Table of Contents

Status

Current state: Under discussion.Accepted

Discussion thread: here  (Not happening yet)

...

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 for several reasons, including 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, and it doesn't allow users to retry to resolve transient issues.  It would be more effective to provide a grace period for retry attempts before ultimately failingwhich 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 . This change ensures that the client doesn't fail at startup due to issues like misconfiguration or network disruptions and allows for retries upon subsequent poll() invocations. The proposed updates include introducing 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.connectionresolve.timeout.ms
• Exception: BootstrapConnectionException BootstrapResolutionException 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 BootstrapResolutionException will be thrown.
• Consumer, Producer, and Admin Clients: The bootstrap code will be changed.

New Configuration

bootstrap.connectionresolve.timeout.ms

The proposed configuration specifies the maximum amount of time clients can spend trying to establish a connection to resolve for the bootstrap server and resolve its IP address. If the connection resolution cannot be established and resolved completed within this timetimeframe, a BootstrapConnectionException BootstrapResolutionException 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 BootstrapResolutionException extends KafkaException

Message: "Unable to establish a connection to the bootstrap server in {}ms.Couldn't resolve server {} from {} as DNS resolution failed for {}"

Type: Non-retriable.

Compatibility, Deprecation, and Migration Plan

• Client Behaviors

  • Bootstrap upon first NetworkClient.poll() instead of in the constructor.

  • Bootstrapping is retriable.

  • KafkaConsumer: Users can retry bootstrapping via poll() if it fails. Each retry will be bounded by either the poll timer or the bootstrap timer, whichever expires first.

  • KafkaAdminClient: Bootstrap exception is thrown when user tries to materialize the result future. The retry is bounded by the API timeout.

  • KafkaProducer: Bootstrap will be done in the background thread. If the client has not been bootstrapped when the user attempts to send a message, it can wait up to the maximum block timer or until the bootstrap is complete, whichever expires first.

  Exception Handling

  • Failed DNS resolution will result in NetworkException

Case Study

Compatibility

• Failed DNS resolution throws BootstraResolutionException: Users are expected to catch the error or the client will be poisioned
• Users who tried to catch ConfigException for DNS resolution error will no longer need this logic.

Deprecation

• There's no deprecation plan

Migration

• There's no migration plan

Case Study

Every time the network client attempts to bootstrap and fails, a warning message will be logged.  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 (For example: misconfiguration)Non-transient case

When the bootstrap timeout expires, the client will throw a BootstrapConnectionException on poll()BootstrapResolutionException.

Case 2: Transient Network Issue (For example: transient DNS failure)Issue

consumer poll won't return any record until the client has been bootstrappedIf there is a transient network issue, such as slow DNS resolution, poll() will continue to return an empty record until the issue is resolved. If the issue cannot be resolved within the bootstrap timeout, a BootstrapConnectionException will be thrown on poll().

KafkaProducer

Case 1: Unable to connect to the bootstrap (For example: misconfiguration)Non-transient case

The BootstrapResolutionException 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 (For example: transient DNS failure)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 (For example: misconfiguration)Non-transient case

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

Case 2: Transient Network Issue (For example: transient DNS failure)Issue

The 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 address is resolved.  Meanwhile, the API calls will expirecan expire.

What should users do after the timeout expires?

The exception is meant to be fatal, so the user should check their network setup, configuration, or adjust the timeout.

The user can continue to retry, but this exception is meant to alert user to take action upon failing to bootstrap.

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

We've discussed many alternatives.  Eventually, we asked ourselves what's the goal of this KIP, i.e., giving people a chance to retry on DNS resolution without poisoning the client. Which came down to two resolutions: 1. giving people a configurable timeout, and 2. adding a fatal error to alert the user.

Here are the 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 users want object instantiation to blockto be blocked on instantiating the client? I don't think it is a good like this 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.
4. Making BootstrapResolutionException retriable
   1. Pros: For the transient case, we might not even need a timeout, people are expected to retry on catching this exception
   2. Cons: Then we reply on alerting mechanism to alert users the issue. If it is indeed a configuration issue, then it is harder to discover
5. Combine DNS resolution and connection into a single timeout
   1. Pros: Using a single timer to account for the connection time.
   2. Cons: Should we make connection retry fatal after the timeout? Maybe not.
6. 5min default timeout
   1. We've decided to reduce it to 2min to stay coherent to the delivery.timeout.ms
   2. 5min can be too long