Adapted client-server.md from ABCI directory (#8510)

* Copied over 'client server' section from ABCI spec

* Adapted the ABCI text in 'Client and Server' section

* Minor changes to README

* Removed TODO from Readme

* Update spec/abci++/abci++_client_server_002_draft.md

Co-authored-by: Daniel <daniel.cason@usi.ch>

* Update spec/abci++/abci++_client_server_002_draft.md

Co-authored-by: Daniel <daniel.cason@usi.ch>

* Update spec/abci++/abci++_client_server_002_draft.md

Co-authored-by: Daniel <daniel.cason@usi.ch>

* Update spec/abci++/abci++_client_server_002_draft.md

Co-authored-by: Daniel <daniel.cason@usi.ch>

* Update spec/abci++/abci++_client_server_002_draft.md

Co-authored-by: Daniel <daniel.cason@usi.ch>

* Update spec/abci++/abci++_client_server_002_draft.md

Co-authored-by: Daniel <daniel.cason@usi.ch>

* Addressed comments

* Moved GRPC link out of the Tendermint-specific occurrence

* Fixed merge

Co-authored-by: Daniel <daniel.cason@usi.ch>

spec/abci++/README.md

@@ -25,19 +25,15 @@ This allows Tendermint to run with applications written in many programming lang
 
 This specification is split as follows:
 
-- [Overview and basic concepts](./abci++_basic_concepts_002_draft.md) - interface's overview and concepts needed to understand other parts of this specification.
+- [Overview and basic concepts](./abci++_basic_concepts_002_draft.md) - interface's overview and concepts
+  needed to understand other parts of this specification.
 - [Methods](./abci++_methods_002_draft.md) - complete details on all ABCI++ methods
   and message types.
 - [Requirements for the Application](./abci++_app_requirements_002_draft.md) - formal requirements
   on the Application's logic to ensure Tendermint properties such as liveness. These requirements define what
-  Tendermint expects from the Application.
+  Tendermint expects from the Application; second part on managing ABCI application state and related topics.
 - [Tendermint's expected behavior](./abci++_tmint_expected_behavior_002_draft.md) - specification of
   how the different ABCI++ methods may be called by Tendermint. This explains what the Application
   is to expect from Tendermint.
-
->**TODO** Re-read these and remove redundant info
-
-- [Applications](../abci/apps.md) - how to manage ABCI application state and other
-  details about building ABCI applications
 - [Client and Server](../abci/client-server.md) - for those looking to implement their
   own ABCI application servers

spec/abci++/abci++_client_server_002_draft.md

@@ -0,0 +1,102 @@
+---
+order: 5
+title: Client and Server
+---
+
+# Client and Server
+
+This section is for those looking to implement their own ABCI Server, perhaps in
+a new programming language.
+
+You are expected to have read all previous sections of ABCI++ specification, namely
+[Basic Concepts](./abci%2B%2B_basic_concepts_002_draft.md),
+[Methods](./abci%2B%2B_methods_002_draft.md),
+[Application Requirements](./abci%2B%2B_app_requirements_002_draft.md), and
+[Expected Behavior](./abci%2B%2B_tmint_expected_behavior_002_draft.md).
+
+## Message Protocol and Synchrony
+
+The message protocol consists of pairs of requests and responses defined in the
+[protobuf file](../../proto/tendermint/abci/types.proto).
+
+Some messages have no fields, while others may include byte-arrays, strings, integers,
+or custom protobuf types.
+
+For more details on protobuf, see the [documentation](https://developers.google.com/protocol-buffers/docs/overview).
+
+As of v0.36 requests are synchronous. For each of ABCI++'s four connections (see
+[Connections](./abci%2B%2B_app_requirements_002_draft.md)), when Tendermint issues a request to the
+Application, it will wait for the response before continuing execution. As a side effect,
+requests and responses are ordered for each connection, but not necessarily across connections.
+
+## Server Implementations
+
+To use ABCI in your programming language of choice, there must be an ABCI
+server in that language. Tendermint supports four implementations of the ABCI server:
+
+- in Tendermint's repository:
+    - In-process
+    - ABCI-socket
+    - GRPC
+- [tendermint-rs](https://github.com/informalsystems/tendermint-rs)
+- [tower-abci](https://github.com/penumbra-zone/tower-abci)
+
+The implementations in Tendermint's repository can be tested using `abci-cli` by setting
+the `--abci` flag appropriately.
+
+See examples, in various stages of maintenance, in
+[Go](https://github.com/tendermint/tendermint/tree/master/abci/server),
+[JavaScript](https://github.com/tendermint/js-abci),
+[C++](https://github.com/mdyring/cpp-tmsp), and
+[Java](https://github.com/jTendermint/jabci).
+
+### In Process
+
+The simplest implementation uses function calls in Golang.
+This means ABCI applications written in Golang can be linked with Tendermint Core and run as a single binary.
+
+### GRPC
+
+If you are not using Golang,
+but [GRPC](https://grpc.io/) is available in your language, this is the easiest approach,
+though it will have significant performance overhead.
+
+Please check GRPC's documentation to know to set up the Application as an
+ABCI GRPC server.
+
+### Socket
+
+Tendermint's socket-based ABCI interface is an asynchronous,
+raw socket server which provides ordered message passing over unix or tcp.
+Messages are serialized using Protobuf3 and length-prefixed with a [signed Varint](https://developers.google.com/protocol-buffers/docs/encoding?csw=1#signed-integers).
+
+If GRPC is not available in your language, your application requires higher
+performance, or otherwise enjoy programming, you may implement your own
+ABCI server using the Tendermint's socket-based ABCI interface.
+The first step is to auto-generate the relevant data
+types and codec in your language using `protoc`.
+In addition to being proto3 encoded, messages coming over
+the socket are length-prefixed. proto3 doesn't have an
+official length-prefix standard, so we use our own. The first byte in
+the prefix represents the length of the Big Endian encoded length. The
+remaining bytes in the prefix are the Big Endian encoded length.
+
+For example, if the proto3 encoded ABCI message is `0xDEADBEEF` (4
+bytes long), the length-prefixed message is `0x0104DEADBEEF` (`01` byte for encoding the length `04` of the message). If the proto3
+encoded ABCI message is 65535 bytes long, the length-prefixed message
+would start with 0x02FFFF.
+
+Note that this length-prefixing scheme does not apply for GRPC.
+
+Note that your ABCI server must be able to support multiple connections, as
+Tendermint uses four connections.
+
+## Client
+
+There are currently two use-cases for an ABCI client. One is testing
+tools that allow ABCI requests to be sent to the actual application via
+command line. An example of this is `abci-cli`, which accepts CLI commands
+to send corresponding ABCI requests.
+The other is a consensus engine, such as Tendermint Core,
+which makes ABCI requests to the application as prescribed by the consensus
+algorithm used.