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Thibault Saunier 2fdd87e282 Import all GStreamer design doc and convert them to markdown

https://bugzilla.gnome.org/show_bug.cgi?id=775667

2016-12-06 14:01:11 -03:00

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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:

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.

To make the state changes non-blocking and cancellable.

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 cancellable, causing application lockups.

We would like to make the state change function, instead, start a separate thread that performs the blocking operations in a cancellable 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 programmatically 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 cancellable 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.

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