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Background

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1. Database triggers - Oracle example
2. Substrate weight and fee handling use cases in `pre-dispatch` and `post-dispatch` stages of transaction processing
3. Smart contracts in other blockchains
4. Event driven architecture

Problem

Currently we can submit transactions that can modify global state if they pass validation. These transactions can contain `Query`s inside of them, allowing them limited capability to execute logic on-chain.
Transactions can include:

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Other use cases for permissioned blockchains might include (these use cases are proposed to be handled outside of the context of triggers, see `Outside of Triggers Scope` section):

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Before discussing Triggers in detail it is important to consider Events, which might play a big role in Trigger execution.

Events can originate from 2 places:

1. Triggers can decide to emit events
2. Events will be automatically emitted for WSV changes
   1. The events are based on WSV diff after particular transaction execution.
   2. Examples: AccountCreated(_), AcccountMetadataFieldSet(_), AccountBalanceChanged(_)  etc.

Events emitted during execution of entities in the current block are added to the block. Though Triggers can react to them only in the next block.

Representation

As you can see in the quote from the whitepaper the Triggers were initially planned to be represented as ISI.
But with the recent decision to introduce WASM, it is suggested to use it for Triggers. So that Triggers can be written in Rust* and compiled into WASM.
This way Triggers will benefit from a high level language features of an already established language and will be able to solve all the listed use cases with ease.
As discussed previously, solving listed use cases with ISI was impossible or very difficult and needed significant work on the language design side.

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4 - If block got enough votes - all peers execute block transactions and triggers in their order at block commit.

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Other Triggers might just influence other transactions in a way that they are executed before them, but do not directly do anything to other transactions.

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• System Level - Influence the whole blockchain system rules and features
  Can fail transactions
  • Do not pay fees for execution*
  • Permissions are not checked for them (can freely modify WSV state)
  • In comparison with Substrate based chains: Similar to runtime in Substrate
• User Level - Provide services to users and other apps
  • Pay fees for execution*
  • Act on behalf of their technical accounts and their permissions
  • In comparison with Substrate based chains: Similar to smart contracts in Substrate

*- for more on this see Execution Limits

Also Registration process is different for them.

By wake up condition:

• Time-based - (System Level and User Level )
  
  • At timestamp
  • Each `duration`
  • For complex use cases Trigger might register itself again to execute at another timestamp
  • We should limit the granularity of possible timestamps, as network has a limit of how fast it can produce and commit blocks
  • Time based Triggers seem to be difficult task for synchronization between peers. The following approach is suggested:

    • Recommendations to execute them should be added by the leader to the block (even if there are no transactions - leader should generate a block with trigger execution recommendations for them)

    • They are best effort - Iroha does not guarantee them to be executed at exact time. But it guarantees execution at closest possible time if network is operational.

    • If a leader does not produce a block with appropriate time based triggers in the defined time period after those trigger's selected timestamps - view change happens - similar to how if leader does not produce block for ordinary transactions

    • Next leader in this case will need to include the previous time based trigger in a block (when a view change happens it will be given a new time window to do this)

  • Example: Periodic rewards, scheduled batch processing
• Block number-based  - (System Level and User Level )
  
  • At block
  • Each `n_blocks`
  • For complex use cases Trigger might register itself again to execute at another height
• Transaction-based
  
  • Before (transaction execution)
    
    • Only for System Level Triggers
    • Have the ability to check and fail or allow transaction
  • After (transaction execution)
    • After each
    • After transaction that triggered a change in WSV that meets arbitrary condition - this is the most general option
    • In both cases they have access to transaction contents and at least read access to WSV
    • Have the ability to check and fail or allow transaction - if System Level
• • Might not be needed as there are already time based triggers, and block time is not fixed in Iroha v2.
  • Example: Per block rewards, scheduled batch processing
    
• Triggers that are triggered by specific ISI call - ExecuteTrigger(Params) - (System Level and User Level )
  
  • Similar to smart contracts as they are in Ethereum blockchain
  • Execute as part of the transaction instructions execution
  • For System Level - only admin can call the Trigger or through democracy pallet
  • Example: Swap with liquidity pool, locking funds (for bridges or stable coins for example)
• Event-based - (System Level and User Level )
  
  • Triggered by events emitted from other triggers or transaction execution.
  • The triggers react to the events emitted in the previous block as shown in the `Execution` section.
  • If leader does not supply the needed trigger execution recommendations based on events from previous block -
    the block will be rejected and the next leader will have to do this.
  • Leader has to produce a block with event based triggers in time, even if it will contain no transactions.
    Or the view change will happen and the scenario proceeds similar to time based triggers.
  • Example: Reacting to currency flow, reacting to system events (events from system level triggers), keeping metadata in sync

Persistent State

Complex Triggers (such as Swap triggers managing Liquidity Pools) might need to store data between their invocations. For this purpose Triggers will have following persistent storage:

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ExecuteTrigger ISI might be designed to return some value, therefore introducing a class of `Library Triggers` that are used by other Triggers for various features.

Triggers can also communicate by defining and emitting events at their execution.

Execution Limits

Trigger execution uses validators' processing resources and therefore has to be limited for the network to stay operational.
Limits would mainly apply to User Level triggers as they are registered freely and are not part of the system itself.

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It might be good to provide Triggers with a way to `Unregister` themselves. Especially for public blockchain use cases.

Implementation Considerations

Outside of Triggers Scope

The following topics are proposed not to be handled with Triggers but instead considered separately. Both of the topics need a separate RFC, but they are mentioned here in the context of their relation to Triggers.

In Triggers we wanted to both of these as part of Transaction Based Triggers (After/Before Transaction) - this proposal effectively removes this type of Triggers from design.

Instruction and Query Permissions

We considered moving our permission check into Triggers (for them to be runtime upgradeable), but this does not seem like a good idea due to the following reason.

If Triggers are used to check instructions' permissions during transaction execution, then how should Trigger permissions be checked.
If it is decided to:

1. Handle transaction permission checks and trigger permission checks separately - this will create inconsistency.
2. Use triggers to check execution of other triggers - this introduces a lot of complexity into the design.

Instead it is proposed to have Permissions as WASM plugin which is not a Trigger, but a separate part of the system. This will allow us to upgrade them at runtime, without the complexity that might be there with Triggers.

It is possible to either migrate current permission system simply to WASM and provide instructions to update them, or we can work on diff based permissioning - which will check WSV diffs after instruction/transaction/trigger execution.

Network Fees

We also considered using Triggers for handling network fees (transaction and trigger fees), but again there is a following issue.

If transaction based Triggers are used to calculate transaction fees, how should we calculate Trigger execution fees.
Here again there might arise either consistency or complexity problems.

Instead it is proposed to have either a highly configurable system where fee configuration can be done at runtime, or again do it through a separate WASM plugin (if fee handling logic might be changed at runtimeThe scope of work proposed here is huge. It might be good to approach it iteratively, focusing on use cases needed for permission-ed blockchains at first (e.g. start with System Level triggers that check transactions).

Additional Information

• Set of OOB ISIs
• https://github.com/hyperledger/iroha/blob/iroha2-dev/docs/source/iroha_2_whitepaper.md#251-event-listeners
• Permissions
• Passive triggers - Re: Triggers