Aggregates `value`

from `tff.CLIENTS`

to `tff.SERVER`

.

```
tff.federated_aggregate(
value,
zero,
accumulate,
merge,
report
)
```

This generalized aggregation function admits multi-layered architectures that involve one or more intermediate stages to handle scalable aggregation across a very large number of participants.

The multi-stage aggregation process is defined as follows:

Clients are organized into groups. Within each group, a set of all the member constituents of

`value`

contributed by clients in the group are first reduced in a manner similar to`tff.federated_reduce`

using reduction operator`accumulate`

with`zero`

as the zero in the algebra. As described in the documentation for`tff.federated_reduce`

, if members of`value`

are of type`T`

, and`zero`

(the result of reducing an empty set) is of type`U`

, the reduction operator`accumulate`

used at this stage should be of type`(<U,T> -> U)`

. The result of this stage is a set of items of type`U`

, one item for each group of clients.Next, the

`U`

-typed items generated by the preceding stage are merged using the binary commutative associative operator`merge`

of type`(<U,U> -> U)`

. This can be interpreted as a`tff.federated_reduce`

using`merge`

as the reduction operator, and the same`zero`

in the algebra. The result of this stage is a single top-level`U`

that emerges at the root of the hierarchy at the`tff.SERVER`

. Actual implementations may structure this step as a cascade of multiple layers.Finally, the

`U`

-typed result of the reduction performed in the preceding stage is projected into the result value using`report`

as the mapping function (for example, if the structures being merged consist of counters, this final step might include computing their ratios).

#### Args:

: A value of a TFF federated type placed at`value`

`tff.CLIENTS`

to aggregate.: The zero of type`zero`

`U`

in the algebra of reduction operators, as described above.: The reduction operator to use in the first stage of the process. If`accumulate`

`value`

is of type`{T}@CLIENTS`

, and`zero`

is of type`U`

, this operator should be of type`(<U,T> -> U)`

.: The reduction operator to employ in the second stage of the process. Must be of type`merge`

`(<U,U> -> U)`

, where`U`

is as defined above.: The projection operator to use at the final stage of the process to compute the final result of aggregation. If the intended result to be returned by`report`

`tff.federated_aggregate`

is of type`R@SERVER`

, this operator must be of type`(U -> R)`

.

#### Returns:

A representation on the `tff.SERVER`

of the result of aggregating `value`

using the multi-stage process described
above.

#### Raises:

: if the arguments are not of the types specified above.`TypeError`