Introduction

Oracle service builds the bridge between blockchain and the real world, and we are going to introduce an oracle service called Oraclize.

The following image describes the underlying mechanism of the Oraclize.

It uses the TLSNotary to make its serivce trust-worthy, and pre-fetch the real world data for all the nodes within the network to achieve consensus, which cannot be easiliy done with Ethereum smart contract.

Example explanation

The following example shows a simple contract using the orcalize serivce

//this import should be replaced by oraclizeAPI.sol
import "dev.oraclize.it/api.sol";
contract YoutubeViews is usingOraclize {

    uint public viewsCount;

    function YoutubeViews() {
        //OAR = OraclizeAddrResolverI(resolve_addr);  //add this line if you are using Oraclize in private chain environment
        update(0);
    }

    function __callback(bytes32 myid, string result) {
        if (msg.sender != oraclize_cbAddress()) throw;
        viewsCount = parseInt(result, 0);
        // do something with viewsCount

        update(60*10); // update viewsCount every 10 minutes
    }

    function update(uint delay) {
        oraclize_query(delay, 'URL', 'html(https://www.youtube.com/watch?v=9bZkp7q19f0).xpath(//*[contains(@class, "watch-view-count")]/text())');
    }

}

The example shows three steps to utilize the oraclize service:

• Constructor will first call the update function, which will trigger the orclize_query transaction and commence data fetching process. You also have to specify the delay (in seconds) from the current time or the timestamp in the future as first argument.
• Once the data query returns the result, it will call the __callback function with the result passed into the function.
• We can now do some operation using the code defined in __callback function, and you can also call update functionagain if you want to execute the query once in a while.

Caution! Please note that in order for the future timestamp to be accepted by Oraclize it must be within 60 days from the current time.

Private chain scenarios

If you are currently using private chain, you need to use the Ethereum-Bridge API, which is a log listener listens to the oraclize query from private chain and connect to oraclize service. The following list the steps of using Ethereum-Bridge:

• Download the repository of Ethereum-Bridge by using git clone https://github.com/oraclize/ethereum-bridge, and execute npm install in the nodejs folder.
• Unlock certain account (in this example we will use acoounts[0]) by using geth --unclock 0, and execute node bridge -H localhost:8545 -a 0. (make sure the account you unlock matches the account input to the bridge)
• The bridge will automatically deploys the Oraclize Address Resolver (OAR) and the Oraclize Connector in the private chain, which interface your contract with the Oraclize service. Please note that your contract shouldn't be deployed with the same address used by the Ethereum Bridge (in this example the address 0).

Scheduler

Since smart contract can only be triggered by transaction, it is hard to have code executed in a specified timestamp. However, there are already some solutions to this issue, for instance, the Ethereum Alarm Clock service. This part we are gonna use Oraclize as a scheduler instead of the alarm clock service. How ? The following example will show you

import "dev.oraclize.it/api.sol";
contract YoutubeViews is usingOraclize {

    uint public viewsCount;

    function YoutubeViews() {
        update(0);
    }

    function __callback(bytes32 myid, string result) {
        if (msg.sender != oraclize_cbAddress()) throw;
        viewsCount = parseInt(result, 0);
        // do something 

        update(10); // do something every 10 seconds
    }

    function update(uint delay) {
        oraclize_query(delay, 'URL', '');   // This query can still work without the URL parameters
    }

}

As you can see, the oraclize service can be a convenient scheduler if no URL parameters are inputted.

轉自：

https://github.com/johnhckuo/Oraclize-Tutorial