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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

Briefly list any new interfaces that will be introduced as part of this proposal or any existing interfaces that will be removed or changed. The purpose of this section is to concisely call out the public contract that will come along with this feature.

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

Describe the new thing you want to do in appropriate detail. This may be fairly extensive and have large subsections of its own. Or it may be a few sentences. Use judgement based on the scope of the change.

1. Use a field named "location" (already used in JobExceptionsInfoWithHistory) that represents TaskManager location in the form of "${hostname}:${port}" in a consistent manner.
2. Update the front-end to adopt the new location field.
3. Rename the column name from "Host" to "Location" on the Web UI to reflect the change that both hostname and port are displayed.
4. 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

• What impact (if any) will there be on existing users?
• If we are changing behavior how will we phase out the older behavior?
• If we need special migration tools, describe them here.
• When will we remove the existing behavior?

The the old "host" fields in REST API is left untouched for compatibility, they can be removed in the next major version (2.0).

Test Plan

Describe in few sentences how the FLIP will be tested. We are mostly interested in system tests (since unit-tests are specific to implementation details). How will we know that the implementation works as expected? How will we know nothing broke?

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

Rejected Alternatives

If there are alternative ways of accomplishing the same thing, what were they? The purpose of this section is to motivate why the design is the way it is and not some other way.

Not applicable.