Class: Wal::Replicator

Inherits:
Object
  • Object
show all
Includes:
PG::Replication::Protocol
Defined in:
lib/wal/replicator.rb

Overview

Responsible to hook into a Postgres logical replication slot and stream the changes to a specific ‘Watcher`. Also it supports inject “contexts” into the replication events.

Defined Under Namespace

Classes: Column, Table

Instance Method Summary collapse

Constructor Details

#initialize(replication_slot:, use_temporary_slot: false, db_config: ActiveRecord::Base.configurations.configs_for(name: "primary").configuration_hash) ⇒ Replicator

Returns a new instance of Replicator.



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# File 'lib/wal/replicator.rb', line 9

def initialize(
  replication_slot:,
  use_temporary_slot: false,
  db_config: ActiveRecord::Base.configurations.configs_for(name: "primary").configuration_hash
)
  @db_config = db_config
  @replication_slot = replication_slot
  @use_temporary_slot = use_temporary_slot
end

Instance Method Details

#replicate(watcher, publications:) ⇒ Object 



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# File 'lib/wal/replicator.rb', line 26

def replicate(watcher, publications:)
  watch_conn = PG.connect(
    dbname: @db_config[:database],
    host: @db_config[:host],
    user: @db_config[:username],
    password: @db_config[:password].presence,
    port: @db_config[:port].presence,
    replication: "database",
  )

  begin
    watch_conn.query(<<~SQL)
      CREATE_REPLICATION_SLOT #{@replication_slot} #{@use_temporary_slot ? "TEMPORARY" : ""} LOGICAL "pgoutput"
    SQL
  rescue PG::DuplicateObject
    # We are fine, we already have created this slot in a previous run
  end

  tables = {}
  context = {}
  transaction_id = nil

  watch_conn.start_pgoutput_replication_slot(@replication_slot, publications, messages: true).filter_map do |msg|
    case msg
    in XLogData(data: PG::Replication::PGOutput::Relation(oid:, name:, columns:, namespace:))
      tables[oid] = Table.new(
        # TODO: for now we are forcing an id column here, but that is not really correct
        primary_key_colums: columns.any? { |col| col.name == "id" } ? ["id"] : [],
        schema: namespace,
        name:,
        columns: columns.map do |col|
          Column.new(
            name: col.name,
            decoder: ActiveRecord::Base.connection.lookup_cast_type_from_column(
              # We have to create this OpenStruct because weird AR API reasons...
              # And the `sql_type` param luckly doesn't really matter for our use case
              ::OpenStruct.new(oid: col.oid, fmod: col.modifier, sql_type: "")
            ),
          )
        end
      )
      next

    in XLogData(lsn:, data: PG::Replication::PGOutput::Begin(xid:, timestamp:, final_lsn:))
      transaction_id = xid
      context = {}
      BeginTransactionEvent.new(
        transaction_id:,
        lsn:,
        final_lsn:,
        timestamp:,
      ).tap { |event| watcher.on_event(event) }

    in XLogData(lsn:, data: PG::Replication::PGOutput::Commit(timestamp:))
      CommitTransactionEvent.new(
        transaction_id:,
        lsn:,
        context:,
        timestamp:,
      ).tap do |event|
        watcher.on_event(event)
        watch_conn.standby_status_update(write_lsn: lsn)
      end

    in XLogData(lsn:, data: PG::Replication::PGOutput::Message(prefix: "wal_ping"))
      watch_conn.standby_status_update(write_lsn: [watch_conn.last_confirmed_lsn, lsn].compact.max)
      next

    in XLogData(data: PG::Replication::PGOutput::Message(prefix:, content:)) if watcher.valid_context_prefix? prefix
      begin
        context = JSON.parse(content).presence || {}
        next
      rescue JSON::ParserError
        # Invalid context received, just ignore
      end

    in XLogData(lsn:, data: PG::Replication::PGOutput::Insert(oid:, new:))
      table = tables[oid]
      next unless watcher.should_watch_table? table.full_table_name
      new_data = table.decode_row(new)
      record_id = table.primary_key(new_data)
      next unless record_id

      InsertEvent.new(
        transaction_id:,
        lsn:,
        context:,
        schema: table.schema,
        table: table.name,
        primary_key: record_id,
        new: new_data,
      ).tap { |event| watcher.on_event(event) }

    in XLogData(lsn:, data: PG::Replication::PGOutput::Update(oid:, new:, old:))
      table = tables[oid]
      next unless watcher.should_watch_table? table.full_table_name
      old_data = table.decode_row(old)
      new_data = table.decode_row(new)
      record_id = table.primary_key(new_data)
      next unless record_id

      UpdateEvent.new(
        transaction_id:,
        lsn:,
        context:,
        schema: table.schema,
        table: table.name,
        primary_key: record_id,
        old: old_data,
        new: new_data,
      ).tap { |event| watcher.on_event(event) }

    in XLogData(lsn:, data: PG::Replication::PGOutput::Delete(oid:, old:, key:))
      table = tables[oid]
      next unless watcher.should_watch_table? table.full_table_name
      old_data = table.decode_row(old.presence || key)
      record_id = table.primary_key(old_data)
      next unless record_id

      DeleteEvent.new(
        transaction_id:,
        lsn:,
        context:,
        schema: table.schema,
        table: table.name,
        primary_key: record_id,
        old: old_data,
      ).tap { |event| watcher.on_event(event) }

    else
      next
    end
  rescue
    watch_conn&.close
    raise
  end
end

#replicate_forever(watcher, publications:) ⇒ Object 



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# File 'lib/wal/replicator.rb', line 19

def replicate_forever(watcher, publications:)
  replication = replicate(watcher, publications:)
  loop { replication.next }
rescue StopIteration
  nil
end