node.js redis async query

node.js, redis

Solution

`hgetall` is an asynchronous call: when it receives a reply from the redis server, it will eventually call your callback `function (target) { ... }`. But within your script, it actually returns immediately. Since `hgetall` returns very fast, Node will immediately run the next statement, `smembers`. But at this point the `sadd` statements haven’t run yet (even if your system is very fast because there hasn’t been a context switch yet).

What you need to do is to make sure `smembers` isn’t called before all the possible `sadd` calls have executed. redis_client provides the `multi` function to allow you to queue up all the `sadd` calls and run a callback when they’re all done. I haven’t tested this code, but you could try this:

var redis_client = redis.createClient(REDIS_PORT, REDIS_URL);

redis_client.hgetall(target_hash, function(e, o) {
  var multi = redis_client.multi();
  var keys = Object.keys(o);
  var i = 0;

  keys.forEach(function (target) {
    // get the "name" from the hash     
    redis_client.hget(o[target], "name", function(e, o) {
      i++;
      if (e) {
        console.log("Error occurred getting key: " + e);
      } else {
        multi.sadd("newset", o);
      }

      if (i == keys.length) {
        multi.exec(function (err, replies) {
          console.log("MULTI got " + replies.length + "replies");
          redis_client.smembers("newset", redis.print);
        });
      }
    });
  });
});

Some libraries have a built-in equivalent of `forEach` that allows you to specify a function to be called when the loop is all done. If not, you have to manually keep track of how many callbacks there have been and call `smembers` after the last one.

Problem

Hope someone can assist with a (simple) async question on node-redis. I'm trying to load a set from a hash in the redis db and then use that populated set further on. Here's the code snippet :- ``` var redis_client = redis.createClient(REDIS_PORT, REDIS_URL); redis_client.hgetall(target_hash,function(e,o){ Object.keys(o).forEach(function(target){ // get the "name" from the hash redis_client.hget(o[target],"name",function(e,o){ if (e){ console.log("Error occurred getting key: " + e); } else { redis_client.sadd("newset",o); } }); }); // the following line prints nothing - why ?? redis_client.smembers("newset",redis.print); ``` When I examine the contents of "newset" in redis it is populated as expected, but at runtime it displayed as empty. I'm sure it's an async issue - any help much appreciated !

Original source