Bootstrapping a database | Tarantool
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Bootstrapping a database

Let’s discuss game initialization. In start() method, we need to populate Tarantool spaces with pokémon data. Why not keep all game data in memory? Why use a database? The answer is: persistence. Without a database, we risk losing data on power outage, for example. But if we store our data in an in-memory database, Tarantool takes care to persist it on disk whenever it’s changed. This gives us one more benefit: quick startup in case of failure. Tarantool has a smart algorithm that quickly loads all data from disk into memory on startup, so the warm-up takes little time.

We’ll be using functions from Tarantool built-in box module:

  • box.schema.create_space('pokemons') to create a space named pokemon for storing information about pokémons (we don’t create a similar space for players, because we intend to only send/receive player information via API calls, so we needn’t store it);
  • box.space.pokemons:create_index('primary', {type = 'hash', parts = {1, 'unsigned'}}) to create a primary HASH index by pokémon ID;
  • box.space.pokemons:create_index('status', {type = 'tree', parts = {2, 'str'}}) to create a secondary TREE index by pokémon status.

Notice the parts = argument in the index specification. The pokémon ID is the first field in a Tarantool tuple since it’s the first member of the respective Avro type. So does the pokémon status. The actual JSON document may have ID or status fields at any position of the JSON map.

The implementation of start() method looks like this:

-- create game object
start = function(self)
    -- create spaces and indexes
    box.once('init', function()
        box.schema.create_space('pokemons')
        box.space.pokemons:create_index(
            "primary", {type = 'hash', parts = {1, 'unsigned'}}
        )
        box.space.pokemons:create_index(
            "status", {type = "tree", parts = {2, 'str'}}
        )
    end)

    -- create models
    local ok_m, pokemon = avro.create(schema.pokemon)
    local ok_p, player = avro.create(schema.player)
    if ok_m and ok_p then
        -- compile models
        local ok_cm, compiled_pokemon = avro.compile(pokemon)
        local ok_cp, compiled_player = avro.compile(player)
        if ok_cm and ok_cp then
            -- start the game
            <...>
        else
            log.error('Schema compilation failed')
        end
    else
        log.info('Schema creation failed')
    end
    return false
end
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