Jakob Borg 916ec63af6 cmd/stdiscosrv: New discovery server (fixes #4618 )

This is a new revision of the discovery server. Relevant changes and
non-changes:

- Protocol towards clients is unchanged.

- Recommended large scale design is still to be deployed nehind nginx (I
  tested, and it's still a lot faster at terminating TLS).

- Database backend is leveldb again, only. It scales enough, is easy to
  setup, and we don't need any backend to take care of.

- Server supports replication. This is a simple TCP channel - protect it
  with a firewall when deploying over the internet. (We deploy this within
  the same datacenter, and with firewall.) Any incoming client announces
  are sent over the replication channel(s) to other peer discosrvs.
  Incoming replication changes are applied to the database as if they came
  from clients, but without the TLS/certificate overhead.

- Metrics are exposed using the prometheus library, when enabled.

- The database values and replication protocol is protobuf, because JSON
  was quite CPU intensive when I tried that and benchmarked it.

- The "Retry-After" value for failed lookups gets slowly increased from
  a default of 120 seconds, by 5 seconds for each failed lookup,
  independently by each discosrv. This lowers the query load over time for
  clients that are never seen. The Retry-After maxes out at 3600 after a
  couple of weeks of this increase. The number of failed lookups is
  stored in the database, now and then (avoiding making each lookup a
  database put).

All in all this means clients can be pointed towards a cluster using
just multiple A / AAAA records to gain both load sharing and redundancy
(if one is down, clients will talk to the remaining ones).

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4648

2018-01-14 08:52:31 +00:00

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Custom types

Custom types is a gogo protobuf extensions that allows for using a custom struct type to decorate the underlying structure of the protocol message.

How to use

Defining the protobuf message

message CustomType {
  optional ProtoType Field = 1 [(gogoproto.customtype) = "T"];
}

message ProtoType {
  optional string Field = 1;
}

or alternatively you can declare the field type in the protocol message to be bytes:

message BytesCustomType {
  optional bytes Field = 1 [(gogoproto.customtype) = "T"];
}

The downside of using bytes is that it makes it harder to generate protobuf code in other languages. In either case, it is the user responsibility to ensure that the custom type marshals and unmarshals to the expected wire format. That is, in the first example, gogo protobuf will not attempt to ensure that the wire format of ProtoType and T are wire compatible.

Custom type method signatures

The custom type must define the following methods with the given signatures. Assuming the custom type is called T:

func (t T) Marshal() ([]byte, error) {}
func (t *T) MarshalTo(data []byte) (n int, err error) {}
func (t *T) Unmarshal(data []byte) error {}

func (t T) MarshalJSON() ([]byte, error) {}
func (t *T) UnmarshalJSON(data []byte) error {}

// only required if the compare option is set
func (t T) Compare(other T) int {}
// only required if the equal option is set
func (t T) Equal(other T) bool {}
// only required if populate option is set
func NewPopulatedT(r randyThetest) *T {}

Check t.go for a full example

Warnings and issues

Warning about customtype: It is your responsibility to test all cases of your marshaling, unmarshaling and size methods implemented for your custom type.

Issues with customtype include:

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