect common
|
|
operations and incorporate their progress into the GUI.
|
|
- Applications can cancel pending operations by doing regular state changes.
|
|
- Applications should be able to wait for completion of async operations.
|
|
|
|
We allow for the following scenarios:
|
|
|
|
- Elements want to inform the application about asynchronous DNS lookups and
|
|
pending network requests. This includes starting and completing the lookup.
|
|
- Elements opening devices and resources asynchronously.
|
|
- Applications having more freedom to implement timeout and cancelation of
|
|
operations that currently block the state changes or happen invisibly behind
|
|
the scenes.
|
|
|
|
|
|
Rationale
|
|
~~~~~~~~~
|
|
|
|
The main reason for adding these extra progress notifications is twofold:
|
|
|
|
1) to give the application more information of what is going on
|
|
|
|
When there are well defined progress information codes, applications
|
|
can let the user know about the status of the progress. We anticipate to
|
|
have at least DNS resolving and server connections and requests be well
|
|
defined.
|
|
|
|
2) To make the state changes non-blocking and cancelable.
|
|
|
|
Currently state changes such as going to the READY or PAUSED state often do
|
|
blocking calls such as resolving DNS or connecting to a remote server. These
|
|
operations often block the main thread and are often not cancelable, causing
|
|
application lockups.
|
|
|
|
We would like to make the state change function, instead, start a separate
|
|
thread that performs the blocking operations in a cancelable way. When going
|
|
back to the NULL state, all pending operations would be canceled immediately.
|
|
|
|
For downward state changes, we want to let the application implement its own
|
|
timeout mechanism. For example: when stopping an RTSP stream, the clients
|
|
needs to send a TEARDOWN request to the server. This can however take an
|
|
unlimited amount of time in case of network problems. We want to give the
|
|
application an opportunity to wait (and timeout) for the completion of the
|
|
async operation before setting the element to the final NULL state.
|
|
|
|
Progress updates are very similar to buffering messages in the same way that the
|
|
application can decide to wait for the completion of the buffering process
|
|
before performing the next state change. It might make sense to implement
|
|
buffering with the progress messages in the future.
|
|
|
|
|
|
Async state changes
|
|
~~~~~~~~~~~~~~~~~~~
|
|
|
|
GStreamer currently has a GST_STATE_CHANGE_ASYNC return value to note to the
|
|
application that a state change is happening asynchronously.
|
|
|
|
The main purpose of this return value is to make the pipeline wait for preroll
|
|
and delay a future (upwards) state changes until the sinks are prerolled.
|
|
|
|
In the case of async operations on source, this will automatically force sinks
|
|
to stay async because they will not preroll before the source can produce data.
|
|
|
|
The fact that other asynchronous operations happen behind the scenes is
|
|
irrelevant for the prerolling process so it is not implemented with the ASYNC
|
|
state change return value in order to not complicate the state changes and mix
|
|
concepts.
|
|
|
|
|
|
Use cases
|
|
~~~~~~~~~
|
|
|
|
* RTSP client (but also HTTP, MMS, ...)
|
|
|
|
When the client goes from the READY to the PAUSED state, it opens a socket,
|
|
performs a DNS lookup, retrieves the SDP and negotiates the streams. All these
|
|
operations currently block the state change function for an indefinite amount
|
|
of time and while they are blocking cannot be canceled.
|
|
|
|
Instead, a thread would be started to perform these operations asynchronously
|
|
and the state change would complete with the usual NO_PREROLL return value.
|
|
Before starting the thread a PROGRESS message would be posted to mark the
|
|
start of the async operation.
|
|
|
|
As the DNS lookup completes and the connection is established, PROGRESS
|
|
messages are posted on the bus to inform the application of the progress. When
|
|
something fails, an error is posted and a PROGRESS CANCELED message is posted.
|
|
The application can then stop the pipeline.
|
|
|
|
If there are no errors and the setup of the streams completed successfully, a
|
|
PROGRESS COMPLETED is posted on the bus. The thread then goes to sleep and the
|
|
asynchronous operation completed.
|
|
|
|
The RTSP protocol requires to send a TEARDOWN request to the server
|
|
before closing the connection and destroying the socket. A state change to the
|
|
READY state will issue the TEARDOWN request in the background and notify the
|
|
application of this pending request with a PROGRESS message.
|
|
|
|
The application might want to only go to the NULL state after it got confirmation
|
|
that the TEARDOWN request completed or it might choose to go to NULL after a
|
|
timeout. It might also be possible that the application just want to close the
|
|
socket as fast as possible without waiting for completion of the TEARDOWN request.
|
|
|
|
* Network performance measuring
|
|
|
|
DNS lookup and connection times can be measured by calculating the elapsed
|
|
time between the various PROGRESS messages.
