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Certification in Percona XtraDB Cluster

Percona XtraDB Cluster replicates actions executed on one node to all other nodes in the cluster, and makes it fast enough to appear as if it is synchronous (virtually synchronous).

The following types of actions exist:

  • DDL actions are executed using Total Order Isolation (TOI). We can ignore Rolling Schema Upgrades (ROI).
  • DML actions are executed using normal Galera replication protocol.

Note

This manual page assumes the reader is aware of TOI and MySQL replication protocol.

DML (INSERT, UPDATE, and DELETE) operations effectively change the state of the database, and all such operations are recorded in XtraDB by registering a unique object identifier (key) for each change (an update or a new addition).

  • A transaction can change an arbitrary number of different data objects. Each such object change is recorded in XtraDB using an append_key operation. An append_key operation registers the key of the data object that has undergone change by the transaction. The key for rows can be represented in three parts as db_name, table_name, and pk_columns_for_table (if pk is absent, a hash of the complete row is calculated).

    This ensures that there is quick and short meta information about the rows that this transaction has touched or modified. This information is passed on as part of the write-set for certification to all the nodes in the cluster while the transaction is in the commit phase.

  • For a transaction to commit, it has to pass XtraDB/Galera certification, ensuring that transactions don’t conflict with any other changes posted on the cluster group/channel. Certification will add the keys modified by a given transaction to its own central certification vector (CCV), represented by cert_index_ng. If the said key is already part of the vector, then conflict resolution checks are triggered.

  • Conflict resolution traces the reference transaction (that last modified this item in the cluster group). If this reference transaction is from some other node, that suggests the same data was modified by the other node, and changes of that node have been certified by the local node that is executing the check. In such cases, the transaction that arrived later fails to certify.

Changes made to database objects are bin-logged. This is similar to how MySQL does it for replication with its Master-Slave ecosystem, except that a packet of changes from a given transaction is created and named as a write-set.

Once the client/user issues a COMMIT, Percona XtraDB Cluster will run a commit hook. Commit hooks ensure the following:

  • Flush the binary logs.

  • Check if the transaction needs replication (not needed for read-only transactions like SELECT).

  • If a transaction needs replication, then it invokes a pre-commit hook in the Galera ecosystem. During this pre-commit hook, a write-set is written in the group channel by a replicate operation. All nodes (including the one that executed the transaction) subscribe to this group-channel and read the write-set.

  • gcs_recv_thread is the first to receive the packet, which is then processed through different action handlers.

  • Each packet read from the group-channel is assigned an id, which is a locally maintained counter by each node in sync with the group. When any new node joins the group/cluster, a seed-id for it is initialized to the current active id from group/cluster.

    There is an inherent assumption/protocol enforcement that all nodes read the packet from a channel in the same order, and that way even though each packet doesn’t carry id information, it is inherently established using the locally maintained id value.

Common Situation

The following example shows what happens in a common situation. act_id is incremented and assigned only for totally ordered actions, and only in primary state (skip messages while in state exchange).

rcvd->id = ++group->act_id_;

Note

This is an amazing way to solve the problem of the id coordination in multi-master systems. Otherwise a node will have to first get an id from central system or through a separate agreed protocol, and then use it for the packet, thereby doubling the round-trip time.

Conflicts

The following happens if two nodes get ready with their packet at same time:

  • Both nodes will be allowed to put the packet on the channel. That means the channel will see packets from different nodes queued one behind another.

  • The following example shows what happens if two nodes modify same set of rows. Nodes are in sync until this point:

    create -> insert (1,2,3,4)
    
    • Node 1: update i = i + 10;
    • Node 2: update i = i + 100;

    Let’s associate transaction ID (trx-id) for an update transaction that is executed on Node 1 and Node 2 in parallel. Although the real algorithm is more involved (with uuid + seqno), it is conceptually the same, so we are using trx_id.

    • Node 1: update action: trx-id=n1x
    • Node 2: update action: trx-id=n2x

    Both node packets are added to the channel, but the transactions are conflicting. The protocol says: FIRST WRITE WINS.

    So in this case, whoever is first to write to the channel will get certified. Let’s say Node 2 is first to write the packet, and then Node 1 makes changes immediately after it.

    Note

    Each node subscribes to all packages, including its own package.

    • Node 2 will see its own packet and will process it. Then it will see the packet from Node 1, try to certify it, and fail.

    • Node 1 will see the packet from Node 2 and will process it.

      Note

      InnoDB allows isolation, so Node 1 can process packets from Node 2 independent of Node 1 transaction changes

      Then Node 1 will see its own packet, try to certify it, and fail.

      Note

      Even though the packet originated from Node 1, it will undergo certification to catch cases like these.

Resolving Certification Conflicts

The certification protocol can be described using the previous example. The central certification vector (CCV) is updated to reflect reference transaction.

  • Node 2 sees its own packet for certification, adds it to its local CCV and performs certification checks. Once these checks pass, it updates the reference transaction by setting it to n2x.

    Node 2 then gets the packet from Node 1 for certification. The packet key is already present in CCV, with the reference transaction set it to n2x, whereas write-set proposes setting it to n1x. This causes a conflict, which in turn causes the transaction from Node 1 to fail the certification test.

  • Node 1 sees the packet from Node 2 for certification, which is then processed, the local CCV is updated, and the reference transaction is set to n2x.

    Using the same case as explained above, Node 1 certification also rejects the packet from Node 1.

This suggests that the node doesn’t need to wait for certification to complete, but just needs to ensure that the packet is written to the channel. The applier transaction will always win and the local conflicting transaction will be rolled back.

The following example shows what happens if one of the nodes has local changes that are not synced with the group:

create (id primary key) -> insert (1), (2), (3), (4);
node-1: wsrep_on=0; insert (5); wsrep_on=1
node-2: insert(5).

The insert(5) statement will generate a write-set that will then be replicated to Node 1. Node 1 will try to apply it but will fail with duplicate-key-error, because 5 already exist.

XtraDB will flag this as an error, which would eventually cause Node 1 to shutdown.

Incrementing GTID

GTID is incremented only when the transaction passes certification, and is ready for commit. That way errant packets don’t cause GTID to increment.

Also, group packet id is not confused with GTID. Without errant packets, it may seem that these two counters are the same, but they are not related.

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