Crate active_standby[−][src]

A concurrency library for high concurrency reads with a single writer.

This library is named after the 2 (identical) tables that we hold internally:

  • Active - this is the table that all Readers view. This table will never be write locked, so readers never face contention.
  • Standby - this is the table the the Writer mutates. A writer should face minimal contention retrieving this table for mutation since Readers move to reading the Active table when they are swapped.

The cost of providing no contention to readers, and minimal contention to writers is:

  1. Memory - Internally we hold 2 copies of the underlying type the user created. This is needed to allow there to always be a table that Readers can check out without contention.
  2. Writer thread CPU usage - The writer must apply all updates twice, once to each table. Lock contention for the writer should be less than with a plain RwLock due to Readers using the active_table.

The usage is meant to be similar to a RwLock. Instead of multiple threads holding an RwLock though and calling read/write, there is a single Writer that acquire a write guard to the tables, and N Readers which can acquire read guards to the tables. Some of the inspiration came from the left_right crate, so feel free to check that out. We don’t implement aliasing, so each table is a true deepcopy of the other. We also don’t optimize for startup.

Minimizing lock contention also makes batching a more effective strategy for Reader performance. Now you can grab a ReadGuard, and handle multiple requests without worrying about starving the writer since it will be able to work on the standby table. This means multiple requests can be handled without having to relock the active_table. Similarly you can batch with the Writer without starving the Readers.

Creation is done through the Writer, which can then spawn Readers (Readers are clonable).

We provide 2 modules:

  1. primitives - these are building blocks that can be used similarly to a RwLock.
  2. collections - these are common collections that use primitives to offer users an interface very similar to the collections themselves, but conforming to the requirements of active_standby.

Modules

collections
primitives