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The cache filter consists of two separate components; the cache itself that evaluates whether a particular query is subject to caching and the actual cache storage. The storage is loaded at runtime by the cache filter. Currently using a custom mechanism; once the new plugin loading macros/mechanism is in place, I'll see if that can be used. There are a few open questions/issues. - Can a GWBUF delivered to the filter contain more MySQL packets than one? If yes, then some queueing mechanism needs to be introduced. Currently the code is written so that the packets are processes in a loop, which will not work. - Currently, the storage API is synchronous. That may work with a storage built upon RocksDB, that writes asynchronously to disk, but not with memcached that can be (and in MaxScale's case would have to be) used asynchronously. Reading may be problematic with RocksDB as values are returned synchronously. So that will stall the thread being used. However, as RocksDB uses in-memory caching and it is possible to arrange so that e.g. selects targeting the same table are stored together, it is not obvious what the impact would be. So as not to block the MaxScale worker threads, there'd have to be a separate thread-pool for RocksDB access and then arrange the data to be moved across. But initially, the inteface is synchronous. - How is the cache configured? The original requirement mentions all sorts of parameters - database name, table name, column name, presence of WHERE clause, regexp, date/time of query, user - but it's not alltogether clear exactly how they should be specified and how they should interract. So initially all selects will be subject to caching with a TTL.
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#Cache
Overview
The cache filter is capable of caching the result of SELECTs, so that subsequent identical SELECTs are served directly by MaxScale, without being routed to any server.
Configuration
The cache filter is straightforward to configure and simple to add to any existing service.
[Cache]
type=filter
module=cache
ttl=5
storage=...
storage_args=...
[Cached Routing Service]
type=service
...
filters=Cache
Each configured cache filter uses a storage of its own. That is, if there are two services, each configured with a specific cache filter, then, even if queries target the very same servers the cached data will not be shared.
Two services can use the same cache filter, but then either the services should use the very same servers or a completely different set of servers, where the used table names are different. Otherwise there can be unintended sharing.
Filter Parameters
The cache filter has one mandatory parameter - storage - and a few
optional ones.
storage
The name of the module that provides the storage for the cache. That
module will be loaded and provided with the value of storage_args as
argument. For instance:
storage=storage_rocksdb
storage_args
A comma separated list of arguments to be provided to the storage module,
specified in storage, when it is loaded. Note that the needed arguments
depend upon the specific module. For instance,
storage_args=path=/usr/maxscale/cache/rocksdb
allowed_references
Specifies whether any or only fully qualified references are allowed in queries stored to the cache.
allowed_references=[fully-qualified|any]
The default is fully-qualified, which means that only queries where
the database name is included in the table name are subject to caching.
select col from db.tbl;
If any is specified, then also queries where the table name is not
fully qualified are subject to caching.
select col from tbl;
Care should be excersized before this setting is changed, because, for instance, the following is likely to produce unexpected results.
use db1;
select col from tbl;
...
use db2;
select col from tbl;
The setting can be changed to any, provided fully qualified names
are always used or if the names of tables in different databases are
different.
maximum_resultset_size
Specifies the maximum size a resultset can have, measured in kibibytes, in order to be stored in the cache. A resultset larger than this, will not be stored.
maximum_resultset_size=64
The default value is TBD.
ttl
Time to live; the amount of time - in seconds - the cached result is used before it is refreshed from the server.
If nothing is specified, the default ttl value is 10.
ttl=60
#Storage