Move content to principles.adoc file

fineract-doc/src/docs/en/chapters/architecture/principles.adoc

@@ -61,3 +61,100 @@ Within each vertical slice is some common packaging structure:
 * domain - OO concepts for the functional area
 * data - Data concepts for the area
 * serialization - ability to convert from/to API JSON for functional area
+
+
+== Read/Write/Batch API Operation Separation
+
+Fineract has three different instance types:
+
+* Read instance
+* Write instance
+* Batch instance
+
+In cases where Fineract has to deal with high load, it can cause a performance problem for a single Fineract instance. To overcome this problem, Fineract instances can be started in different instance types for better scalability and performance in a multi-instance environment:
+
+Detailed documentation on instance types can be found in this document: https://github.com/apache/fineract/blob/develop/fineract-doc/src/docs/en/chapters/deployment/instance-type.adoc
+
+The linked document also explains deployment steps and specifications.
+
+=== Background
+
+Instances, where we're planning to deploy Fineract to a highly-available setup with the highest-throughput because we can scale individual parts of it.
+
+
+Reading data happens much more often than actually writing data so the ability to do this type of deployment will greatly improve what we can achieve in terms of performance.
+
+Also, with the separation in place, we could utilize read-replica databases as well where write and batch instances connect to a master DB and the read instances connect to read-replicas therefore reducing the load on the database as well.
+
+All three instances come with different types of restrictions in terms of what they are capable of. For example a read instance is going to be only capable of serving read (GET) APIs. Write APIs will be able to serve both read and write APIs. Batch instance is only able to serve batch job related APIs and run the batch jobs themselves.
+
+=== Context
+
+===== Instance types
+
+**Read instance** only accepts `GET` request. Optionally, there are database connection details for this mode.
+
+**Write instance** accepts `GET/POST/PUT/DELETE` requests. It uses the original database connection details.
+
+**BATCH instance** accepts `POST` request related to run jobs. It will start the Quartz scheduler at startup time and run the batch jobs.
+
+=== Modes
+
+* Full mode (default)
+* Read mode
+* Write mode
+* Batch mode
+
+=== Setup
+
+==== Environment Variable
+
+Using environment variables, the three instance types performance enhancing operation separation can be performed:
+
+* FINERACT_READ_MODE_ENABLED
+* FINERACT_WRITE_MODE_ENABLED
+* FINERACT_BATCH_MODE_ENABLED
+
+These three are the boolean variables to enable or disable the instance modes.
+
+=== Database Settings
+
+Optional database connection details for reading mode are:
+
+.Database connection settings
+|===
+^| Key ^| Type ^| Value
+
+| schema_server | character varying (100) |  127.0.0.1
+| schema_name | character varying (100) | fineract_default
+| schema_server_port | character varying (10) | 5432
+| schema_username | character varying (100) | mifosadmin
+| schema_password | character varying (100) | password
+| readonly_schema_server | character varying(100) | [null]
+| readonly_schema_name | character varying(100) | [null]
+| readonly_schema_server_port | character varying(10) | [null]
+| readonly_schema_usernname | character varying (100) | [null]
+| readonly_schema_password | character varying (100) | [null]
+| readonly_schema_connection_parameters | text | [null]
+|===
+
+=== Use Case Scenarios
+
+==== APIs
+
+* Read APIs are allowed only for read and write instances
+* Write APIs are allowed only for write instances
+* Batch job APIs are allowed only for batch instances
+
+==== Batch jobs
+
+* Batch job scheduling is allowed only for batch instances
+
+==== Read-only instance type restrictions
+
+* Events are disabled for read-only instances
+* Read-only tenant connection support
+
+==== Batch-only instance type restrictions
+
+* Receiving events is disabled for batch-only instances