A log is an append-only sequence of records. “logs” are binary- encoded messages meant for other programs to read. When you append a record to a log, the log assigns the record a unique and sequential offset number that acts like the ID for that record. A log is like a table that always orders the records by time and indexes each record by its offset and time created.
This fundamental working of a log is implemented in the log package.
Since disks have a size limit, we can’t append to the same file forever, so we split the log into a list of segments. The active segment is the segment we actively write to. When we’ve filled the active segment, we create a new segment and make it the active segment.
Each segment comprises a store file and an index file. Store file is where we store the record data; we continually append records to this file. The Index file maps the record offsets to their position in the store file, thus speeding up reads. Reading a record given its offset is a two-step process: first you get the entry from the index file for the record, which tells you the position of the record in the store file, and then you read the record at that position in the store file. Index files are small enough, so we memory-map (read more) them and make operations on the file as fast as operating on in-memory data.
Structure of the log package is as follows:
gRPC is used for network interfacing of Loghouse. The gRPC server is implemented in server package.
The protobuf definitions are in api/v1/log.proto. To generate go
code from proto, execute make compile and the structs and gRPC method stubs will
get generated. These stubs are then implemented in server.go.
Since there’s a hardware limitation on the size of data we can store on a single server, at some point the only option is to add more servers to host this Loghouse service. With the addition of nodes, challenges of server-to-server Service discovery, Replication, Consensus and Load Balancing arise, which are tackled as follows:
Once we add a new node running our service, we need a mechanism to connect it to the rest of the cluster. This Server-to-server service discovery is implemented using Serf — a library that provides decentralized cluster membership, failure detection, and orchestration.
The discovery package integrates Serf where Membership is the type wrapping Serf to provide discovery and cluster membership to this service.
Apart from scalability, distributing a service improves the availability as in if any node goes down, another one will take its place and serve requests. To provide availability, we need to ensure that the log written to one node get copied or replicated to other nodes as well.
We might easily implement a replicator by making each node subscribe to other nodes using the ConsumeStream rpc. However, this will cause an infinite replication of each log unless we introduce some kind of coordinator which will keep a track of each log’s offset, ensure that all nodes replicate that log at the same offset exactly once (form consensus on the log’s offset) and prevent infinite replication.
There are many algorithms in distributed systems to solve this most popular being Paxos and Raft. Raft is used in this project and utilizes this awesome implementation by Hashicorp. This piece of code sits in distributed_log.go where we set up Raft. Raft library uses a FSM interface to execute the business logic of Loghouse as well as the Snapshotting and Restoring logic. This FSM is implemented in fsm.go
Membership and distributed_log are stitched together in agent.go. Agent is the starting point of the service and will set up everything - distributed log, grpc server, membership handling and authorization.
This is a single-writer, multiple-reader distributed service and the leader server is the only server that can append to the log.
Client-server connection is authenticated using mTLS. To generate the certificates execute
make gencert. This uses cfssl to generate
client and server self-signed certificates from the config files present in resources.
These are then used to authenticate and encrypt gRPC and Raft connections. RSA-2048 is used
for encryption on both client and server.
Authorization is implemented using ACLs. Casbin is used to provide the ACL functionality. In authorizer.go we set up Casbin to enforce the policies defined in policy.csv as per the model: model.conf. Authorization takes place during Produce/Consume RPCs in server.go.
Client-side loadbalancing is used. gRPC provides a way to do this via resolvers and pickers. resolver.go fetches the server addresses from raft config and updates the client connection with the list of addresses. picker.go handles the RPC balancing logic by picking a server from the servers discovered by the resolver. Depending on the rpc name, Produce requests are routed to the leader and Consume to one of the followers in a round-robin manner.