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Cleanup fanout docs and ASCII art

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Greg Fitzgerald 2019-01-16 10:15:18 -07:00 committed by Grimes
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book/art/data-plane.bob
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| | | | +--------------------------------------------+ |
| | | | | | | |
| | | +----------------------+ | | |
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v v v v v v v v
+-+----------------+-+ +-+----------------+-+ +-+----------------+-+ +-+----------------+-+
+--------------------+ +--------------------+ +--------------------+ +--------------------+
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| Neighborhood 1 | | Neighborhood 2 | | Neighborhood 3 | | Neighborhood 4 |
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book/src/data-plane-fanout.md
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# Data Plane Fan-out
# Data Plane Fanout
This Article describes the current single layer broadcast and retransmit mechanisms as well as proposed changes to add a multi-layer retransmit via an Avalanche mechanism.
This article describes the current single-layer broadcast and retransmit mechanisms as well as proposed changes to add a multi-layer retransmit via an Avalanche mechanism.
## Current Design
There's two basic parts to the current Data Plane's Fan-out design.
#### Broadcast stage
In this stage, the leader distributes its data across the Layer-1 nodes. Currently, Layer-1 nodes are all known "tvu peers" (ClusterInfo::tvu_peers).
There's two basic parts to the current data plane's fanout design.
#### Broadcast Service
In this service, the leader distributes its data across the Layer-1 nodes. Currently, Layer-1 nodes are all known "TVU peers" (`ClusterInfo::tvu_peers`).
The leader performs a round-robin broadcast where it sends each blob of data to only one validator at a time. That way each Layer-1 node only receives partial data from the leader and
the Retransmit Stage in each Layer-1 node's TVU will ensure all data is shared between its Layer-1 peers and a complete window is received.
#### Retransmit stage
The Retransmit stage *forwards* data from a Layer-1 node to all of _its_ "retransmit peers" (list of tvu peers excluding the leader). So as nodes start seeing complete windows they can send their votes back to the leader.
#### Retransmit Stage
The Retransmit stage *forwards* data from a Layer-1 node to all of _its_ "retransmit peers" (list of TVU peers excluding the leader). So as nodes start seeing complete windows they can send their votes back to the leader.
Validators know to only forward blobs that came from the leader by checking the signatures against the current leader.
**Cluster_info -> retransmit** = Used by TVUs to retransmit. Currently Layer-1 sends this to all tvu peers, leader is automatically excluded.
**Cluster_info -> retransmit** = Used by TVUs to retransmit. Currently Layer-1 sends this to all TVU peers, leader is automatically excluded.
**Cluster_info -> broadcast** = Used by leader to broadcast to layer-1 nodes.
**BroadcastStage -> run** = Used by leader (TPU) to broadcast to all validators. Currently all TVU Peers are considered layer-1. But blobs are transmitted sort of round robin. See Cluster_info->Broadcast.
**BroadcastService -> run** = Used by leader (TPU) to broadcast to all validators. Currently all TVU Peers are considered layer-1. But blobs are transmitted sort of round robin. See Cluster_info->Broadcast.
## Proposed Design
@ -27,7 +30,7 @@ The leader broadcasts its blobs to the layer-1 (neighborhood 0) nodes exactly li
Each neighborhood has `NEIGHBORHOOD_SIZE` nodes and `fanout/2` neighborhoods are allowed per layer.
Nodes in a layer will broadcast their blobs to exactly 1 node in each next-layer neighborhood and each node in a neighborhood will perform retransmits amongst its neighbors (just like layer-1 does with its layer-1 peers).
This means any node has to only send its data to its neighbors and each neighborhood in the layer below instead of every single tvu peer it has.
This means any node has to only send its data to its neighbors and each neighborhood in the layer below instead of every single TVU peer it has.
The retransmit mechanism also supports a second, `grow`, mode of operation that squares the number of neighborhoods allowed per layer which dramatically reduces the number of layers needed to support a large network but can also have a negative impact on the network pressure each node in the lower layers has to deal with.
A good way to think of the default mode (when `grow` is disabled) is to imagine it as `chain` of layers where the leader sends blobs to layer-1 and then layer-1 to layer-2 and so on, but instead of growing layer-3 to the square of number of nodes in layer-2, we keep the `layer capacities` constant, so all layers past layer-2 will have the same number of nodes until the whole network is covered. When `grow` is enabled, this
quickly turns into a traditional fanout where layer-3 will have the square of the number of nodes in layer-2 and so on.
@ -44,22 +47,24 @@ Since multiple neighborhoods exist in the upper layer and a node will receive bl
<img alt="Inner workings of a neighborhood" src="img/data-plane-neighborhood.svg" class="center"/>
#### A weighted selection mechanism
#### A Weighted Selection Mechanism
To support this mechanism, there needs to be a agreed upon way of dividing the network amongst the nodes. To achieve this the `tvu_peers` are sorted by stake and stored in a list. This list can then be indexed in different ways to figure out neighborhood boundaries and retransmit peers.
For example, the leader will simply select the first `DATA_PLANE_FANOUT` nodes as its layer 1 nodes. These will automatically be the highest stake holders allowing the heaviest votes to come back to the leader first.
The same logic determines which nodes each node in layer needs to retransmit its blobs to. This involves finding its neighbors and lower layer peers.
#### Broadcast stage
Broadcast Stage uses a bank to figure out stakes and hands this off to ClusterInfo to figure out the top `DATA_PLANE_FANOUT` stake holders.
#### Broadcast Service
Broadcast service uses a bank to figure out stakes and hands this off to ClusterInfo to figure out the top `DATA_PLANE_FANOUT` stake holders.
These top stake holders will be considered Layer 1. For the leader this is pretty straightforward and can be achieved with a `truncate` call on a sorted list of peers.
#### Retransmit stage
#### Retransmit Stage
The biggest challenge in updating to this mechanism is to update the retransmit stage and make it "layer aware"; i.e using the bank each node can figure out which layer it belongs in and which lower layer peers nodes to send blobs to with minimal overlap across neighborhood boundaries.
Overlaps will be minimized based on `((node.layer_index) % (layer_neighborhood_size) * cur_neighborhood_index)` where `cur_neighborhood_index` is the loop index in `num_neighborhoods` so that a node only forwards blobs to a single node in a lower layer neighborhood.
Each node can receive blobs froms its peer in the layer above as well as its neighbors. As long as the failure rate is less than the number of erasure codes, blobs can be repaired without the network failing.
#### Constraints
`DATA_PLANE_FANOUT` - The size of layer 1 is determined by this. Subsequent layers have `DATA_PLANE_FANOUT/2` neighborhoods when `grow` is inactive.
`NEIGHBORHOOD_SIZE` - The number of nodes allowed in a neighborhood. Neighborhoods will fill to capacity before new ones are added, i.e if a neighborhood isn't full, it _must_ be the last one.