# KVStore API¶

Topics:

## Basic Push and Pull¶

Provides basic operation over multiple devices (GPUs) on a single device.

### Initialization¶

Let’s consider a simple example. It initializes a (int, NDArray) pair into the store, and then pulls the value out.

val kv = KVStore.create("local") // create a local kv store.
val shape = Shape(2,3)
kv.init(3, NDArray.ones(shape)*2)
val a = NDArray.zeros(shape)
kv.pull(3, out = a)
a.toArray
// Array[Float] = Array(2.0, 2.0, 2.0, 2.0, 2.0, 2.0)


### Push, Aggregation, and Updater¶

For any key that’s been initialized, you can push a new value with the same shape to the key, as follows:

kv.push(3, NDArray.ones(shape)*8)
kv.pull(3, out = a) // pull out the value
a.toArray
// Array[Float] = Array(8.0, 8.0, 8.0, 8.0, 8.0, 8.0)


The data that you want to push can be stored on any device. Furthermore, you can push multiple values into the same key, where KVStore first sums all of these values, and then pushes the aggregated value, as follows:

val gpus = Array(Context.gpu(0), Context.gpu(1), Context.gpu(2), Context.gpu(3))
val b = Array(NDArray.ones(shape, gpus(0)), NDArray.ones(shape, gpus(1)), \
NDArray.ones(shape, gpus(2)), NDArray.ones(shape, gpus(3)))
kv.push(3, b)
kv.pull(3, out = a)
a.toArray
// Array[Float] = Array(4.0, 4.0, 4.0, 4.0, 4.0, 4.0)


For each push command, KVStore applies the pushed value to the value stored by an updater. The default updater is ASSIGN. You can replace the default to control how data is merged.

val updater = new MXKVStoreUpdater {
override def update(key: Int, input: NDArray, stored: NDArray): Unit = {
println(s"update on key \$key")
stored += input * 2
}
override def dispose(): Unit = {}
}
kv.setUpdater(updater)
kv.pull(3, a)
a.toArray
// Array[Float] = Array(4.0, 4.0, 4.0, 4.0, 4.0, 4.0)
kv.push(3, NDArray.ones(shape))
// update on key 3
kv.pull(3, a)
a.toArray
// Array[Float] = Array(6.0, 6.0, 6.0, 6.0, 6.0, 6.0)


### Pull¶

You’ve already seen how to pull a single key-value pair. Similar to the way that you use the push command, you can pull the value into several devices with a single call.

val b = Array(NDArray.ones(shape, gpus(0)), NDArray.ones(shape, gpus(1)),\
NDArray.ones(shape, gpus(2)), NDArray.ones(shape, gpus(3)))
kv.pull(3, outs = b)
b(1).toArray
// Array[Float] = Array(6.0, 6.0, 6.0, 6.0, 6.0, 6.0)


## List Key-Value Pairs¶

All of the operations that we’ve discussed so far are performed on a single key. KVStore also provides the interface for generating a list of key-value pairs. For a single device, use the following:

val keys = Array(5, 7, 9)
kv.init(keys, Array.fill(keys.length)(NDArray.ones(shape)))
kv.push(keys, Array.fill(keys.length)(NDArray.ones(shape)))
// update on key: 5
// update on key: 7
// update on key: 9
val b = Array.fill(keys.length)(NDArray.zeros(shape))
kv.pull(keys, outs = b)
b(1).toArray
// Array[Float] = Array(3.0, 3.0, 3.0, 3.0, 3.0, 3.0)