Update README.md

Add important links, background and implementation details, describe compability aspects and extendability

Change-Id: Ie38300a0d33b347c5d0d82b3e8547039e54829f3
Issue-ID: DCAEGEN2-798
Signed-off-by: Izabela Zawadzka <izabela.zawadzka@nokia.com>

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 ONAP component for collecting high volume data, eg, RTPM (Real Time Performance Management).
 
+##General information
+* Repository: https://gerrit.onap.org/r/#/admin/projects/dcaegen2/collectors/hv-ves
+* Changes: https://gerrit.onap.org/r/#/q/project:dcaegen2/collectors/hv-ves
+* [HV-VES landing page on ONAP Wiki](https://wiki.onap.org/display/DW/High+Volume+VES+Collector)
+
+##Background
+VES-HV collector has been proposed, based on a need to process high-volumes of data generated frequently by a large number of NFs.
+The driving use-case is the 5G RAN, where it is expected that up to 10k NF instances report the data, per DCAE platform deployment.
+The network traffic generated in simulations - based on 4G BTS Real-Time PM data has shown, that GPB serialization is 2-3 times more effective, than JSON serialization utilized in VES collector.
+Results have been published within ONAP presentation in Casablanca Release Developer Forum: [Google Protocol Buffers versus JSON - 5G RAN use-case - comparison](https://wiki.onap.org/download/attachments/25434845/Casablanca_Dev_Forum_GPB_comparison_20180621.pptx?version=1&modificationDate=1530275050000&api=v2)
+
+The goal of the collector is to support high volume data. It uses plain TCP connections tunneled in SSL/TLS. Connections are stream-based (as opposed to request-based) and long running. Payload is binary-encoded (currently we are using Google Protocol Buffers). HV-VES uses direct connection to DMaaP's Kafka. All these decisions were made in order to support high-volume data with minimal latency.
+
+For more details on the rationale, please read a [high-level feature description](https://wiki.onap.org/display/DW/5G+-+Real+Time+PM+and+High+Volume+Stream+Data+Collection).
+
+##Description
+**Compatibility aspects (VES-JSON)**
+* VES-HV has been designed as a high-volume variant of the existing VES(JSON) collector, and not a completely new collector
+* VES-HV follows the VES-JSON schema - as much as possible
+    * It uses a PROTO representation of the VES Common Header
+    * The PROTO files tend to use most encoding effective types defined by GPB to cover Common Header fields.
+    * It makes routing decisions based mostly on the content of the "Domain" parameter
+    * It allows to embed Payload of different types (by default hvMeas domain is included)
+* VES-HV publishes events on DMaaP-Kafka bus, using native Kafka Interfaces
+* Analytics applications impacts
+    * An analytics application operating on high-volume data needs to be prepared to read directly from Kafka
+    * An analytics application need to operate on GPB encoded data in order to benefit from GPB encoding efficiencies
+    * It is assumed, that due to the nature of high volume data, there would have to be dedicated applications provided, able to operate on such volumes of data.
+
+**Extendability**
+
+VES-HV was designed to allow for extendability - by adding new domain-specific PROTO files.
+
+The PROTO file, which contains the VES CommonHeader, comes with a binary-type Payload parameter, where domain-specific data shall be placed. Domain-specific data are encoded as well with GPB, and they do require a domain-specific PROTO file to decode the data. This domain-specific PROTO needs to be shared with analytics applications - VES-HV is not analyzing domain-specific data. In order to support the RT-PM use-case, VES-HV includes a "hvMeas" domain PROTO file, as within this domain, the high volume data is expected to be reported to VES-HV collector.
+Still, there are no limitations to define additional domains, based on existing VES domains (like Fault, Heartbeat) or completely new domains. New domains can be added "when needed".
+
+In case of new domains, it is necessary to extend the Common Header PROTO "Domain" enumeration with new values covering this new domain(s).
+GPB PROTO files are backwards compatible, and such a new domain could be added without affecting existing systems.
+
+Analytics applications will have to be as well equipped with this new domain-specific PROTO file.
+Currently, these additional, domain specific proto files could be simply added to respective repos of VES-HV collector.
+
+##Implementation details
+**Technology stack**
+* [Project Reactor](https://projectreactor.io/) is used as a backbone of the internal architecture.
+* [Netty](http://netty.io/) is used by means of [reactor-netty](https://projectreactor.io/docs/netty/release/api/) library.
+* We are using [Kotlin](https://kotlinlang.org/) so we can write very concise code with great interoperability with existing Java libraries.
+* Types defined in [Λrrow](https://arrow-kt.io/) library are also used when it improves readability or general cleanness of the code.
+
+**Rules**
+* Do not block. Use non-blocking libraries. Do not use block* Reactor calls inside the core of the application.
+* Pay attention to memory usage.
+* Do not decode the payload - it can be of a considerable size. The goal is to direct the event into a proper Kafka topic. The routing logic should be based only on VES Common Header parameters.
+* All application logic should be defined in hv-collector-core module and tested on a component level by tests defined in hv-collector-ct. The core module should have a clean interface (defined in boundary package: api and adapters).
+* Use [Either](https://arrow-kt.io/docs/datatypes/either/) functional data type when designing fail-cases inside the main [Flux](https://projectreactor.io/docs/core/release/api/reactor/core/publisher/Flux.html). Using exceptions is a bit like using [goto](https://en.wikipedia.org/wiki/Goto#Criticism) + it adds some performance penalty: collecting stack trace might be costly but we do not usually need it in such cases. RuntimeExceptions should be treated as application bugs and fixed.
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