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

Motivation

The TaskManager location of subtasks are important for identifying TM-related problems. There are a number of places in REST API and Web UI where TaskManager location is returned/displayed: SubtaskCurrentAttemptDetailsHandler, JobVertexTaskManagersHandler and JobExceptionsHandler to name a few. The current implementation to represent TaskManager location has the following problems:

1. Only hostname is provided to represent TaskManager location in some places (e.g. SubtaskCurrentAttemptDetailsHandler). However, in a containerized era, it is common to have multiple TMs on the same host, and port info is crucial to distinguish different TMs.
2. Inconsistent naming of the field to represent TaskManager location: "host" is used in most places but "location" is also used in JobExceptions-related places.
3. Inconsistent semantics of the "host" field: The semantics of the host field are inconsistent, sometimes it denotes hostname only while in other times it denotes hostname + port (which is also inconsistent with the name of "host"). To provide some examples:

As a result, users find it difficult to identify TM-related problems and confusing to deal with the inconsistent naming and semantics of the fields in REST API. Similar issues have been raised independently multiple times, trying to address it:

• [FLINK-24957] show `host:port` information in `subtasks` tab - ASF JIRA (apache.org)
• [FLINK-25371] Include data port as part of the host info for subtask detail panel on Web UI - ASF JIRA (apache.org)
• [FLINK-32984] Display the host port information on the suttasks page of SubtaskExecution - ASF JIRA (apache.org) 

Public Interfaces

Any handlers/interfaces in the entire REST API and Web UI that contains TaskManager location info.

• SubtasksAllAccumulatorsHandler
• SubtasksTimesHandler
• SubtaskCurrentAttemptDetailsHandler (corresponds to the subtasks details panel of a job vertex on UI)

• JobVertexTaskManagersHandler (corresponds to theTaskManger details panel of a job vertex on UI)

• JobExceptionsHandler

Proposed Changes

1. Add a new string formatter method getEndpoint() to TaskManagerLocation and ArchivedTaskManagerLocation that prints in the form of "${hostname}:${port}" to align the string formatter used by REST API.
2. Use a field named "endpoint" that represents TaskManager endpoint (host + port) using the newly added string formatter method.
3. Update the front-end to adopt the new endpoint field.
4. Rename the column name from "Host" to "Endpoint" on the Web UI to reflect the change that both hostname and port are displayed.
5. Keep the old "host" field untouched for compatibility. Notice that the info stored in the old "host" field is inconsistent, sometimes only host is stored, sometimes host + port is stored, we 'll leave them as they were with a few comments to keep being compatible.

Compatibility, Deprecation, and Migration Plan

The the old "host" fields in REST API is left untouched for compatibility, they will be marked as deprecated and can be removed in the next major version (after being kept for at least 2 minot releases).

Test Plan

UTs and simple human-involved tests on UI should cover it well.

Rejected Alternatives

Not applicable.