Node.JS：for循环中的异步Postgres查询

Question

I want to query some data from the database, from a for loop. The problem is, the query is processed after the end of the loop.

This code:

ret = [];
first = rows[0];
first.device_firsttime = first.device_lasttime;
first.alerts = [];


for(var i = 0; i < 5; i++)
{
    console.log("!"+i);
    (function(i) {
        query("SELECT EXTRACT('epoch' FROM alert_time)::integer alert_time, alert_id, alert_lat, alert_lon, alert_str, alert_cc, alert_distance FROM blitz_device_former_alerts WHERE alert_locid = $1", [first.device_locid], function(error_a,rows_a,result_a)
        {
            console.log(i+"!");
            ret.push(i);
        });
    })(i);
}

console.log("-END---"+JSON.stringify(ret));
ret.push(first);
res.end(JSON.stringify(ret));

Writing this to the console:

!0
!1
!2
!3
!4
-END---[]
POST /userlogs 200 140.110 ms - -
0!
1!
2!
3!
4!

The correct output should be

!0
0!
!1
1!
!2
2!
!3
3!
!4
4!
-END---[0,1,2,3,4]
POST /userlogs 200 xxx.xxx ms - -

Answer 1

The query is asynchronous. The loop doesn't wait for those queries to finish. It's easier to use some flow control library such as 'async'.

var async = require('async');
async.eachSeries([0, 1, 2, 3, 4], function(i, callback) {
  console.log("!"+i);
  query("SELECT EXTRACT('epoch' FROM alert_time)::integer alert_time, alert_id, alert_lat, alert_lon, alert_str, alert_cc, alert_distance FROM blitz_device_former_alerts WHERE alert_locid = $1", [first.device_locid], function(error_a,rows_a,result_a) {
    console.log(i+"!");
    ret.push(i);
    callback(null);  // null -> no error
  });

}, function(err) {
  console.log("-END---"+JSON.stringify(ret));
  ret.push(first);
  res.end(JSON.stringify(ret));
});

Answer 2

You may want to use the async library for that job: essentially it will run your functions in sequence. It is also a good practice to use named functions, in this case a second-order function getExtractor() : a function that returns another function, usable by async . async likes to have functions that accept callbacks, which is where the result of each execution is added.

The code would be something like this:

var async = require('async');
first = rows[0];
first.device_firsttime = first.device_lasttime;
first.alerts = [];

var tasks = [];    
for(var i = 0; i < 5; i++)
{
    tasks.push(getExtractor(i));
}

async.series(function(result) {
    console.log("-END---"+JSON.stringify(result));
    result.push(first);
    res.end(JSON.stringify(result));
});

function getExtractor(i) {
    return function(callback) {
        console.log("!"+i);
        query("SELECT EXTRACT('epoch' FROM alert_time)::integer alert_time, alert_id, alert_lat, alert_lon, alert_str, alert_cc, alert_distance FROM blitz_device_former_alerts WHERE alert_locid = $1", [first.device_locid], function(error_a,rows_a,result_a)
        {
            console.log(i+"!");
            return callback(null, i);
        });
    });
}

A few notes:

• Each function pushed to the tasks array accepts a callback of the form function(error, result) . The first parameter is an optional error.
• I have not dealt with errors in the code, for simplicity, but it is important to do it.
• If for any reason you want to run all functions in parallel you can do it just using async.parallel() .
• The final bit (when all pieces have been gathered) is executed in the callback passed to async.series() .
• When async receives an array of functions, it sends to its callback an array of results. That is why we get the result automatically constructed by async .
• You can do the same without async but frankly, the code is quite complex and I don't recommend it.