[dlt] README.md

src/dlt/README.md

+```
+     NEC Laboratories Europe GmbH
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+     PROPRIETARY INFORMATION
+
+ The software and its source code contain valuable trade secrets and
+ shall be maintained in confidence and treated as confidential
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+          Copyright (c) 2021 NEC Laboratories Europe GmbH
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+ Authors: Konstantin Munichev <konstantin.munichev@neclab.eu>
+
+
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+ ```
+
+# DLT module guide
+
+## General information
+The DLT module is used to provide access to the underlying Fabric deployment. It allows clients
+to add, retrieve, modify and delete blockchain-backed data, essentially working as a key-value
+database. External clients should use REST API to communicate with this service, its detailed
+description available below.
+
+## Code structure
+The whole DLT module consists of several packages:
+- fabric package
+- http package
+- proto package
+- client example
+
+### Fabric package
+The most important class in this package is `FabricConnector`. First, it establishes connection
+with the underlying Fabric network using Java Gateway SDK. After that, it could be used as a
+CRUD interface.
+Other files contain auxiliary code for `FabricConnector` which allows it to register/enroll
+users and to obtain smart contract instances.
+
+### HTTP package
+Contains server side HTTP handler. It accepts requests from the outside and performs the
+requested operation. For the detailed description see API description section.
+
+### Proto package
+The proto package contains `Config.proto` file which contains messages for REST API. The most
+important ones are `DltConfig` (it defines the whole DLT configuration) and `DltRecord` which
+represents data to store in the blockchain.
+
+### Client example
+This code is not necessary to the service, but it could be used to test the service. It contains
+a sample REST client which connects the service and perform all the CRUD operations. 
+
+## REST API description
+| Method | URL | Input | Response code | Output |
+| --- | ----------- | --- | --- | --- |
+| POST | /dlt/configure | Configuration object | 201 or 400 | Status value | 
+| GET | /dlt/configure | - | 200 or 404 | Configuration object |
+| POST | /dlt/record | Record object | 200, 201, 400 or 404 | Status value |
+| GET | /dlt/record | Record id | 200 or 404 | Record object |
+
+Record and configuration object are defined in `proto` package.
+