Missing API for GATT Write w/o Response

[martijnthe]

There does not seem to be a way to specify that the GATT Write w/o Response operation should be used when calling writeValue().

I don't think this should be "abstracted away" / left up to the UA implementation because this is the only GATT operation by which GATT clients can have more than one GATT packets in flight towards the GATT server. The other write operations have to be ack'd before the next write can be sent out, making it very slow.

Most applications that I've seen that need to optimize for throughput (Pebble smartwatch, Parrot's drones, ...), usually use GATT Write w/o Response in combination with GATT Value Notifications for data in the other direction.

Also related: #26

[jyasskin]

Do you have devices that allow both Write with Response and Write w/o Response on the same characteristic?

What behavior do you want when the requested write type isn't among the characteristic's properties?

I suspect the API shape will be writeValue(value, {type: 'without-response'})

[martijnthe]

Do you have devices that allow both Write with Response and Write w/o Response on the same characteristic?

Is possible and allowed by the BT spec.
I'm sure someone will come up with a good use case for having both enabled for the same characteristic.

What behavior do you want when the requested write type isn't among the characteristic's properties?

I'd say if with-response was explicitly specified, error out the way you would do normally when an operation is attempted that isn't allowed. If it's not explicitly specified, fallback to write with response?

I suspect the API shape will be writeValue(value, {type: 'without-response'})

Sounds good to me!

[360fun]

Hi, I think I may got stuck in this issue: basically I'm trying to cache the GATT server connection, but when I write some data it executes the command and it disconnects right after! So the second time that I try to write a command using the cached service variable, I get the error "Uncaught (in promise) DOMException: GATT Service no longer exists.".
The trick seems to re-connect every time the GATT server, but this adds latency and this is a problem in applications like drive drones, cars, robots...since they need a real time feedback! :(

I started to guess why this happens and then I read that the kind of command in this case, for my device, is "without response", so, guessing, when the API doesn't get anything after the promise, it disconnects the GATT server. If this is right...is there a workaround or we need to wait an update in the API? :\

[g-ortuno]

It would be great if you could open a Chromium issue with your problem. Here are the instructions:

https://www.chromium.org/developers/how-tos/file-web-bluetooth-bugs

[beaufortfrancois]

For info, @360fun also reported this issue at poshaughnessy/web-bluetooth-parrot-drone#1 (comment)

[360fun]

Yes I did :) I'm kind of lost...but now I've a new way to investigate the issue, thanks! :D I never worked with Bluetooth before, so is good to learn! ;)

[beaufortfrancois]

We'll continue conversation at poshaughnessy/web-bluetooth-parrot-drone#1 (comment) to not pollute this thread and come back if it's a Web Bluetooth API issue.

[Emill]

I'm also in favor for adding this option.

From the Bluetooth spec:

This sub-procedure is used to write a Characteristic Value to a server when the client knows the Characteristic Value Handle and the client does not need an acknowledgement that the write was successfully performed.

It basically informs that Write Without Response should be chosen when the client does not need an acknowledgement. In this case I guess the "client" should refer to the application (and not the stack or browser) since the application is the only one who knows if an acknowledgement is really needed or not. The Bluetooth spec does not state that Write Without Response can only be used if no other write sub-procedures are allowed according to the characteristic's permissions.

If the client wants to write many values and it ignores Write Responses, and the browser by itself chooses Write With Response, it results in wasteful significant delays.

[360fun]

In the end I found that the problem was in my device: a watchdog is killing the connection after 500ms, so I just have to use a loop to keep alive the connection! :) But was nice to discover this thing anyway and know a bit more about Bluetooth. :D

[Vudentz]

If the client wants to write many values and it ignores Write Responses, and the browser by itself chooses Write With Response, it results in wasteful significant delays.

It goes both ways, if the remote device does not support this mode for some reason then the values would be ignored so at least it shouldn't be the default behavior.

[beaufortfrancois]

Just to be clear, this option would only be useful if one single bluetooth characteristic supports Write w Response AND Write w/o Response at the same time. In this case, UA may choose the bad one by default I agree.
If not, UA will always choose the right option, the one that is specified by the writable bluetooth characteristic's properties.

@beaufortfrancois I have a device with a characteristic which supports both write and write without response. The device is the BBC micro:bit. It has an event bus which is exposed over Bluetooth via input and output event characteristics. For the input event, you can use either write or write without response. The point is that events are a generalised mechanism for communicating with the micro:bit and the decision as to whether acknowledged or unacknowledged writes are required has to be taken on an application by application basis. We definitely need this.

Some of the standard SIG defined Bluetooth services allow characteristics which support both variants too btw. For example:

Automation IO:
https://www.bluetooth.com/specifications/gatt/viewer?attributeXmlFile=org.bluetooth.service.automation_io.xml

Human Interface Device (HID):

https://www.bluetooth.com/specifications/gatt/viewer?attributeXmlFile=org.bluetooth.service.human_interface_device.xml

[timmyhadwen]

Hi All,

Also need this for Silicon Labs Bluetooth over the air updates which have both Write and Write without response. Any updates on whether this can be added in the near future?

Cheers
Tim