Status

Current state: Under discussion.

Discussion thread: here  (Not happening yet)

JIRA: here

Motivation

Instantiating a new client may result in a fatal failure if the bootstrap server cannot be resolved due to misconfiguration or transient network issues such as slow DNS. This is suboptimal because of the fact that it might take a long time for the address to become available at the DNS server and users will need to continue to retry.  Also, the ConfigException exception type does not accurately reflect the root cause of the problem, which makes it hard to handle this failure case.  We think it is reasonable to allow users to have a grace period to retry if the address cannot be resolved immediately. Also, poisoning the clients during the construction can be obstructive; I think it is better to fail the client on its first attempt to connect to the network.

Proposed Changes

This KIP proposes moving bootstrapping logic from the constructor to the NetworkClient poll for two purposes,

1. not failing the client upon instantiation. In many cases, this behavior also kills the app, which might not be desirable.

2. piggybacking onto the client poll is a more natural way to retry.

We propose to add a new configuration option for timing out the bootstrapping process, a new exception type for handling bootstrap-related issues, and additional logging to aid in diagnosing bootstrapping failures.

Public Facing Changes

• Timeout Configuration: bootstrap.connection.timeout.ms
• Exception: BootstrapConnectionException extends KafkaException
• Logging: log.WARN("Unable to bootstrap after {} ms.", elapsedMs)

Internal Changes

• Client Constructor: The constructor will only parse the bootstrap configuration.

• NetworkClient:

  • Bootstrapping will now occur in the poll method before attempting to update the metadata. This includes resolving the addresses and bootstrapping the metadata.
  • An error message will be logged in the event of a failed bootstrap process.
  • If the timeout exceeds, a non-retriable BootstrapConnectionException will be thrown.
• Consumer, Producer, and Admin Clients: The bootstrap code will be changed.

New Configuration

bootstrap.connection.timeout.ms

The proposed configuration specifies the maximum amount of time clients can spend trying to establish a connection to the bootstrap server and resolve its IP address. If the connection cannot be established and resolved within this time, a BootstrapConnectionException will be thrown.

Note that the default value for this configuration option is open for discussion. It can be set to 0, which is the same as the current behavior of exiting upon the first failure.

New Error

Name: BootstrapConnectionException extends KafkaException

Message: "Unable to establish a connection to the bootstrap server in {}ms."

Type: Non-retriable.

Compatibility, Deprecation, and Migration Plan

Compatibility

• There shouldn't be a compatibility problem as the bootstrap logic changes only affect the failure scenario.
• The user can use a timeout (bootstrap.connection.timeout.ms) of 0 to mimic the current behavior, i.e. fatal upon the first failure.

Deprecation

• There's no deprecation plan

Migration

• There's no migration plan

Case Study

In this section, I outlined how clients can react to bootstrap failures. In particular, I want to cover two common cases:

1. Misconfiguration or non-transient issues with the network
2. Transient network issues, e.g., slow DNS resolution.

KafkaConsumer

Case 1: Unable to connect to the bootstrap

When the bootstrap timeout expires, the client will throw a BootstrapConnectionException.

Case 2: Transient Network Issue

consumer poll won't return any record until the client has been bootstrapped. If the issue cannot be resolved within the bootstrap timeout, a BootstrapConnectionException will be thrown.

KafkaProducer

Case 1: Unable to connect to the bootstrap

The BootstrapConnectionException will be thrown in send() and partitionsFor() when the bootstrap timeout expires. If the max.block.ms elapsed before the timeout expires, a TimeoutException will be thrown instead.

Case 2: Transient Network Issue

The send() and partitionsFor() methods will be blocked on bootstrap until either the max.block.ms or the bootstrap timeout elapses.

AdminClient

Case 1: Unable to connect to the bootstrap

The API call results will either timeout if the request times out first or be completed exceptionally with a BootstrapConnectionException

Case 2: Transient Network Issue

If there is a transient network issue, such as a transient DNS failure, the user won't be able to get the results back until the bootstrap issue is resolved.  Meanwhile, the API calls will expire.

Test Plan

1. NetworkClient

   1. Test DNS resolution upon its initial poll

   2. Test if the right exception type is thrown

2. Existing clients (Consumer, Producer, AdminClient)

   1. Test successful bootstrapping upon retrying

Rejected Alternatives

1. Maintain the current code behavior and add a retry loop with a timeout.

   1. Pros: Same logic, less code change.

   2. Cons: Do people want object instantiation to block? I don't think it is a good idea.

2. Throw DNS resolution upon failing but no retry

   1. Pros: No additional config is needed

   2. Cons: This is a behavioral change, and the application owner might need to rewrite the exception handling, i.e. catching the DNS failure logic.

3. No retry.  The network client will continue to retry until it is interrupted.
   
   1. Pros: No compatibility break. No additional exception handling logic, the network client will just log the error and continue to retry upon the next poll
   2. Cons: I think we should have some failure mechanism to notify users.