doc: improve developing connectors

docs/2-developing/0-connectors/0-connectors.mdx

@@ -1,21 +1,280 @@
 ---
-title: "Developing custom connectors"
+title: "Guidelines for writing a connector"
+toc_max_heading_level: 6
 ---
 
+:::info
+This page describes patterns and processes that have been successfully used
+in many Conduit connectors over the past few years. The document assumes basic
+knowledge about how Conduit works.
+:::
+
 Conduit connectors can be built in any programming language that supports gRPC.
 To make it easier to write connectors we provide
 a [Connector SDK](https://github.com/ConduitIO/conduit-connector-sdk) written in
 Go. Using the SDK is the recommended way of writing a Conduit connector. If you
 are implementing a connector in a different language you should refer to
 the [Connector Protocol](https://github.com/ConduitIO/conduit-connector-protocol).
 
-## Conduit connector template
+
+## Start with the connector template
 
 The easiest way to start implementing your own Conduit connector is by using the
 [Conduit connector template](/docs/developing/connectors/conduit-connector-template).
 It contains the basic project structure as well as some additional utilities
-like GitHub actions and a Makefile.
+like GitHub actions and a Makefile with commonly used targets and GitHub workflows
+for linting the code, running unit tests, automatically merging minor dependabot
+upgrades, etc.
+
+## Research the 3rd party system
+
+Researching the 3rd party system for which a Conduit connector is built helps
+understand the capabilities and limitations of the system, which in turn results
+in a better connector design and better work organization.
+
+Some questions that typically need to be answered:
+
+1. **How is the data organized in the system?**
+
+   The way data is organized in the system will affect how the connector is
+   configured (e.g. what data is a user able to read or write to) and how the
+   hierarchy is mapped to records and collections in Conduit.
+
+2. **What parameters are needed to connect to the system?**
+
+   We generally recommend using connection strings/URIs, if available. The
+   reason for this is that modifying a connector's parameters is a matter of
+   changing an existing string, and not separate configuration parameters in a
+   connector.
+
+3. **What APIs or drivers are available?**
+
+   The choice is influenced by a number of factors, such as:
+   - Is there a driver available? If a public API and a driver are available, we
+     recommend using a driver, since it's a higher level of abstraction.
+   - In which language should the connector be written?
+   - If the language of choice is Go, is a pure Go driver available? A CGo
+     driver may make the usage and deployment of a connector more complex.
+   - Does the driver expose all the functionalities needed?
+   - How mature is the driver?
+
+4. **What authentication methods should be supported?**
+
+   If multiple authentication methods are available, then a decision needs to be
+   made about the methods that should be implemented first. Additionally, it
+   should be understood how expired credentials should be handled. For example,
+   a connector won't be able to handle an expired password, but a token can
+   sometimes be refreshed.
+
+5. **Can the connector be isolated from other clients of the system?**
+
+   In some cases a connector, as a client using a system, might affect other
+   clients, for example when getting messages from a message broker's queue, the
+   message will be delivered to the connector, while other clients might expect
+   the message.
+
+6. **Are the source/destination specific features supported?**
+
+   Source and destination connectors may have specific requirements (some of
+   them are outlined in later sections). When researching, attention should be
+   paid if those requirements can be fulfilled.
+
+## Development
+
+### Familiarize with the Connector SDK/connector protocol
+
+Conduit's Connector SDK is the Go software development kit for implementing a
+connector for Conduit. If you want to implement a connector in another language
+please check
+the [connector protocol](https://github.com/conduitio/conduit-connector-protocol).
+
+### Write a test pipeline
+
+Having a test pipeline helps see the results of development sooner rather than
+later. When working on a source connector, a file or log destination can be used
+to build a test pipeline. When working on a destination, a file or generator
+source can be used to manually or automatically generate some test data for the
+destination connector.
+
+### Use built-in connectors as examples
+
+Conduit's [built-in connectors](/docs/using/connectors/list)
+are great references that can be used when writing connectors. They are also the
+first ones to be updated with latest SDK and protocol changes.
+
+### Configuration
+
+A connector's configuration is a map in which keys are parameter names and
+values are parameter values. Every source and destination needs to return the
+parameters that it accepts, and, optionally, the validations that need to be run
+on those.
+
+Conduit
+offers [ParamGen](https://github.com/ConduitIO/conduit-commons/tree/main/paramgen),
+a tool that generates the parameters map from a configuration struct. The SDK
+contains a function, `sdk.Util.ParseConfig`, that can parse and validate a
+user's configuration map into the configuration struct.
+
+### Middleware
+
+Conduit's Connector SDK adds default middleware that, by default, enables some
+functionality. A connector developer should be familiar with the middleware,
+especially with
+the [schema related middleware](/docs/using/connectors/configuration-parameters/schema-extraction).
+
+### Source connectors
+
+The following section summarizes best practices that are specific to source
+connectors.
+
+#### Deciding how should a record position look like
+
+Every record read by a source connector has
+a [position](/docs/using/opencdc-record#fields) attached.
+If a connector is stopped after that record is read, its position will be given
+to the connector's `Open()` method the next time it starts. Hence, a position
