The aim of this RFC is to address the following list of problems formulated at MRG planning meeting:   - protocol backward compatibility to enable cluster upgrade w/o     downtime   - consistency of data on replica and leader   - switch from leader to replica without data loss   - up to date replicas to run read-only requests   - ability to switch async replicas into sync ones   - guarantee of rollback on leader and sync replicas   - simplicity of cluster orchestration   What this RFC is not:     - high availability (HA) solution with automated failover, roles     assignments an so on   - master-master configuration support   Quorum-confirmed commit details   The main idea behind the proposal is to reuse existent machinery as much                                                   as possible. It will ensure the well-tested and proven functionality across many instances in MRG and beyond is used. The transaction rollback mechanism is in place and works for WAL write failure. If we substitute  the WAL success with a new situation which is named 'quorum’ later in this  document then no changes to the machinery is needed. The same is true for  snapshot machinery that allows to create a copy of the database in memory for the whole period of snapshot file write. Adding quorum here also minimizes changes.   Currently replication represented by the following scheme:       To introduce the 'quorum' we have to receive confirmation from replicas to make a decision on whether the quorum is actually present. Leader collects necessary amount of replicas confirmation plus its own WAL success. This state is named 'quorum' and gives leader the right to complete the customers' request. So the picture will change to:     The quorum should be collected as a table for a list of transactions  waiting for quorum. The latest transaction that collects the quorum is considered as complete, as well as all transactions prior to it, since all transactions should be applied in order. Leader writes a ‘quorum’ message to the WAL and it is delivered to Replicas.   Replica should report a positive or a negative result of the TXN to the  Leader via the IPROTO explicitly to allow Leader to collect the quorum or anti-quorum for the TXN. In case negative result for the TXN received from minor number of Replicas, then Leader has to send an error message to each Replica, which in turn has to disconnect from the replication the same way as it is done now in case of conflict.   In case Leader receives enough error messages to do not achieve the quorum it should write the ‘rollback’ message  in the WAL. After that Leader and Replicas will perform the rollback  for all TXN that didn’t receive quorum.   Recovery and failover.   Tarantool instance during reading WAL should postpone the commit until the quorum is read. In case the WAL eof is achieved, the instance should keep rollback for all transactions that are waiting for a quorum entry until the role of the instance is set. In case this instance become a  Replica there are no additional actions needed, sine all info about  quorum/rollback will arrive via replication. In case this instance is  assigned a Leader role, it should write ‘rollback’ in it’s WAL and perform rollback for all transactions waiting for a quorum.   In case of a Leader failure a Replica with the biggest LSN with former leader’s ID is elected as a new leader. The replica should record ‘rollback’ in its WAL which effectively means that all transactions without quorum should be rolled back. This rollback will be delivered to all replicas and they will perform rollbacks of all transactions waiting for quorum.   Snapshot generation.   We also can reuse current machinery of snapshot generation. Upon receiving a request to create a snapshot an instance should request a readview for the current commit operation. Although start of the snapshot generation should be postponed until this commit operation receives its quorum. In case operation is rolled back, the snapshot generation should be aborted and restarted using current transaction after rollback  is complete.   After snapshot is created the WAL should start from the first operation that follows the commit operation snapshot is generated for. That means WAL will contain a quorum message that refers to a transaction that is not present in the WAL. Apparently, we have to allow this for the case quorum refers to a transaction with LSN less than the first entry in  t he WAL and only once.   Asynchronous replication.   Along with synchronous Replicas the cluster can contain asynchronous Replicas. That means async Replica doesn’t reply to the Leader with errors since they’re not contributing into quorum. Still, async Replicas have to follow the new WAL operation, such as keep rollback info until ‘quorum’ message is received. This is essential for the case of ‘rollback’ message appearance in the WAL. This message assumes Replica is able to perform all necessary rollback by itself.  Cluster information should contain explicit notification of each Replica operation mode.    Synchronous replication enabling.   Synchronous operation can be required for a set of spaces in the data scheme. That means only transactions that contain data modification for these  spaces should require quorum. Such transactions named synchronous. As soon as last operation of synchronous transaction appeared in Leader’s WAL, it will cause all following transactions — no matter if they are synchronous or not — wait for the quorum. In case quorum is not achieved the ‘rollback’ operation will cause rollback of all transactions after the synchronous one. It will ensure the consistent state of the data both on Leader and Replicas.  In case user doesn’t require synchronous operation for any space then  no changes to the WAL generation and replication will appear.   Cluster description should contain explicit attribute for each Replica to denote it participates in synchronous activities. Also the description should contain criterion on how many Replicas responses are  needed to achieve the quorum.