TableDB

table database with replication

NPL Raft

The implementation is basically a port from jraft to NPL

Raft consensus implementation in NPL

The core algorithm is implemented based on the TLA+ spec, whose safety is proven, and liveness is highly depended on the pseudo random number sequence, which could be fine if different servers in the same cluster are generating random numbers with different seeds.

Supported Features,

• Core Algorithm(Leader election,Log replication), safety is proven
• Configuration Change Support, add or remove servers one by one without limitation
• Client Request Support
• Urgent commit, see below
• log compaction

Urgent Commit, is a new feature introduced by this implementation, which enables the leader asks all other servers to commit one or more logs if commit index is advanced. With Urgent Commit, the system's performance is highly improved and the heartbeat interval could be increased to seconds,depends on how long your application can abide when a leader goes down, usually, one or two seconds is fine.

About this implementation

it's always safer to implement such kind of algorithm based on Math description other than natural languge description. there should be an auto conversion from TLA+ to programming languages, even they are talking things in different ways, but they are identical

Threading model

Now the implementation is Single Thread model.

Logic

The Core Raft algorithm logic is in RaftServer, whose implementation is straight forward.

NOTE: logic will be modified in Phase 2, to adapt to the WAL Raft log.

TableDB Raft

Interface

The client interface keeps unchanged, we provide a script/TableDB/RaftSqliteStore.lua, which is a StorageProvider. And we also changed the TableDatabase internal to adapt to the new Raft API, RaftSqliteStore will send the Log Entry above to the raft cluster in each interface.

Config

To use TableDB Raft, u need to add a tabledb.config.xml file in the Tabledb rootFolder.

<tabledb>
	<providers>
		<provider type="TableDB.RaftSqliteStore" name="raft" file="(g1)npl_mod/TableDB/RaftSqliteStore.lua">./,localhost,9004,4,rtdb
		</provider>
	</providers>
	<tables>
		<table provider="raft" name="User1"/>
		<table provider="raft" name="User2"/>
		<table provider="raft" name="default" />
	</tables>
</tabledb>

Above is a example of the Raft provider config.

• provider have 4 config,
  • type is used in commonlib.gettable(provider.attr.type).
  • file is used in NPL.load(provider.attr.file).
  • name is the name of the provider, which is used in the table config.
  • values is the init args used in provider type's init method.
• table have 2 config,
  • provider is the name of the provider, correspond to the name in provider,
  • name is the table name. If the name is default, the corresponding provider will be set to be default provider of the database. If default is not set, default provider will be sqlite.

Callbacks

Like the original interfaces, callbacks is also implemented in a async way. But we make the connect(which is called in RaftSqliteStore:init) to be sync to get the Cluster's leader, this will boost the performance and alleviate the retry in subsequent request.

Snapshot

We use sqlite's Online Backup API to make snapshot.

Test TableDB/NPL Raft

Start a 3 Raft Nodes cluster

cd setup && setup.bat will start 3 Raft nodes. The 3 nodes will automatically elect a leader. we can stop one node(whether it is leader or not) and the cluster can still function correctly. Raft can tolerate N/2 + 1 nodes failue, where N is the total nodes in cluster.

Send Commands to the Cluster

Create setup/client/temp/test_raft_database directory, place the tabledb.config.xml above to the directory, and run setup.bat client appendEntries. This will start 1 client node whose tabledb rootFolder is temp/test_raft_database and default provider is raft.The client node will send commands to the cluster and automaticlly retry in a backoff way if the command not succeed. All 3 Raft nodes will recv the same commands. Whether succeed or not the client's callback will be called.

Add a server to the Cluster

addsrv 5 will start a node whose id is 5.(Note: you need to delete the server5 folder). To add the node to the cluster, execute setup.bat client addServer 5. The cluster will automatically sync the logs previously commited to this new added server. Now if you start a client to send commands to the cluster, the new server will also recv these commands.

Note: every time you start a new client(whether Send Commands, Add server or Remove server), you should stop the previous client.

Remove a server in the Cluster

setup.bat client removeServer 5 will remove the node 5 in the cluster. But you can not request to remove the leader.

The Raft cluster is fault tolerate and highly available. You can stop the cluster and client with stopNPL.bat.

Unit Test

NPL.load("(gl)script/ide/UnitTest/luaunit.lua");
NPL.load("(gl)npl_mod/Raft/test/TestClusterConfiguration.lua");
NPL.load("(gl)npl_mod/Raft/test/TestSnapshotSyncRequest.lua");
NPL.load("(gl)npl_mod/Raft/test/TestFileBasedSequentialLogStore.lua");
NPL.load("(gl)npl_mod/Raft/test/TestServerStateManager.lua");
NPL.load("(gl)npl_mod/TableDB/test/TestRaftLogEntryValue.lua");
LuaUnit:run('TestRaftLogEntryValue');
LuaUnit:run('TestFileBasedSequentialLogStore');
LuaUnit:run('TestClusterConfiguration');
LuaUnit:run('TestSnapshotSyncRequest');
LuaUnit:run('TestServerStateManager');
ParaGlobal.Exit(0)

Welcome for more feedbacks.:)