+needs to contain enough information for a source connector to resume reading
+records from where it exactly stopped.
+
+A position is a slice of bytes that can represent any data structure. In Conduit
+connectors, it's common to see that a position is actually a `struct`, that's
+marshalled into a JSON string.
+
+#### Implementing snapshots
+
+Firstly, it should be clarified if supporting snapshots is a requirement or if
+it's possible to do at all. If a connector is required to support snapshots,
+then it's recommended to make it possible to turn off snapshots through the
+connector configuration.
+
+The following things need to be taken into account when implementing a snapshot
+procedure:
+
+- The snapshot needs to be consistent.
+- The set of the existing data can be quite large.
+- Restarting a connector during a snapshot should **not** require re-reading all
+  the data again. This is because in some destination connectors it may cause
+  data duplication, and it could be a significant performance overhead.
+
+#### Implementing change data capture (CDC)
+
+Change Data Capture (CDC) should be implemented so that the following criteria
+is met:
+
+1. If a snapshot is needed, changes that happened while the
+   snapshot was running should be captured too.
+2. Different types of changes might be possible (new data inserted, existing
+   data updated or deleted).
+3. Changes that happened while a connector was stopped need to be read by the
+   connector when it starts up (assuming that the changes are still present in
+   the source system).
+
+Some source systems may provide a change log (e.g. WAL in PostgreSQL, binlog in
+MySQL, etc.). Others may not and a different way to capture changes is needed.
+Specifically, for connectors written for SQL databases, two patterns can be
+used:
+
+1. **Triggers**
+
+   With triggers, it's possible to capture all types of changes (creates,
+   updates and deletes). A trigger will write the event with necessary
+   metadata (operation performed, timestamp, etc.) into a "trigger table", that
+   will be read by the connector.
+
+   An advantage of this approach is that all types of operations can be
+   captures. However, it may incur a performance penalty.
+2. **A timestamp-based query**
+
+   If the table a source connector is reading from has a timestamp column that
+   is updated whenever a new row is inserted or an existing row is updated, then
+   a query can be used to fetch changes. A disadvantage of this approach is that
+   delete operations cannot be captured.
+
+#### Iterator pattern
+
+A pattern that we found to be useful when writing source connectors is the
+iterator pattern. The basic idea is that a source connector's `Read()` method
+reads records through an iterator:
+
+```go
+type Iterator interface {
+	HasNext() bool
+	Next() opencdc.Record
+}
+
+type Source struct {}
+func (s *Source) Read(ctx context.Context) (opencdc.Record, error) {
+	if s.iterator.HasNext(ctx) {
+        return opencdc.Record{}, sdk.ErrBackoffRetry
+   }
+
+    return s.iterator.Next(ctx)
+}
+```
+
+There are three implementations of the iterator interface:
+
+- `SnapshotIterator`: used when performing a snapshot
+- `CDCIterator`: used in CDC
+- `CombinedIterator`: an iterator that combines the above two and is able to
+  perform a snapshot and then switch to CDC.
+
+The iterator to be used is determined in the source's `Open()` method based on
+the connector's configuration: if a snapshot is required, then a
+`CombinedIterator` will be used, if not, then the `CDCIterator` will be used.
+
+There are multiple advantages of this approach. The source connector remains "
+focused" on the higher level operations, such as configuration, opening the
+connectors, tearing down, etc. The iterators contain the code that deals with
+the source system itself and convert the data into `opencdc.Record`s. They also
+take care of switching from snapshot mode to CDC mode.
+
+### Destination connectors
+
+#### Batching
+
+Batching can considerably improve the write performance and should be considered
+when developing a destination connector. The Connector SDK
+provides [batching middleware](/docs/using/connectors/configuration-parameters/batching)
+that automatically builds batches.
+
+When writing batches into a destination system, the order of records should
+**not** be changed, even if grouping records would be useful in a way. For
+example, in some connectors, it's useful to group records by operation, because
+the destination system has different procedures for those operations.
+
+If a destination connector receives the following batch:
+
+```text
+[create_1, update_1, update_2, create_2]
+```
+
+then this batch can be split into following batches:
+
+```text
+Batch 1: [create_1]
+Batch 2: [update_1, update_2]
+Batch 3: [create_2]
+```
+
+This way, ordering is preserved and the connector can still take advantage of
+batching.
+
+### Acceptance tests
+
+Conduit's Connector SDK contains a set of acceptance tests that verify many of a
+connector's methods, such as:
+
+- Are missing configuration parameters detected?
+- Can a source that is stopped during CDC be successfully restarted?
+- Can a destination write a batch of records?
+
+An example of implemented acceptance tests can be found in
+the [Kafka connector](https://github.com/ConduitIO/conduit-connector-kafka/blob/3233c37392e7249e9d64c9cae0374a0b47ca840f/acceptance_test.go).
+
+### Debugging the connector
 
-Find out more about the template and how to use it in the readme.
+The steps for debugging a standalone connector have been
+described [here](https://github.com/ConduitIO/conduit/discussions/1724#discussioncomment-10178484).
 
-![scarf pixel conduit-site-docs-developing-connectors](https://static.scarf.sh/a.png?x-pxid=3ada0949-fa61-40d6-a44a-76447ea4e39f)
+![scarf pixel conduit-site-docs-developing-connectors](https://static.scarf.sh/a.png?x-pxid=3ada0949-fa61-40d6-a44a-76447ea4e39f)

docs/2-developing/1-processors/index.mdx

@@ -1,5 +1,5 @@
 ---
-title: 'Developing custom processors'
+title: 'Guidelines for writing a processor'
 ---
 
 These are processors you'd write yourself in cases the [Built-in](/docs/using/processors/builtin/) ones don't meet your needs.