Add ReplicationConfigs to SessionConfig for excluding keyspaces from replica precompute

[michaelhly]

We add ReplicationOptions which includes a vector of keyspace names to SessionConfig. When instantiating ClusterData, we exclude keyspaces from provided by ReplicationOptions when computing PrecomputedReplicas to conserve memory/compute resources.

Fixes: #671

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• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• I added relevant tests for new features and bug fixes.
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[michaelhly]

@piodul @wprzytula @havaker would you mind taking a quick look? Happy to write tests and finish up docs if this looks like it's in the right track.

[piodul]

@Lorak-mmk please review

[Lorak-mmk]

You can already choose which keyspaces to precompute (see keyspaces_to_fetch) in SessionConfig, but:

• it is an allowlist, while your approach is denylist. I'm wondering if maybe the best approach is to allow both (make enum Allowlist(Vec) and Denylist(Vec) instead of just Vec).
• It prevents info about other keyspaces to be fetched at all (so it's not available in ClusterData), while your solution only avoids precomputing replication strategies.

So I think this change may be valuable. I'll try to post initial review soon.

[wprzytula]

@michaelhly Sorry for so delayed response.
I really like the idea. The taken approach is correct: ReplicationConfig (a small name change to be in line with other Configs) should be passed down to ClusterData::new().
As @Lorak-mmk noted, there should be both an Allowlist and Denylist variant for specyfing keyspaces to precompute.

[wprzytula]

Let's not make it an Option; ReplicationConfig should be always passed to where ReplicaLocator is constructed.

[wprzytula]

Better not to allocate here. Let's just use the borrowed Vec there.

Suggested change

	// Exclude these keyspaces from replica set precomputation
	let keyspaces_to_exclude = match replication_opts {
	Some(opts) => opts.keyspaces_to_exclude().to_vec(),
	None => vec![],
	};
	// Exclude these keyspaces from replica set precomputation
	let keyspaces_to_exclude = replication_config.keyspaces_to_exclude();

[wprzytula]

Even better, let's allow for both Allowlist and Denylist variant:

Suggested change

	// Exclude these keyspaces from replica set precomputation
	let keyspaces_to_exclude = match replication_opts {
	Some(opts) => opts.keyspaces_to_exclude().to_vec(),
	None => vec![],
	};
	let replicas_precomputation_config = replication_config.replicas_precomputation_config();

[wprzytula]

Suggested change

	let keyspace_strategies = keyspaces
	.iter()
	.filter(|(k, _)| !keyspaces_to_exclude.contains(k))
	.map(|(_, ks)| &ks.strategy);
	let keyspace_strategies = keyspaces
	.iter()
	.filter(|(k, _)| match replicas_precomputation_config {
	Denylist(disallowed_keyspaces) => !disallowed_keyspaces.contains(k),
	AllowList(allowed_keyspaces) => allowed_keyspaces.contains(k),
	)
	.map(|(_, ks)| &ks.strategy);

[wprzytula]

Suggested change

	pub struct ReplicationConfigs {
	pub struct ReplicationConfig {

[wprzytula]

Suggested change

	pub fn exclude_keyspace(&mut self, keyspace: String) {
	pub fn exclude_keyspace(&mut self, keyspace: impl Into<String>) {

[wprzytula]

Let's create a ReplicasPrecomputationList enum:

enum ReplicasPrecomputationList {
    Allowlist { allowed_keyspaces: Vec<String> },
    Denylist { disallowed_keyspaces: Vec<String> },
}

and store it ReplicationConfig as Option<ReplicaPrecomputationList>.

[wprzytula]

SessionBuilder needs the corresponding setters as well, as it's the intended way to construct a Session.

[wprzytula]

Thank you for contributing the idea @michaelhly!