Background
Whitepaper gave some information about Triggers and initial requirements about this functionality, but a lot of open questions comes into play when implementation details were discussed.
The design of Iroha Triggers mainly takes inspiration from:
- Database triggers - Oracle example
- Substrate weight and fee handling use cases in `pre-dispatch` and `post-dispatch` stages of transaction processing
Problem
Iroha Special Instructions can not provide the fully fledged toolbox for all possible use cases because they lack an ability to react on changes in Iroha state. Iroha Triggers provide this functionality and in addition to Iroha Special Instructions give users full-scale functionality for almost any business scenario.
Triggers execution, in contrast to transaction execution, depends on condition.
Triggers can be either based on time or on a confirmed transaction (event). This is very powerful and enables Turing complete computation for Iroha's smart contracts.
- Trigger at timestamp (every 5 minutes or after one hour)
- Trigger at blockchain (new block, reach of some height, etc.)
- Trigger at condition (new account, transfer of an asset, etc.)
Also, for cases like Iroha Permissions Module we will need to be able to manage Triggers execution order - for example to execute them before block/transaction/instruction or after.
Order Book case
Users send bid and ask limit orders into the order book. Every order in the order book should be executed when counter order with the same price will be received. Speaking other word it should happen when bid meets ask at the same price. In this case limit order should be notified that there is a matching order and swap should be executed with resulting funds sent to the owners of both orders. Looks like this notification is reasonable to implement using triggers.
- Challenging here is that there might be really a lot of orders in the order book and every order should have a trigger and every trigger should be checked somehow if price counter order exists or not.
- Another challenge is that after one order executed using a certain amount of funds of counter order, other orders having the same price cannot be executed on the same amount of the counter order funds. Other orders even for the same price should wait for new free fund amounts of counter orders.
Impure Triggers and Instructions
Because impure (I/O to 3rd party systems) Instructions and Triggers will not be supported by Iroha 2 - Iroha Events can be used to receive information by Cloud Events Consumers.
Solution
Peer can be a place to store Iroha Triggers. We can register new Triggers by Register<Peer, Trigger> Iroha Special Instruction.
Because we depend on triggers a lot, we will have tight coupling between transactions processing and their execution. Every Iroha Special Instruction that changes Iroha state (domain related instructions category from Set of OOB ISIs) can Trigger, so we need to check Iroha Triggers that depend on conditions every instruction, Iroha Triggers that depend on blockchain every block and Iroha Triggers that depend on timestamp every block also.
It is suggested to:
- Store Triggers under Peer entity
- Have the following Triggers categories:
- Time-based
- Block number-based
- Transaction-based (can have a condition of whether they need to execute, similar to Oracle `when` condition)
- Triggers that are triggered by specific ISI call - ExecuteTrigger(Params)
- Have an ability to configure trigger order for Transaction Based triggers:
- Before (transaction execution) - have the ability to check and fail or allow transaction
- After (transaction execution)
- Triggers will be build on top of Iroha Special Instructions and WASM
- For instruction-based Triggers condition will be of `Instruction::Execute(Query)` type
- All changes are declared as a set of Iroha Special Instructions or WASM
Decisions
Alternatives
- Store Triggers as Assets - to much complexity and runtime dependent code
- Execute Triggers sending new set of transactions - require networking communications, can end up in missing triggers
Concerns
- Some complexity for end users
- Open question about parameters propagation (condition → execution)
Assumptions
- We do not have impurity inside Iroha and all Triggers can be replayed on different peers resulting in the same state of each consensus participant
- For triggers we assume that time on different peers in some sync state with delays less or equal to block build time
Risks
- Users will not be able to use Iroha Triggers in their business scenarios `[2; 8]`