Instead of lengthy blurbs, switch to single-line, machine-readable
standardized (https://spdx.dev) license identifiers. The Linux kernel
switched long ago, so there is strong precedent.
Three cases are handled: AGPL-only, Apache-only, and dual licensed.
For the latter case, I chose (AGPL-3.0-or-later and Apache-2.0),
reasoning that our changes are extensive enough to apply our license.
The changes we applied mechanically with a script, except to
licenses/README.md.
Closes#9937
Unfortunately, defining metrics in Scylla requires some code
duplication, with the metrics declared in one place but exported in a
different place in the code. When we duplicated this code in Alternator,
we accidentally dropped the first metric - for BatchGetItem. The metric
was accounted in the code, but not exported to Prometheus.
In addition to fixing the missing metric, this patch also adds a test
that confirms that the BatchGetItem metric increases when the
BatchGetItem operation is used. This test failed before this patch, and
passes with it. The test only currently tests this for BatchGetItem
(and BatchWriteItem) but it can be later expanded to cover all the other
operations as well.
Fixes#9406
Signed-off-by: Nadav Har'El <nyh@scylladb.com>
Message-Id: <20210929121611.373074-1-nyh@scylladb.com>
This patch moves the alternator latencies histograms to use the time_estimated_histogram.
The changes requires changing the defined type and use the simpler
insertion method.
Signed-off-by: Amnon Heiman <amnon@scylladb.com>
In this patch, we re-implement the three read-modify-write operations -
PutItem, UpdateItem, DeleteItem. All three operations may need to read the
item before writing it to support conditional updates (the "Expected"
parameter) and UpdateItem may also need the previous item's value for
its update expression (e.g., a user may ask to "set a=a+1" or "set a=b").
Before this patch, the implementation of RMW operations simply did a read,
and then a write - without any attempt to protect concurrent operations.
In this patch, Scylla's LWT mechanism (storage_proxy::cas()) is used
instead, to ensure that concurrent update operations are correctly
isolated even if they are conditional. This means that Alternator now
requires the experimental LWT feature to be enabled (and refuses to
boot if it isn't).
The version presented here is configured to always use LWT for *every*
write, regardless of whether it has a condition or not. So it will
will significantly slow down write-only workloads like YCSB. But the code
in this patch actually includes three other modes, which can be chosen by
setting an enum constant in the code. In the future we will want to let the
user configure this mode, globally, per table or per attribute.
Note that read requests are NOT modified, and work exactly as they did
before: i.e., strongly-consistent reads are done using a normal
CL=LOCAL_QUORUM read - not via LWT. I believe this is good enough given
Dynamo's guarantees, and critical for our read performance.
Also note that patch doesn't yet fix the BatchWriteItem operation.
Although BatchWriteItem does not support any RMW operations - just pure
writes - we may still need to do those pure writes using LWT. This
should be fixed in a follow-up patch.
Unfortunately, this patch involves a large amount of code movement and
reorganization, because:
1. The cas operation requires each operation to be made into an object,
with a separate apply() function, forcing a lot of code to move.
2. Moreover, we need to do this for three different operations (PutItem,
UpdateItem, DeleteItem) so to avoid massive code duplication, I had
to move some common code.
3. The cas operation also forced us to change some of the utility functions'
APIs.
The end result is that this patch focuses more on a compact and
understandable *end result* than it does on an easy to understand *patch*,
so reviewers - sorry about that.
All alternator-test/ tests pass with this patch (and also with all of the
different optional modes enabled). However, other than that, I did not yet
do any real isolation tests (are concurrent operations really isolated
correctly? or is LWT just faking it? :-) ), performance tests or stress
tests - and I'll definitely need to do those as well.
Fixes#5054
Signed-off-by: Nadav Har'El <nyh@scylladb.com>
This patch make two chagnes to the alternator stats:
1. It add estimated_histogram for the get, put, update and delete
operation
2. It changes the metrics naming, so the operation will be a label, it
will be easier to handle, perform operation and display in this way.
Signed-off-by: Amnon Heiman <amnon@scylladb.com>
Three metrics related to filtering are added to alternator:
- total rows read during filtering operations
- rows read and matched by filtering
- rows read and dropped by filtering
Read-before-write stat counters were already introduced, but the metrics
needs to be added to a metric group as well in order to be available
for users.
his patch adds a statistics framework to Alternator: Executor has (for
each shard) a _stats object which contains counters for various events,
and also is in charge of making these counters visible via Scylla's regular
metrics API (http://localhost:9180/metrics).
This patch includes a counter for each of DynamoDB's operation types,
and we increase the ones we support when handled. We also added counters
for total operations and unsupported operations (operation types we don't
yet handle). In the future we can easily add many more counters: Define
the counter in stats.hh, export it in stats.cc, and increment it in
where relevant in executor.cc (or server.cc).
Signed-off-by: Nadav Har'El <nyh@scylladb.com>
