Added proposal for receive split (#3489)

* Added proposal for receive split

Signed-off-by: Chans321 <tsschand@gmail.com>

* Modified the proposal for receive split.

Signed-off-by: Chans321 <tsschand@gmail.com>

docs/proposals/receive_split.md

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+---
+title: Thanos Remote Write
+type: proposal
+menu: proposals
+status: accepted
+---
+
+### Related Tickets
+
+* Hashring improvements: https://github.com/thanos-io/thanos/issues/3141
+
+### Summary
+
+This document describes the motivation and design of splitting the receive component into receive-route and receive.
+Additionally, we touch possibility for adding a consistent hashing mechanism and buffering the incoming requests.
+
+### Motivation
+
+1. Splitting functionality can be used to make resharding events easier.
+2. Right now the receiver handles ingestion, routing and writing, thus leading to too many responsibilities in a single process.
+This also makes the receiver more difficult to operate and understand for Thanos users.
+Splitting the receiver component into two different components could potentially have the following benefits:
+    1. Resharding events become faster and cause no downtime in ingestion.
+    2. Deployment becomes easier to understand for Thanos users.
+    3. Each component consists of less code.
+    4. The new architecture enables further performance improvements.
+
+### Goals
+
+1. Split the receive component into receive-route and receive (and ensure ease of resharding events).
+2. Evaluate any effects on performance by simulating scenarios and collecting and analyzing metrics.
+3. Use consistent hashing to avoid reshuffling time series after resharding events. The exact consistent hashing mechanism to be used needs some further research.
+
+### Drawbacks of the project
+There is no possible way to have a single-process receiver. The user must have a router + a receiver running.
+##### Solution
+We document how the new architecture can be set up to have the same general deployment of the old architecture. (We run router and receiver on the same node).