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Status

Current state: Under Discussion

Discussion thread: here [Change the link from the KIP proposal email archive to your own email thread]

JIRA: here [Change the link from KAFKA-1 to your own ticket]

Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).

Motivation

During the discussion for KIP-980, which proposed the creation of connectors in STOPPED state, there was a suggestion to also allow setting the initial offset for a connector in the connector creation API. The proposal was deemed valid (point no.4) but was deferred to a future KIP. This KIP proposes to implement that change.

This feature will be a helpful to users who want create a connector and start from a specific offset.

To do this today, user will have to

1. first create a connector in STOPPED state and check the response for errors
2. alter the offsets for said connector and check the response for errors
3. finally set the connector state to RUNNING.

Public Interfaces

"POST /connectors" REST API endpoint

A new optional field "initial_offset"  will be added to the request body format for the POST /connectors endpoint. The value for this field has to adhere to format specified in KIP-875 else a 400 Bad Request response will be returned. Please note that sink and source connectors have different schemas for offset.

If the field is omitted in the request body, the current behaviour of creating connectors will be preserved. An example request body would look like:

CreateConnectorRequest

This method will be supported in both the distributed mode as well as the standalone mode.

KIP-980 had introduced an option to pass JSON config to bin/connect-standalone.sh. This new field will be supported there as well.

Proposed Changes

When Connect runtime receives a request with initial_offset to create a connector via POST /connectors, it will perform the following steps

1. Create a connector in STOPPED state
2. Validate the offset using the same checks performed while altering connector offsets (PATCH /$connector/offsets ) as specified in KIP-875
3. Update the offset of the connector created in step 1 to the value of “initial_offset”
4. If “initial_state” property of the connector is set in the request, the runtime will set the state of the connector to that value. If “initial_state” is not specified, the runtime will set the state of the connector to RUNNING

If Connect runtime encounters an error in any of these steps, it will delete the connector and return an error response.

Compatibility, Deprecation, and Migration Plan

Since “initial_offset” is an optional field and when unspecified will default to current behaviour, we do not foresee a compatibility issue while downgrading the Connect cluster. This new feature will use the current OffsetStorageWriter. Hence the connector will be indistinguishable from one which started with no initial offset specified and in due course committed an offset to the store.

Test Plan

The following scenarios will be tested using Unit, Integration or system tests

• Create connector with initial_offset in standalone mode
• Create connector with initial_state and initial_offset in standalone mode
• Create connector with initial_offset in distributed mode
• Create connector with initial_state and initial_offset in distributed mode
• Failure while creating connector with initial_offset will clean up and return error response
• Standalone mode cli accepts initial_offset in JSON config file

Rejected Alternatives

Restrict initial_offset usage to cases where initial_state == STOPPED

We could enforce intial_offset usage only to cases when a connector is created in STOPPED state. This would’ve reduced one point of failure while performing the action. However,  we feel that this will impact the usability of this new API and will not offer any major benefit from the steps which a user has to run currently , namely, creating a connector in STOPPED state , altering the offset and then setting the state to RUNNING.

Validate initial_offset before creating the connector in STOPPED state

We could’ve validated the initial_offset _before_ creating the connector and thereby avoid a clean-up if the offset is not valid. However, validating offset after connector creation helps in the following ways

• Multiple concurrent connector creation requests with the same connector name with and without initial_offset : this would’ve resulted in a race between the request which gets to write the offset first and the request which writes the connector state first
• Reuse existing alter offset code : This code assumes that the connector already exists and is in STOPPED state