how to use couchbase as fifo queue - queue

Question

With Java client, how can I use couchbase to implement FIFO queue, thread safe? There can be many threads popping from the queue, and pushing into the queue. Each object in the queue is a string[].

Answer 1

Couchbase doesn't have any built-in functionality for creating queues, but you can do that by yourself.
I'll explain how to do that in short example below.
I.e. we have queue with name queue and it will have items with names item:<index>. To implement queue you'll need to store your values with key like: <queue_name>:item:<index>, where index will be separate key queue:index, that you need to increment while pushing to queue, and decrement while popping.
In couchbase you could use increment and decrement operations to implement queue, because that operations are atomic and threadsafe.
So code of your push and pop functions will be like:
void push(string queue, string[] value){
int index = couchbase.increment(queue + ':index');
couchbase.set(queue + ':item:' + index, value);
}
string[] pop(string queue){
int index = couchbase.get(queue + ':index');
string[] result = couchbase.get(queue + ':item:' + index);
couchbase.decrement(queue + ':index');
return result;
}
Sorry for code, Ive used java and couchbase java client a long time ago. If now java client have callbacks, like nodejs client, you can rewrite that code to use callbacks. It will be better, I think.
Also you can add additional check into set operation - use add (in C# client it called StoreMode.Add) operation that will throw exception if item with given key has already exists. And you can catch that exception and call push function again for same arguments.
UPD: I'm sorry, it was too early in the morning, so I couldn't think clear.
For fifo, as #avsej said you'll need two counters: queue:head and queue:tail. So for fifo:
void push(string queue, string[] value){
int index = couchbase.increment(queue + ':tail');
couchbase.set(queue + ':item:' + index, value);
}
string[] pop(string queue){
int index = couchbase.increment(queue + ':head') - 1;
string[] result = couchbase.get(queue + ':item:' + index);
return result;
}
Note: code can look slightly different depending on start values of queue:tail and queue:head (will it be zero or one or something else).
Also you can set some max value for counters, after reaching it, queue:tail and queue:head will be reseted to 0 (just to limit number of documents). Also you can set expire value to each document, if you actually need this.

Answer 2

Couchbase already CouchbaseQueue data structure.
Example usage: taken from the below SDK documentation
Queue<String> shoppingList = new CouchbaseQueue<String>("queueDocId", collection, String.class, QueueOptions.queueOptions());
shoppingList.add("loaf of bread");
shoppingList.add("container of milk");
shoppingList.add("stick of butter");
// What does the JSON document look like?
System.out.println(collection.get("queueDocId").contentAsArray());
//=> ["stick of butter","container of milk","loaf of bread"]
String item;
while ((item = shoppingList.poll()) != null) {
System.out.println(item);
// => loaf of bread
// => container of milk
// => stick of butter
}
// What does the JSON document look like after draining the queue?
System.out.println(collection.get("queueDocId").contentAsArray());
//=> []
Java SDK 3.1 CouchbaseQueue Doc