Migrating from Kafka to Raft

Note: this document presumes a high degree of expertise with channel configuration update transactions. As the process for migration involves several channel configuration update transactions, do not attempt to migrate from Kafka to Raft without first familiarizing yourself with the Add an Organization to a Channel tutorial, which describes the channel update process in detail.

For users who want to transition channels from using Kafka-based ordering services to Raft-based ordering services, v1.4.2 allows this to be accomplished through a series of configuration update transactions on each channel in the network.

This tutorial will describe this process at a high level, calling out specific details where necessary, rather than show each command in detail.

Assumptions and considerations

Before attempting migration, take the following into account:

  1. This process is solely for migration from Kafka to Raft. Migrating between any other orderer consensus types is not currently supported.
  2. Migration is one way. Once the ordering service is migrated to Raft, and starts committing transactions, it is not possible to go back to Kafka.
  3. Because the ordering nodes must go down and be brought back up, downtime must be allowed during the migration.
  4. Recovering from a botched migration is possible only if a backup is taken at the point in migration prescribed later in this document. If you do not take a backup, and migration fails, you will not be able to recover your previous state.
  5. All channels must be migrated during the same maintenance window. It is not possible to migrate only some channels before resuming operations.
  6. At the end of the migration process, every channel will have the same consenter set of Raft nodes. This is the same consenter set that will exist in the ordering system channel. This makes it possible to diagnose a successful migration.
  7. Migration is done in place, utilizing the existing ledgers for the deployed ordering nodes. Addition or removal of orderers should be performed after the migration.

High level migration flow

Migration is carried out in five phases.

  1. The system is placed into a maintenance mode where application transactions are rejected and only ordering service admins can make changes to the channel configuration.
  2. The system is stopped, and a backup is taken in case an error occurs during migration.
  3. The system is started, and each channel has its consensus type and metadata modified.
  4. The system is restarted and is now operating on Raft consensus; each channel is checked to confirm that it has successfully achieved a quorum.
  5. The system is moved out of maintenance mode and normal function resumes.

Preparing to migrate

There are several steps you should take before attempting to migrate.

  • Design the Raft deployment, deciding which ordering service nodes are going to remain as Raft consenters. You should deploy at least three ordering nodes in your cluster, but note that deploying a consenter set of at least five nodes will maintain high availability should a node goes down, whereas a three node configuration will lose high availability once a single node goes down for any reason (for example, as during a maintenance cycle).
  • Prepare the material for building the Raft Metadata configuration. Note: all the channels should receive the same Raft Metadata configuration. Refer to the Raft configuration guide for more information on these fields. Note: you may find it easiest to bootstrap a new ordering network with the Raft consensus protocol, then copy and modify the consensus metadata section from its config. In any case, you will need (for each ordering node):
    • hostname
    • port
    • server certificate
    • client certificate
  • Compile a list of all channels (system and application) in the system. Make sure you have the correct credentials to sign the configuration updates. For example, the relevant ordering service admin identities.
  • Ensure all ordering service nodes are running the same version of Fabric, and that this version is v1.4.2 or greater.
  • Ensure all peers are running at least v1.4.2 of Fabric. Make sure all channels are configured with the channel capability that enables migration.
    • Orderer capability V1_4_2 (or above).
    • Channel capability V1_4_2 (or above).

Entry to maintenance mode

Prior to setting the ordering service into maintenance mode, it is recommended that the peers and clients of the network be stopped. Leaving peers or clients up and running is safe, however, because the ordering service will reject all of their requests, their logs will fill with benign but misleading failures.

Follow the process in the Add an Organization to a Channel tutorial to pull, translate, and scope the configuration of each channel, starting with the system channel. The only field you should change during this step is in the channel configuration at /Channel/Orderer/ConsensusType. In a JSON representation of the channel configuration, this would be .channel_group.groups.Orderer.values.ConsensusType​.

The ConsensusType is represented by three values: Type, Metadata, and State, where:

  • Type is either kafka or etcdraft (Raft). This value can only be changed while in maintenance mode.
  • Metadata will be empty if the Type is kafka, but must carry valid Raft metadata if the ConsensusType is etcdraft. More on this below.
  • State is either NORMAL, when the channel is processing transactions, or MAINTENANCE, during the migration process.

In the first step of the channel configuration update, only change the State from NORMAL to MAINTENANCE. Do not change the Type or the Metadata field yet. Note that the Type should currently be kafka.

