In the traditional world, applications often use logs to capture and describe what’s going on at a specific moment. These logs are often used to
debug applications, detect specific events
notify the viewer of the logs that something happened
interacting with smart contracts!
So how does Ethereum do it?
The EVM currently has 5 opcodes （创建log record）for emitting event logs: LOG0, LOG1, LOG2, LOG3, and LOG4.
A log record can be used to describe an event within a smart contract, like a token transfer or a change of ownership.
Each log record consists of both topics and data.
Topic：32-byte (256 bit) “words” that are used to describe what’s going on in an event. LOG**x：**x代表可以最多包含的topic数量
Data：携带data比topic应廉价。没有size限制。所以可以携带大量结构复杂的数据
The first part of a log record consists of an array of topics. These topics are used to describe the event. The first topic usually consists of the signature (a keccak256 hash) of the name of the event that occurred, including the types (uint256, string, etc.) of its parameters. One exception where this signature is not included as the first topic is when emitting anonymous events. Since topics can only hold a maximum of 32 bytes of data, things like arrays or strings cannot be used as topics reliably. Instead, it should be included as data in the log record, not as a topic. If you were to try including a topic that’s larger than 32 bytes, the topic will be hashed instead. As a result, this hash can only be reversed if you know the original input. In conclusion, topics should only reliably be used for data that strongly narrows down search queries (like addresses). In conclusion, topics can be seen as indexed keys of the event that all map to the same value, which we will talk about next.
The following contract implements the Transfer event, used by ERC20-compliant token contracts:
Since this is not an anonymous event, the first topic will consist of the event signature:
Now, let’s take a look at the arguments (from, to, value) of this Solidity event:
Since the first 2 arguments are declared as indexed, they are treated like additional topics. Our final argument will not be indexed, which means it will be attached as data (instead of a separate topic). This means we are able to search for things like “*find all Transfer logs from address 0x0000… to address *0x0000…” or even “find all logs to address 0x0000…”, but not for things like “find all Transfer logs with value x”. We know this event will have 3 topics, which means this logging operation will use the LOG3 opcode.
Now, we just need to understand how data (like our final argument) can be included. LOG3 requires 5 arguments:
LOG3(memoryStart, memoryLength, topic1, topic2, topic3)
Event data is read from memory in the following fashion:
memory[memoryStart...(memoryStart + memoryLength)]
Luckily, higher-level smart contract programming languages like Solidity, Vyper, or Bamboo will handle writing event data to memory for us, which means you can usually pass data directly as a parameter when emitting logs.
Using web3, a popular JavaScript library used to interact with a local or remote Ethereum node, we are able to subscribe to new event logs:
This code will alert us whenever a new SAI token transfer has occurred, which can be useful for various applications. For example, a wallet interface could alert you whenever you receive tokens on your Ethereum address.
The base cost of logging operations is 375 gas. On top of that, every included topic costs an additional 375 gas. Finally, each byte of data costs 8 gas.
That’s actually pretty cheap! Let’s imagine an ERC-20 token transfer. First, we have a base cost of 375 gas. Second, the Transfer event contains 3 topics, which is an additional 375 * 3 = 1125 gas. Finally, we add 8 gas for each byte of data included. Since the data only contains the value of the ERC-20 transfer, which can be a maximum of 32 bytes (²²⁵⁶), the maximum amount of gas needed to account for the data of the logging operation would be 8 * 32 = 256 gas. This adds up to a total gas cost of 1756 gas. For reference, a standard ether (non-token) transfer costs 21000 gas, or more than 10x that!
If we assume a gas price of 1 gwei, the total cost of the operation would be 1756 gwei, the equivalent of 0.000001756 ETH. If the current price of ETH is around $200, this would only add up to $.0003512. Keep in mind that this is the price of storing data on the blockchain worldwide, likely forever.
Disclaimer: This is just the cost of a logging operation itself. Any Ethereum transaction starts at 21000 gas, and the transaction’s input data costs up to 16 gas per byte. Typically, to transfer & log an ERC-20 token, it costs between 40,000–60,000 gas.
Logs 是一个比较好的办法长期的存储一些数据在blockchain上，同时价格低廉。同时也是让其他contract监听者接收到事件的一种方式。这样就没必要让其他应用程序反复来查询该contract的状态。
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