Due to:
* Cluster RPC MFA is not idempotent!
* There is a lack of rollback for callback's side-effects
For instance, when two nodes try to add a cluster-singleton
concurrently, one of them will have to wait for the table lock
then try to catch-up, then try to apply MFA.
The catch-up will have the singleton created, but the initiated
initiated multicall apply will fail causing the commit to rollback,
but not to 'undo' the singleton creation.
Later, the retries will fail indefinitely.
Prior to EMQX 5.0, the edge edition's main difference comaring
to standard edition are:
* Less number of plugins in the release (smaller package size)
* Smaller number is vm.args (for lower memory usage)
Starting from 5.0 most of the plugins are included as features,
the tuned vm.args arguments does not justify a special release edition.
Also as nanomq is getting mature,
EMQ's recommendtation for MQTT broker in edge is https://nanomq.io/
This commit adds a default timeout of 10 seconds to the rule engine's
`jq/2` function, and adds a new function `jq/3` (where the last parameter is
a timeout value). The default timeout can be configured with the setting
"rule_engine.jq_function_default_timeout".
Having a timeout when executing jq code in the rule engine is important
as jq code can potentially run forever. Also, the Erlang jq library
limits the number of jq programs that can execute concurrently so a jq
program that loops forever could potentially also prevent a "non-buggy"
jq program from ever starting.
This commit adds a function to the rule engine that alows users
to transform text or JSON objects using [jq filter programs][1].
[jq][1] is a command line tool that can be used to transform
and filter JSON text using jq's built-in language. The rule engine
function that is added with this commit uses the
[Erlang jq NIF library][2] that wraps the jq C library in an
Erlang NIF function.
[1]: https://stedolan.github.io/jq/
[2]: https://github.com/emqx/jq