While in maintenance mode, normal transactions, config updates unrelated to migration, and Deliver requests from the peers used to retrieve new blocks are rejected. This is done in order to prevent the need to both backup, and if necessary restore, peers during migration, as they only receive updates once migration has successfully completed. In other words, we want to keep the ordering service backup point, which is the next step, ahead of the peer’s ledger, in order to be able to perform rollback if needed. However, ordering node admins can issue Deliver requests (which they need to be able to do in order to continue the migration process).

Verify that each ordering service node has entered maintenance mode on each of the channels. This can be done by fetching the last config block and making sure that the Type, Metadata, State on each channel is kafka, empty (recall that there is no metadata for Kafka), and MAINTENANCE, respectively.

If the channels have been updated successfully, the ordering service is now ready for backup.

Backup files and shut down servers

Shut down all ordering nodes, Kafka servers, and Zookeeper servers. It is important to shutdown the ordering service nodes first. Then, after allowing the Kafka service to flush its logs to disk (this typically takes about 30 seconds, but might take longer depending on your system), the Kafka servers should be shut down. Shutting down the Kafka brokers at the same time as the orderers can result in the filesystem state of the orderers being more recent than the Kafka brokers which could prevent your network from starting.

Create a backup of the file system of these servers. Then restart the Kafka service and then the ordering service nodes.

Switch to Raft in maintenance mode

The next step in the migration process is another channel configuration update for each channel. In this configuration update, switch the Type to etcdraft (for Raft) while keeping the State in MAINTENANCE, and fill in the Metadata configuration​. It is highly recommended that the Metadata configuration​ be identical​ on all channels. If you want to establish different consenter sets with different nodes, you will be able to reconfigure the Metadata configuration​ after the system is restarted into etcdraft mode. Supplying an identical metadata object, and hence, an identical consenter set, means that when the nodes are restarted, if the system channel forms a quorum and can exit maintenance mode, other channels will likely be able do the same. Supplying different consenter sets to each channel can cause one channel to succeed in forming a cluster while another channel will fail.

Then, validate that each ordering service node has committed the ConsensusType change configuration update by pulling and inspecting the configuration of each channel.

Note: the transaction that changes the ConsensusType must be the last configuration transaction before restarting the nodes (in the next step). If some other configuration transaction happens after this step, the nodes will most likely crash on restart, or result in undefined behavior.

Restart and validate leader

Note: exit of maintenance mode must be done after restart.

After the ConsensusType update has been completed on each channel, stop all ordering service nodes, stop all Kafka brokers and Zookeepers, and then restart only the ordering service nodes. They should restart as Raft nodes, form a cluster per channel, and elect a leader on each channel. Make sure to validate that a leader has been elected on each channel by inspecting the node logs (you can see what to look for below). This will confirm that the process has been completed successfully.

When a leader is elected, the log will show, for each channel:

"Raft leader changed: 0 -> ​node-number​ ​channel=​channel-name​
node=​node-number​ ​"

For example:

2019-05-26 10:07:44.075 UTC [orderer.consensus.etcdraft] serveRequest ->
INFO 047 Raft leader changed: 0 -> 1 channel=testchannel1 node=2

In this example ​node 2​ reports that a leader was elected (the leader is ​node 1​) by the cluster of channel ​testchannel1​.

Switch out of maintenance mode

Perform another channel configuration update on each channel (sending the config update to the same ordering node you have been sending configuration updates to until now), switching the State from MAINTENANCE to NORMAL. Start with the system channel, as usual. If it succeeds on the ordering system channel, migration is likely to succeed on all channels. To verify, fetch the last config block of the system channel from the ordering node, verifying that the State is now NORMAL. For completeness, verify this on each ordering node.

When this process is completed, the ordering service is now ready to accept all transactions on all channels. If you stopped your peers and application as recommended, you may now restart them.

Abort and rollback

If a problem emerges during the migration process before exiting maintenance mode, simply perform the rollback procedure below.

  1. Shut down the ordering nodes and the Kafka service (servers and Zookeeper ensemble).
  2. Rollback the file system of these servers to the backup taken at maintenance mode before changing the ConsensusType.
  3. Restart said servers, the ordering nodes will bootstrap to Kafka in maintenance mode.
  4. Send a configuration update exiting maintenance mode to continue using Kafka as your consensus mechanism, or resume the instructions after the point of backup and fix the error which prevented a Raft quorum from forming and retry migration with corrected Raft configuration Metadata.

There are a few states which might indicate migration has failed:

  1. Some nodes crash or shutdown.
  2. There is no record of a successful leader election per channel in the logs.
  3. The attempt to flip to NORMAL mode on the system channel fails.