feat(cluster): Add support for request proxying for scale out (#2385)

* feat(cluster): initial commit for scale-out cluster

Signed-off-by: Ramkumar Chinchani <rchincha@cisco.com>

* feat(cluster): support shared storage scale out

This change introduces support for shared storage backed
zot cluster scale out.

New feature
Multiple stateless zot instances can run using the same shared
storage backend where each instance looks at a specific set
of repositories based on a siphash of the repository name to improve
scale as the load is distributed across multiple instances.
For a given config, there will only be one instance that can perform
dist-spec read/write on a given repository.

What's changed?
- introduced a transparent request proxy for dist-spec endpoints based on
siphash of repository name.
- new config for scale out cluster that specifies list of
cluster members.

Signed-off-by: Vishwas Rajashekar <vrajashe@cisco.com>

---------

Signed-off-by: Ramkumar Chinchani <rchincha@cisco.com>
Signed-off-by: Vishwas Rajashekar <vrajashe@cisco.com>
Co-authored-by: Ramkumar Chinchani <rchincha@cisco.com>

pkg/cli/server/root.go

@@ -457,6 +457,11 @@ func validateConfiguration(config *config.Config, log zlog.Logger) error {
 		}
 	}
 
+	// check validity of scale out cluster config
+	if err := validateClusterConfig(config, log); err != nil {
+		return err
+	}
+
 	return nil
 }
 
@@ -1103,3 +1108,27 @@ func validateSync(config *config.Config, log zlog.Logger) error {
 
 	return nil
 }
+
+func validateClusterConfig(config *config.Config, log zlog.Logger) error {
+	if config.Cluster != nil {
+		if len(config.Cluster.Members) == 0 {
+			log.Error().Err(zerr.ErrBadConfig).
+				Msg("cannot have 0 members in a scale out cluster")
+
+			return zerr.ErrBadConfig
+		}
+
+		// the allowed length is 16 as the siphash requires a 128 bit key.
+		// that translates to 16 characters * 8 bits each.
+		allowedHashKeyLength := 16
+		if len(config.Cluster.HashKey) != allowedHashKeyLength {
+			log.Error().Err(zerr.ErrBadConfig).
+				Str("hashkey", config.Cluster.HashKey).
+				Msg(fmt.Sprintf("hashKey for scale out cluster must have %d characters", allowedHashKeyLength))
+
+			return zerr.ErrBadConfig
+		}
+	}
+
+	return nil
+}