solution: extend docs for architecture

docs/01-architecture-intro.adoc

@@ -14,4 +14,51 @@ currently healthy nodes. It's built using _reactive_ and non-blocking code and p
 API.
 
 Dshackle servers can be connected to each other as well, or work in parallel, to provide scalable and fault tolerant
-connection to blockchains. All connections can be secured and authenticated with client/server certificates.
+connection to blockchains. All connections can be secured and authenticated with client/server certificates.
+
+=== Upstream selection
+
+Dshackle tries to find an optimal upstream for each particular request, to do so it consider following factors of a node:
+
+- Current height
+- Is it fully synchronized, missing few (less than 6) blocks or in process of initial sync?
+- How many peers it has?
+
+And for a request:
+
+- Is it for a concrete data (_block #100_) or latest (_get balance_)?
+- Is result a static value (_just block_) or may vary depending on network and node (_latest nonce_)?
+- Does it need to be repeat over multiple nodes (_broadcast transaction_)?
+- Request can also specify which subset of nodes should be able to execute the request by selecting node _Labels_
+(see "link:08-quorum-and-selectors.adoc[Quorum and Selectors]")
+
+Based on these factors Dshackle execute request on a most optimal node. For most of the simple requests it just gets a node which is synchronized
+to the point that must have response for that particular request. If node failed, returned invalid response, returned
+empty response when shouldn't then Dshackle tries again on another node, or on the same node after awhile (default is
+200ms between failover repeats)
+
+=== gRPC protocol
+
+Dshackle uses gRPC protocol for communications, because:
+
+- provides additional parameters on top of upstreams JSON RPC requests
+- based on HTTP/2 with low latency, compression, server push, pipelining and multiplexing
+- gRPC has binding and code generators for most of the languages and frameworks
+- easy to support TLS encryption and authentication
+
+Dshackle has extra methods and functionality on top of standard APIs of upstreams, and it's more flexible to wrap
+original APIs, such as JSON, into gRPC and Protobuf + have additional data provided by Dshackle. It also allowed
+to push new data from the server, or send responses asynchronously, immediately after it gets executed on an upstream.
+
+=== Distributed Load Balancing
+
+Dshackle servers can connect to each other through a secure encrypted and authenticated connection.
+It allows to build a network of nodes deployed to different regions or run blockchain nodes outside of the main
+network.
+
+Later is especially important for blockchain nodes, as they require open firewall with incoming connections for P2P, and
+to execute untrusted code ("smart contracts") on the same time. With Dshackle it's possible to separate unsecure nodes from
+business application.
+
+But in general Dshackle allows to run a fault tolerant load balancing to build scalable and fail safe systems on
+blockchain.