How do I accomplish lightweight transactions with linearizable consistency? 

A description about lightweight transactions and when to use them.

Distributed databases present a unique challenge when data must be strictly read and written in sequential order. In transactions for creating user accounts or transferring money, race conditions between two potential writes must be regulated to ensure that one write precedes the other. In Cassandra, the Paxos consensus protocol is used to implement lightweight transactions that can handle concurrent operations.

The Paxos protocol is implemented in Cassandra with linearizable consistency, that is sequential consistency with real-time constraints. Linearizable consistency ensures transaction isolation at a level similar to the serializable level offered by RDBMSs. This type of transaction is known as compare and set (CAS); replica data is compared and any data found to be out of date is set to the most consistent value. In Cassandra, the process combines the Paxos protocol with normal read and write operations to accomplish the compare and set operation.

The Paxos protocol is implemented as a series of phases:
  1. Prepare/Promise
  2. Read/Results
  3. Propose/Accept
  4. Commit/Acknowledge
These phases are actions that take place between a proposer and acceptors. Any node can be a proposer, and multiple proposers can be operating at the same time. For simplicity, this description will use only one proposer. A proposer prepares by sending a message to a quorum of acceptors that includes a proposal number. Each acceptor promises to accept the proposal if the proposal number is the highest they have received. Once the proposer receives a quorum of acceptors who promise, the value for the proposal is read from each acceptor and sent back to the proposer. The proposer figures out which value to use and proposes the value to a quorum of the acceptors along with the proposal number. Each acceptor accepts the proposal with a certain number if and only if the acceptor is not already promised to a proposal with a high number. The value is committed and acknowledged as a Cassandra write operation if all the conditions are met.

These four phases require four round trips between a node proposing a lightweight transaction and any cluster replicas involved in the transaction. Performance will be affected. Consequently, reserve lightweight transactions for situations where concurrency must be considered.

Lightweight transactions will block other lightweight transactions from occurring, but will not stop normal read and write operations from occurring. Lightweight transactions use a timestamping mechanism different than for normal operations and mixing LWTs and normal operations can result in errors. If lightweight transactions are used to write to a row within a partition, only lightweight transactions for both read and write operations should be used. This caution applies to all operations, whether individual or batched. For example, the following series of operations can fail:
DELETE ...
INSERT .... IF NOT EXISTS
SELECT ....
The following series of operations will work:
DELETE ... IF EXISTS
INSERT .... IF NOT EXISTS
SELECT .....

Reads with linearizable consistency

A SERIAL consistency level allows reading the current (and possibly uncommitted) state of data without proposing a new addition or update. If a SERIAL read finds an uncommitted transaction in progress, Cassandra performs a read repair as part of the commit.