substrate - 基板中的事件存储成本是多少？

Question

在实现我的链逻辑时，我想知道是否要使用事件，因为它们可能会花费节点额外的存储来存储事件日志。这里涉及的实际存储成本是多少？日志会在某个时候自动清除吗？

Answer 1

运行时事件由 System module 处理。 .在您自己的模块中，您通常实现默认的 deposit_event功能:

从代码内文档:

deposit_event: Helper function for depositing an event. The default behavior is to call deposit_event from the System module. However, you can write your own implementation for events in your runtime. To use the default behavior, add fn deposit_event<T>() = default; to your Module.

如果您查看 System module code ，你会发现最终调用了一个辅助函数来存储事件:

/// Deposits an event into this block's event record adding this event
/// to the corresponding topic indexes.
///
/// This will update storage entries that correspond to the specified topics.
/// It is expected that light-clients could subscribe to this topics.

pub fn deposit_event_indexed(topics: &[T::Hash], event: T::Event) { ... }

该函数修改了三个存储项，您可以在 decl_storage 中找到它们。对于系统模块:

/// Events deposited for the current block.
Events get(events): Vec<EventRecord<T::Event, T::Hash>>;

/// The number of events in the `Events<T>` list.
EventCount get(event_count): EventIndex;

/// Mapping between a topic (represented by T::Hash) and a vector of indexes
/// of events in the `<Events<T>>` list.
EventTopics get(event_topics): double_map hasher(blake2_256) (), blake2_256(T::Hash)
            => Vec<(T::BlockNumber, EventIndex)>;

事件故事的最后部分可以在 initialize 中找到。 System 模块中的函数，其中所有这三个项目都被“清理”:

pub fn initialize( ... ) {
    ...
    <Events<T>>::kill();
    EventCount::kill();
    <EventTopics<T>>::remove_prefix(&());
}

这个 initialize函数在 Executive module 中调用在每个 block 的开头， on_initialize 之前为任何模块调用:

fn initialize_block_impl(
    block_number: &System::BlockNumber,
    parent_hash: &System::Hash,
    extrinsics_root: &System::Hash,
    digest: &Digest<System::Hash>,
) {
    <system::Module<System>>::initialize(block_number, parent_hash, extrinsics_root, digest);
    <AllModules as OnInitialize<System::BlockNumber>>::on_initialize(*block_number);
}

总之，在运行时添加单个 Event 的成本是:

运行 deposit_event_indexed功能。

将新项目添加到运行时存储中的两个向量。

...它们在下一个区 block 开始时被清理，因此存储成本不会增加。