[Bro-Dev] Broker data store use case and questions

[Jon Siwek]

On 5/10/18 3:53 PM, Michael Dopheide wrote:

> 1) My initial gut feeling was that all of the when() calls for insertion 
> could get really expensive on a brand new cluster before the store is 
> populated.

I've not tried to explicitly measure differences yet, though my hunch is 
that the overhead of needing to use when() to drive data store 
communication patterns could be slightly more expensive than just using 
remote events (or &synchronized as the previous implementation used). 
I'm thinking of overhead more in terms of memory here, as it needs to 
save the state of the current frame making the call so it can resume 
later.

Another difference is the data store implementation of known-hosts is 
that it does always require remote communication just to check for 
whether a given key is in the data store yet, which may be a bottleneck 
for some use-cases.

You can also compare/contrast with another implementation of 
known-hosts.bro if you toggle the `Known::use_host_store = F` code path. 
  There, instead of using a data store, it sends remote events via 
Cluster::publish_hrw to uniformly partition the data set across proxies 
in a scalable manner but without persistence.

Yet another idea for an implementation, if you need persistence + 
scalability, would be combining the HRW stuff with data stores.  e.g. 
partitioning the total data set across proxies while using a data store 
on each one for local storage instead of a table/set.

I don't know if there's a general answer to which way is best.  Likely 
varies per use-case / network.

> 2) Correct me if I'm wrong, but it seems like the check for a host 
> already being in known_hosts (now host_store) no longer exists.  As a 
> result, we try to re-insert the host, calling when(), every time we see 
> an established connection with a local host.

Sounds right.

Specifically, it's Broker::put_unique() that hides the following:

(1) tell master data store "insert this key if it does not exist"
(2) wait for master data store to tell us if the key was inserted, and 
thus did not exist before

There's no check against the local cache to first see if the key exists 
as going down that path leads to race conditions.

> 3) How do I retrieve values from the store to test for existence?

Broker::exists() to just check existence of a key or Broker::get() to 
retrieve value at a key.  You can also infer existence from the result 
of Broker::get().

Either requires calling inside 'when()'.  Generally, any function in the 
API you see return a Broker::QueryResult needs to use 'when()'.

> 4) Assuming that requires another Broker call inside a when(), does it 
> make sense to pull the data store into memory at bro_init() and do 
> a Cluster::publish_hrw?

Not sure I follow since, in the current implementation of known-hosts, 
the data store and Cluster::publish_hrw code paths don't interact 
(they're alternate implementations of the same thing as mentioned 
before).  If the question is just whether it makes sense to go the 
Cluster::publish_hrw route instead of using a data store: yes, just 
depends on what you prefer.  IMO, the data store approach has downsides 
that make it less preferable to me.

- Jon