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Alternator supports four ways in which write operations can use quorum writes or LWT or both, which we called "write isolation policies". Until this patch, Alternator defaulted to the most generally safe policy, "always_use_lwt". This default could have been overriden for each table separately, but there was no way to change this default for all tables. This patch adds a "--alternator-write-isolation" configuration option which allows changing the default. Moreover, @dorlaor asked that users must *explicitly* choose this default mode, and not get "always_use_lwt" without noticing. The previous default, "always_use_lwt" supports any workload correctly but because it uses LWT for all writes it may be disappointingly slow for users who run write-only workloads (including most benchmarks) - such users might find the slow writes so disappointing that they will drop Scylla. Conversely, a default of "forbid_rmw" will be faster and still correct, but will fail on workloads which need read-modify-write operations - and suprise users that need these operations. So Dor asked that that *none* of the write modes be made the default, and users must make an informed choice between the different write modes, rather than being disappointed by a default choice they weren't aware of. So after this patch, Scylla refuses to boot if Alternator is enabled but a "--alternator-write-isolation" option is missing. The patch also modifies the relevant documentation, adds the same option to our docker image, and the modifies the test-running script test/alternator/run to run Scylla with the old default mode (always_use_lwt), which we need because we want to test RMW operations as well. Fixes #6452 Signed-off-by: Nadav Har'El <nyh@scylladb.com> Message-Id: <20200524160338.108417-1-nyh@scylladb.com>
Scylla developer documentation
This folder (and its subfolders) contain developer-oriented documentation concerning the Scylla codebase. We also have a wiki, which contains additional developer-oriented documentation. There is currently no clear definition of what goes where, so when looking for something be sure to check both.
Seastar documentation can be found here.
User documentation can be found on docs.scylladb.com
For information on how to build Scylla and how to contribute visit HACKING.md and CONTRIBUTING.md.