|
|
|
|
|
|
|
|
Messages
|
|
~~~~~~~~
|
|
|
|
A new PROGRESS message will be created.
|
|
The following fields will be contained in the message:
|
|
|
|
- "type", GST_TYPE_PROGRESS_TYPE
|
|
|
|
- a set of types to define the type of progress
|
|
|
|
GST_PROGRESS_TYPE_START: A new task is started in the background
|
|
GST_PROGRESS_TYPE_CONTINUE: The previous tasks completed and a new
|
|
one continues. This is done so that the application can follow
|
|
a set of continuous tasks and react to COMPLETE only when the
|
|
element completely finished.
|
|
GST_PROGRESS_TYPE_CANCELED: A task is canceled by the user.
|
|
GST_PROGRESS_TYPE_ERROR: A task stopped because of an error. In case of
|
|
an error, an error message will have been posted before.
|
|
GST_PROGRESS_TYPE_COMPLETE: A task completed successfully.
|
|
|
|
- "code", G_TYPE_STRING
|
|
|
|
A generic extensible string that can be used to programatically determine the
|
|
action that is in progress. Some standard predefined codes will be
|
|
defined.
|
|
|
|
- "text", G_TYPE_STRING
|
|
|
|
A user visible string detailing the action.
|
|
|
|
- "percent", G_TYPE_INT between 0 and 100
|
|
|
|
Progress of the action as a percentage, the following values are allowed:
|
|
- GST_PROGRESS_TYPE_START always has a 0% value.
|
|
- GST_PROGRESS_TYPE_CONTINUE have a value between 0 and 100
|
|
- GST_PROGRESS_TYPE_CANCELED, GST_PROGRESS_TYPE_ERROR and
|
|
GST_PROGRESS_TYPE_COMPLETE always have a 100% value.
|
|
|
|
- "timeout", G_TYPE_INT in milliseconds
|
|
|
|
The timeout of the async operation. -1 if unknown/unlimited..
|
|
This field can be interesting to the application when it wants to display
|
|
some sort of progress indication.
|
|
|
|
- ....
|
|
|
|
Depending on the code, more fields can be put here.
|
|
|
|
|
|
Implementation
|
|
~~~~~~~~~~~~~~
|
|
|
|
Elements should not do blocking operations from the state change function.
|
|
Instead, elements should post an appropriate progress message with the right
|
|
code and of type GST_PROGRESS_TYPE_START and then start a thread to perform
|
|
the blocking calls in a cancelable manner.
|
|
|
|
It is highly recommended to only start async operations from the READY to PAUSED
|
|
state and onwards and not from the NULL to READY state. The reason for this is
|
|
that streaming threads are usually started in the READY to PAUSED state and that
|
|
the current NULL to READY state change is used to perform a blocking check for
|
|
the presence of devices.
|
|
|
|
The progress message needs to be posted from the state change function so that
|
|
the application can immediately take appropriate action after setting the state.
|
|
|
|
The threads will usually perform many blocking calls with different codes
|
|
in a row, a client might first do a DNS query and then continue with
|
|
establishing a connection to the server. For this purpose the
|
|
GST_PROGRESS_TYPE_CONTINUE must be used.
|
|
|
|
Usually, the thread used to perform the blocking operations can be used to
|
|
implement the streaming threads when needed.
|
|
|
|
Upon downward state changes, operations that are busy in the thread are canceled
|
|
and GST_PROGRESS_TYPE_CANCELED is posted.
|
|
|
|
The application can know about pending tasks because they received the
|
|
GST_PROGRESS_TYPE_START messages that didn't complete with a
|
|
GST_PROGRESS_TYPE_COMPLETE message, got canceled with a
|
|
GST_PROGRESS_TYPE_CANCELED or errored with GST_PROGRESS_TYPE_ERROR.
|
|
Applications should be able to choose if they wait for the pending
|
|
operation or cancel them.
|
|
|
|
If an async operation fails, an error message is posted first before the
|
|
GST_PROGRESS_TYPE_ERROR progress message.
|
|
|
|
|
|
Categories
|
|
~~~~~~~~~~
|
|
|
|
We want to propose some standard codes here:
|
|
|
|
"open" : A resource is being opened
|
|
"close" : A resource is being closed
|
|
|
|
"name-lookup" : A DNS lookup.
|
|
|
|
"connect" : A socket connection is established
|
|
|
|
"disconnect" : a socket connection is closed
|
|
|
|
"request" : A request is sent to a server and we are waiting for a
|
|
reply. This message is posted right before the request is sent
|
|
and completed when the reply has arrived completely.
|
|
|
|
"mount" : A volume is being mounted
|
|
|
|
"unmount" : A volume is being unmounted
|
|
|
|
More codes can be posted by elements and can be made official later.
|