Ensuring two redis commands are atomic if the second comman

Question

Using the node_redis client, how can I make two commands execute such that they will be performed atomically when the second command relies on the result of the first.

Here is an example of what I am thinking.

client.multi([
    ["hget", orgId, topicId]
]).exec(function (err, sessions) {
    sessions = JSON.parse(sessions);
    if (sessions === undefined) {
        sessions = [sessionId];
    }
    else {
        sessions.push(sessionId)
    }
    client.hset(orgId, topicId, JSON.stringify(sessions)]);
});

Can someone confirm there would not be a race condition to write back the new value if this ran simultaneously in two places.

Answer 1

The right answer is to "watch" the value. It is also important to make sure each query has its own connection and handles the possibility of a concurrency error.

var atomicReadWriteOperationRecursive = function (hashKey, valueKey, count) {
    var client = redis.createClient();
    client.watch(hashKey, valueKey);
    client.hget(hashKey, valueKey, function (err, response) {
            client.multi()
                .hset(orgId, topicId, newValue);
                .exec(function (err) {
                    client.close();
                    if (err && count < 3) {
                        atomicReadWriteOperationRecursive(hashKey, valueKey, count++);  
                    }
                });
        });
}

See here https://github.com/mranney/node_redis/issues/545.