We've been writing a new client-server protocol as a public API for the creation of new Geode clients. We settled on using Protobuf as the encoding for the protocol as it makes writing clients easier by abstracting away a lot of the encoding details.
One of the big challenges in designing a new protocol has been how to encode values. Like the old binary client protocol, the PDX encoding is complicated, underdocumented, and stateful. However, we need a way for users to send values that are more complex than mere primitives or maps of primitives..
The first approach was to use the JSON-PDX conversion that is already used for the REST API. Many languages have libraries to encode objects as JSON, and it's a familiar format to many developers. However, using JSON for encoding has downsides, in that it's large and slow.
Selecting an Encoding Mechanism
Adding this encoding mechanism to the protocol is not exclusive with allowing JSON or custom encodings – the object encoding can be pluggable, and the user's desired response encoding should be selected during the handshake process.
Regardless of proposal, we should allow users to have a pluggable object encoding that they can register a handler with on the server. This encoder will receive a byte array and return an
Object. This allows users to do custom serialization if desired.
Protobuf Struct Encoding and Extension
This section is intended as useful background on the start of some thoughts about encoding a PDX-like type with Protobuf; for the proposed encoding, see "The Proposed Encoding", below.
Protobuf ships with a
Struct message, defined in
struct.proto, that can be recursively nested to encode JSON.
By extending the same sort of structure, we can encode almost any value with a type that is supported, including adding support for more complex types like dates or UUIDs. This should make it possible to write serializers and should enable driver developers to write auto-serializers that will serialize these objects via reflection to Protobuf.
typeName field can be used for other clients to recognize the same type. Internally, it will be stored in the PDXInstance that this is converted to, but that detail shouldn't need to be exposed to the driver developer.
So for example, the following class and value:
would encode as (using a pseudo-static initializer syntax):
This all gets compiled down to binary for an encoding that is more efficient than JSON.
Ideally, a driver developer would provide annotations or registration for application developers to specify the manner in which a type should be serialized. In languages that use setters and getters by convention, it would probably be more idiomatic to refer to setters getters for reflection rather than the member variables of the object.
The Proposed Encoding
As an optimization to avoid sending field names with every message, allow clients (and servers) to cache metadata for data they are about to send. This is done by registering an ID that can be used in future messages to refer to the metadata without retransmitting that metadata. This encoding will not actually be smaller for single values of a type, but if multiple values of the same type are sent the savings can be significant.
Type registration will be per-connection (meaning IDs cannot be cached between connections). This eliminates the need to keep synchronization on the server, as well as decoupling type registrations from the internal details of PDX. It also means that the drivers only have to keep track of a relatively small amount of data.
The first time a client (or server) sends a type, it will send it with the
NewStructType message, along with a unique ID number, which will lead the other side to cache it. After that, it should reuse that ID number and send
So for example, using the same User from above:
Suppose the client chose ID 42 for this type. Then the first put message using such a value would have a value like so:
A later PutRequest would encode the value like this (enclosing Request omitted for succinctness):
This is the proposed EncodedValue message that will contain values a client sends to the server or the server sends to the client:
Under this EncodedValue scheme, types defined by the server and types defined by the client will use different sets of IDs (though these can refer to the same cached values if they are the same). This is because we intend to add support for asynchronous messages and/or multiplexing of multiple channels of communication over one socket, and this avoids having the server and client race to assign IDs. If IDs were shared, the server would need to send back new IDs when it sent back types the client had not seen before.
Type definitions will encode all values that are not primitives or arrays of primitives as Objects that may be of any type, whereas primitives will be type checked. Clients may do their own validation. This is, in significant part, a leaky abstraction due to the way PDX saves values.
If a client is sending mutually recursive types or types that contain instances of themselves, it should send the type definition the first time one is seen (or in the parent instance) and send the type with ID in each later instance.
Whether a client must send all following values by ID or the values can be sent with a full ID each time should be configurable in the handshake.
In order to avoid arbitrary object serialization (which can lead to gadget chain exploits), we will probably need to constrain valid types to those registered as DataSerializable, or possibly even only those registered with the
ReflectionBasedAutoSerializer. This may also mean that we need a special class of typenames for those types that are put first by a client.
The way that objects are deserialized on the server is dependent on how PDX behaves now.
A driver developer may wish to provide a way for users to register types before sending values. An earlier version of this document described a protocol where the types had to be defined in a separate message before the value in which they are first put. That had separate list of types for the registration method. Because using the same list of types as EncodedValue amounts to pretty much the same as sending a new value, we opted for the method above.
Driver developers will have to make sure that types they want to use in different language clients can be correlated. So package names may or may not make sense. The naming convention is not entirely decided, nor is whether we can register nameless types. It may be wise to reserve a set of names with special meaning ("JSON" perhaps?) and perhaps a set of names that would correspond to classes that have no domain class in Java (leading underscore, or just those with no package name?)
The use of NumericArray for all the integral types is because they all have the same varint encoding and will encode the same way on the wire. It may be advisable to use more restricted types and separate messages to get better typing in the generated Protobuf code.
Each of the primitives maps to the corresponding Java primitive. Arrays map to arrays of Java primitives. Other fields will encode to the corresponding objects.