cumulus_pallet_xcmp_queue

Branch/Release: release-polkadot-v1.10.0

Source Code

Purpose

Responsible for the Queues (both incoming and outgoing) for XCMP messages. This pallet does not actually receive or send messages. Its responsibility is to place the incoming and outgoing XCMP messages in their respective queues and manage these queues.

Config

Pallet-specific configs
- ChannelInfo — Configures two functions get_channel_status and get_channel_info to provide information about channels (ChannelStatus and ChannelInfo).
- VersionWrapper — Means of converting an Xcm into a VersionedXcm. Xcm’s should be versioned in order to pass the validation.
- XcmpQueue — Defines max length of an XCMP message, and how enqueue_message should behave.
- MaxInboundSuspended — The maximum number of inbound XCMP channels that can be suspended simultaneously. Any further channel suspensions will fail and messages may get dropped without further notice. Choosing a high value (1000) is okay.
- ControllerOrigin — The origin that is allowed to resume or suspend the XCMP queue.
- ControllerOriginConverter — The conversion function used to attempt to convert an XCM MultiLocation origin to a superuser origin. This is used for allowing the superuser’s queue to send messages even to paused queues.
- PriceForSiblingDelivery — The price for delivering an XCM to a sibling parachain destination.
Common configs
- RuntimeEvent — The overarching event type.
- WeightInfo — The weight information of this pallet.

Dispatchables

`suspend_xcm_execution`

pub fn suspend_xcm_execution(origin: OriginFor<T>) -> DispatchResult

Suspends all XCM executions for the XCMP queue.

origin must pass the ControllerOrigin check. NOTE: this does not change the status of the in/out bound channels.

Params:

origin: OriginFor<T> — caller of the dispatchable

Errors:

AlreadySuspended — description of conditions, when this error happens

`resume_xcm_execution`

pub fn resume_xcm_execution(origin: OriginFor<T>) -> DispatchResult

Resumes all XCM executions for the XCMP queue.

origin must pass the ControllerOrigin check. NOTE: this does not change the status of the in/out bound channels.

Params:

origin: OriginFor<T> — caller of the dispatchable

Errors:

AlreadyResumed — description of conditions, when this error happens

`update_suspend_threshold`

pub fn update_suspend_threshold(origin: OriginFor<T>, new: u32) -> DispatchResult

Overwrites the number of pages which must be in the queue for the other side to be told to suspend their sending.

origin must be root.

Params:

origin: OriginFor<T> — caller of the dispatchable
new: u32 — new page threshold for suspend

`update_drop_threshold`

pub fn update_drop_threshold(origin: OriginFor<T>, new: u32) -> DispatchResult

Overwrites the number of pages which must be in the queue after which we drop any further messages from the channel.

origin must be root.

Params:

origin: OriginFor<T> — caller of the dispatchable
new: u32 — new page threshold for drop

`update_resume_threshold`

pub fn update_resume_threshold(origin: OriginFor<T>, new: u32) -> DispatchResult

Overwrites the number of pages which the queue must be reduced to before it signals that message sending may recommence after it has been suspended.

origin must be root.

Params:

origin: OriginFor<T> — caller of the dispatchable
new: u32 — new page threshold for resume

Important Mentions and FAQ’s

messages are not ordered when they are received, but they are ordered when they are sent. Signal messages are prioritized over non-signal messages.

Messages and signals are stored in different queues. When the messages to be sent are taken with take_outbound_messages, they will be ordered:

if there are signals present for outbound messages that targeting a parachain, we will only send signals, not messages
messages (that are not signals) won’t be ordered

polkadot/xcm/src/v3 has SendXcm trait, which has 2 blank methods validate and deliver. For each router struct, one can implement SendXcm for it.

deliver method take destination as a parameter, and calls send_fragment function with the target parachain id.
send_fragment puts the message to the queue of the given parachain id.
- unlike it’s name, deliver method does not actually delivers the message. It is calling send_fragment, which places a message fragment on the outgoing XCMP queue for recipient. So, deliver is only putting the message to the respective outgoing xcmp queue