Reading and Writing

We now show how to read and write the replicated log. The replicated log is append-only. To append an entry we call the following:

let write_entry = KeyValue { key: String::from("a"), value: 123 };
omni_paxos.append(write_entry).expect("Failed to append");

This will cause our write_entry to be proposed to get decided in the replicated log. Appends can be pipelined without waiting for preceding entries to be decided. Furthermore, append() can be called on any node. If the calling node is not the leader, the entry will be forwarded.

Reading the Log

Reads are also handled by calling various functions on OmniPaxos. To read the entry at a specific index idx of the log we call omni_paxos.read_entry(idx). We can also read a specific range of log entries with omni_paxos.read_entries().

/*** Read a single entry ***/
let idx = 5;
let read_entry = omni_paxos.read(idx);

/*** Read a range ***/
let read_entries = omni_paxos.read_entries(2..5);

The read functions return Option<LogEntry> and Option<Vec<LogEntry>> respectively, where None is returned if the index or range is out of bounds. LogEntry is an enum with the following variants:

  • Decided(T): The entry is decided and guaranteed to not be reverted. It is thus safe to apply a decided entry to the application state. For instance, in our case, it is safe to update our key-value store when we read a Decided(KeyValue) entry.
  • Undecided(T): The entry is NOT decided and might be removed from the log at a later time. However, it could be useful in applications that allow speculative execution for example.
  • Trimmed(TrimmedIndex): We tried to read an index where the entry has already been trimmed.
  • Snapshotted(SnapshottedEntry<T>): The index we read has already been compacted into a snapshot. We can access the snapshot from the field snapshot in SnapshottedEntry. In our case our this will correspond to KVSnapshot that we defined here.
  • StopSign(StopSign): This Sequence Paxos instance has been stopped for reconfiguration. This implies that this log will not be appended anymore and one should use the new Sequence Paxos instead for writing.

It is also possible to only read decided entries or snapshot from a specific index using read_decided_suffix(idx).

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