CODEWITHSUNDEEP
node.jsredis
Ali
Ali

Reputation: 5476

Redis: How to check if exists in while loop

I'm using Redis in my application and one thing is not clear for me. I save an object with a random generated string as its key. However I would like to check if that key exists. I am planning to use while loop however I am not sure how would I struct it according to Redis. Since if I would like to check for once, I would do;

redisClient.get("xPQ", function(err,result){
   if(result==null)
      exists = false
});

But I would like use the while loop as;

while(exists == false)

However I cannot build the code structure in my head. Would the while be inside the function or outside the function?

Upvotes: 1

Views: 3025

Answers (2)

Sripathi Krishnan
Sripathi Krishnan

Reputation: 31528

In general, you shouldn't check for existence of a key on the client side. It leads to race conditions. For example, another thread could insert the key after the first thread checked for its presence.

You should use the commands ending with NX. For example - SETNX and HSETNX. These will insert the key only if doesn't already exist. It is guaranteed to be atomic.

Upvotes: 2

Didier Spezia
Didier Spezia

Reputation: 73236

I do not understand why you need to implement active polling to check whether a key exists (there are much better ways to handle this kind of situations), but I will try to answer the question.

You should not use a while loop at all (inside or outside the function). Because of the asynchronous nature of node.js, these loops are better implemented using tail recursion. Here is an example:

var redis = require('redis')
var rc = redis.createClient(6379, 'localhost');

function wait_for_key( key, callback ) {
   rc.get( key, function(err,result) {
      if ( result == null ) {
         console.log( "waiting ..." )
         setTimeout( function() {
            wait_for_key(key,callback);
         }, 100 );
      } else {
         callback(key,result);
      }
   });
}

wait_for_key( "xPQ", function(key,value) {
   console.log( key+" exists and its value is: "+value )
});

There are multiple ways to simplify these expressions using dedicated libraries (using continuation passing style, or fibers). For instance you may want to check the whilst and until functions of the async.js package.

https://github.com/caolan/async

Upvotes: 0

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