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Cluster: Cluster-Wide Transactions
Cluster transactions are a way to ensure that certain operations will favor consistency over availability in the CAP theorem.
This page contains:
- Why do we need Cluster Wide Transactions
- How cluster transaction works described by the flow of a cluster transaction request example.
- Cluster Transaction Properties
- Concurrent Cluster Wide and Single-Node Transactions
- Failure modes in cluster wide transactions
- Debug cluster wide transaction
Code examples and client API can be found here.
Why Cluster Wide Transactions
Usually, RavenDB's uses the multi master model and apply a transaction on a single node first then asynchronously replicates the data to other members in the cluster. This ensures that even in the presence of network partitions or hard failures RavenDB is able to accept writes and safely keep them.
The downside of the multi master model is that certain error modes can cause two clients to try to modify the same set of documents on two different database nodes. That can cause Conflicts and make it hard to provide certain guarantees to the application. For example, ensuring the uniqueness of a user's email in a distributed cluster. Just checking for the existence of the email is not sufficient, it might be that two clients talking to separate database nodes and both of them checking that the user does not exists and then create what will end up as a duplicate user.
To handle this (and similar) scenario, RavenDB offers the cluster wide transaction feature. This allow you to explicitly state that you want a particular interaction with the database to favor consistency over availability and ensure that changes are going to be applied in an identical manner across the cluster even in the presence of failures and network partitions.
However, in order to ensure that, RavenDB requires that a cluster wide transaction will contact at least a majority of the voting nodes in the cluster. If it is not able to do so, the cluster wide transaction will fail.
For the rest of this document we are going to refer to single node transactions, applied on a single node and then disseminated using async replication vs. cluster wide transactions that are accepted by a majority of the nodes in the cluster and then applied on each of them.
How Cluster Transaction Works
- A request sent from the client via SaveChanges() will generate a Raft Command and the server will wait for a consensus on it.
- When consensus is achieved, each node will validate the compare exchange values first, if this fails the transaction is rolled back. From the nature of the Raft consensus algorithm the cluster wide transaction should be either eventually accepted on all nodes or fail on all of them.
- Once the validation has passed the request will be stored on the local cluster state machine of every node to be processed in an asynchronous manner by the relevant database.
- The relevant database notice that it have pending cluster transactions and will start to execute them. Since order matters a failure at this stage will halt the cluster transaction execution until it fixed. The possible failure modes for this scenario are listed below.
- Every document that has been added by the cluster transaction gets the
RAFT:int64-sequential-numberChange Vector and will have a precedence if conflict arises between that document and a document from a regular transaction.
- After the database has executed the requested transaction a response will be returned to the client.
On success, the client receives the transaction's Raft Index which will be added to any future requests, so performing an operation against any other node will wait for that index to be applied first, ensuring order of operations.
- In the background the Cluster Observer will track the completed cluster transactions and order to remove the local cluster state machine only when it has been successfully committed on all of the database nodes.
Cluster Transaction Properties
The Cluster transaction feature allows to perform consistent cluster wide ACID transactions and can be composed from two optional parts:
- Compare Exchange values, which will be validated and executed by the cluster.
- Store/Delete operations on documents, which are executed by the database nodes after the transaction has been accepted.
Atomicity - After having a quorum for the cluster transaction request by raft and successful concurrency check for the compare exchange values, it is guaranteed to be executed. Failure during the quorum or the concurrency check will roll back the transaction, while failure during the commit of the documents will halt any further cluster transactions execution on the database until that failure is remedied (failure mode for the documents commits are described later here).
Consistency - Is guaranteed on the requested node. The node will complete the request only when the transaction is completed and the documents are persistent on the node. The response to the client will contain the cluster transaction Raft Index so it be added to any future request in order to ensure that the node has committed that transaction before serving the client.
Durability - Once the transaction has been accepted it is guaranteed to run on all the database's node, even in the case of system (or even cluster-wide) restarts or failures.
Concurrent Cluster and Single-Node Transactions
Case 1: Multiple concurrent cluster transactions
Optimistic concurrency for cluster wide transactions is handled using the compare exchange feature. The transaction compare exchange operations are validated and if they can't be executed because the values has changed since the transaction was initiated, the entire transaction is aborted and an error is returned to the client.
Optimistic concurrency at the document level is not supported for cluster wide transactions. Compare exchange operations should be used to ensure
consistency in that regard. Concurrent cluster wide transactions are guaranteed to appear as if they are run one at time (
serializable isolation level).
Cluster-wide transactions may only contain
DELETE commands. This is required to ensure that we can apply the transaction to each of the
database nodes without regards to the current node state.
If the concurrency check of the compare exchange has passed, the transaction will proceed and will be committed on all the database nodes.
Case 2: Concurrent cluster and non-cluster transaction
When mixing cluster wide transactions and single node transactions, you need to be aware of the rules RavenDB uses to resolve conflicts between them.
Documents changed by the cluster wide transactions will always have precedence in such a conflict and overwrite changes made in a single node transaction. It is common to use cluster wide transactions for certain high value operations such as creation of a new user, sale of a product with strictly limited amount, etc. and use single transactions for the common case.
A single node transaction that operates on data that has been modified by a cluster wide transaction will operate as usual, as the cluster wide transaction has already been applied (either directly on the node or via replication, see sidebar) the cluster wide transaction will not be executed again.
Replication will try to synchronize the data, so in order to avoid conflicts every document that was modified under the cluster transaction will receive the special
RAFT:int64-sequential-number Change Vector and the special flag
FromClusterTx which ensure precedence over a regular change vector.
Case 3: Cluster transaction with an External incoming replication
While the internal replication with the cluster is discussed in the previous case, the case where two clusters are connected via external replication is a bit different.
The logic of documents that were changed by the cluster transaction versus document that were changed be the regular transaction stays the same, but upon the case where a conflict is on a document that was changed by both local cluster transaction and a remote cluster transaction, the local one will have precedence, but the
FromClusterTx flag is removed, so on the next conflict the local is no longer treated as a modified by cluster transaction document.
Cluster wide transaction can operate only with a functional cluster, so if no consensus was acquired for the cluster transaction by the majority of the nodes or currently there is no leader, the transaction will be rolled back.
Concurrency issues for compare exchange operations
Acquiring a consensus doesn't mean the acceptance of the transaction. Once the consensus is acquired, each node will do a concurrency check on the compare exchange values, and if this fails the transaction rolled back.
Failure to apply transaction on database nodes
Once the transaction passed the compare exchange concurrency check the transaction guaranteed to be committed. And any failure at this stage must be remedied.
|Failure||How to fix it|
|Out of disk space||Making free space, will fix the problem and let the cluster transactions to be committed.|
|Creation/Deletion of a document with different collection||Deleting the document on the other collection|
The execution of cluster transaction on the database will be stopped until this type of failure will be fixed.
Debug Cluster Wide Transaction
To view the current state of the cluster transaction that are waited to completed by all of the database node can be found at:
||Get cluster transactions from the raft change vector index.|
||The number of cluster transaction to show.|